FRANK Solutions for Class 10 Chemistry Chapter 5 - Mole Concept And Stoichiometry

Chapter 5 - Mole Concept And Stoichiometry Exercise 115

Question 1

State:
(a) Gay-Lussac's law of combining volumes
(b) Avogadro's law

Solution 1

(a) Gay-Lussac's law: It states that 'when gases react, they do so in volumes which bear a simple ratio to one another, and also to the volume of the gaseous product, provided all the volumes are measured at the same temperature and pressure'.
(b) Avogadro's law : It states that 'Under the same conditions of temperature and pressure, equal volumes of all gases contain the same number of molecules'.

Question 2

(a) Define atomicity of a gas.
(b) Differentiate between 2H and H₂.

Solution 2

Question 3

When stating the volume of a gas, the pressure and temperature should also be given. Why?

Solution 3

When stating the volume of a gas, the pressure and temperature should also be given because the volume of a gas is highly susceptible to slight change in pressure and temperature of the gas.

Question 4

(a) The relative atomic mass of Cl atom is 35.5 a.m.u. Explain this statement.
(b) What is the value of Avogadro's number?
(c) What is the value of molar volume of a gas at STP?

Solution 4

Question 5

Define the terms:
(a) Vapour density.
(b) Molar volume.
(c) Relative atomic mass.
(d) Avogadro's number.
(e) Relative molecular mass.

Solution 5

Question 6

(a) What are the main applications of Avogadro's law?
(b) How does Avogadro's law explain Gay - lussac's law of combining volumes?

Solution 6

Question 7

Explain the terms:
(a) Gram atom
(b) Gram mole
(c) Mole

Solution 7

(a) Gram atom: "The quantity of the element which weighs equal to its gram atomic mass is called one gram atom of that element".
For example: The gram atomic mass of hydrogen is 1g. So, 1g of hydrogen is 1 gram atom of hydrogen.
(b) Gram mole: "A sample of substance with its mass equal to its gram molecular mass is called one gram molecule of this substance or one gram mole".
For example: Gram molecular mass of oxygen is 32 g. So One gram mole of oxygen is 32g.

Question 8

Calculate the relative molecular masses of (use K = 39, Cl = 35.5, O = 16, C = 12, H = 1, Na = 23, N = 14, S= 32)
(a) Potassium chlorate
(b) Sodium acetate
(c) Chloroform
(d) Ammonium sulphate

Solution 8

Question 9

Explain the terms empirical formula and molecular formula.

Solution 9

Empirical formula:"Empirical formula of a compound is the formula which gives the number of atoms of different elements present in one molecule of the compound, in the simplest numerical ratio".
Molecular formula: "Molecular formula of a compound denotes the actual number of atoms of different elements present in one molecule of the compound".

Question 10

Give the empirical formula of:
(a) C₆H₆
(b) C₆H₁₂O₆
(c) C₂H₂
(d) CH₃COOH

Solution 10

(a) The empirical formula of C₆H₆ is: CH
(b) The empirical formula of C₆H₁₂O₆ is: CH₂O.
(c) The empirical formula of C₂H₂ is: CH
(d) The empirical formula of CH₃COOH is: CH₂O.

Question 11

Give three pieces of information conveyed by the formula H₂O.

Solution 11

Three pieces of information conveyed by the formula H₂O is that:
It shows that there are 2hydrogen atoms and 1oxygen atoms present in H₂O.
The hydrogen and oxygen atoms are present in simplest whole number ratio of 2:1.
It represents one molecule of compound water.

Question 12

What do you understand by the term mole? How many elementary units are in one mole of a substance?

Solution 12

Question 13

Fill in the blanks:
(a) Molecular weight of a gas is twice its ______.
(b) Mass of 22.4 litre of a gas at STP is ______.
(c) One gram atom of an element contains ______ atoms.
(d) One a.m.u. is the mass of ______ atom of C^₁₂.
(e) Avogadro's number is equal to ______.

Solution 13

Question 14

Solution 14

Question 15

Give examples of compounds whose:
(a) Empirical formula is the same as the molecular formula.
(b) Empirical formula is different from the molecular formula.

Solution 15

(a) Na₂SO₄.10H₂O.
(b) C₆H₁₂O₆.

Chapter 5 - Mole Concept And Stoichiometry Exercise 116

Question 1

Solution 1

Question 2

Solution 2

Question 3

Solution 3

Question 4

Calculate the percentage composition of oxygen in lead nitrate [Pb(NO₃)₂]. [Pb = 207, N= 14, O = 16]

Solution 4

Question 5

The usefulness of a fertilizer depends upon percentage of nitrogen present in it. Find which of the following is a better fertilizer:
(a) Ammonium nitrate [NH₄NO₃]
(b) Ammonium phosphate [(NH₄)₃PO₄] (N=14,H=1,O=16,P=31)

Solution 5

Question 6

A compound of lead has following percentage composition, Pb = 90.66%, O = 9.34%. Calculate empirical formula of a compound. [Pb = 207, O = 16]

Solution 6

Question 7

Empirical formula of a compound is CH₂O. If its empirical formula is equal to its vapour density, calculate the molecular formula of the compound.

Solution 7

Question 8

Solution 8

Question 9

(a)What is the volume occupied by the following gases at STP?
(i) 48 g of oxygen.
(ii) 16 g of sulphur dioxide.
(b) Determine the  molecular mass of a gas if 5g of it occupy a volume of 4 L at STP.

Solution 9

Question 10

Solution 10

Question 11

Calculate the percentage of water in ferrous sulphate crystals [Fe = 56, S = 32, O =16, H = 1].

Solution 11

Question 12

An organic compound has the following percentage composition: C = 12.76%, H = 2.13%, Br = 85.11%. The vapour density of the compound is 94. Find out its molecular formula.

Solution 12

Question 13

Two oxides of a metal (M) have 20.12% and 11.19% oxygen. The formula of the first oxide is MO. Determine the formula of the second oxide.

Solution 13

Question 14

Calculate the volume of air at STP, required to convert 300 mL of sulphur dioxide to sulphur trioxide. Air contains 21% of oxygen by volume.

Solution 14

Question 15

Solution 15

Question 16

A student puts his signature with graphite pencil. If the mass of carbon in the signature is 10^-12 g. Calculate the number of carbon atoms in the signature.

Solution 16

Chapter 5 - Mole Concept And Stoichiometry Exercise 117

Question 1

Under the same conditions of temperature and pressure you collect 2 L of carbon dioxide, 3L of chlorine, 5 L of hydrogen, 4 L of nitrogen and 1 L of sulphur dioxide. In which gas sample will there be :
(a) The greatest number of molecules.
(b) The least number of molecules.
Justify your answer.

Solution 1

Question 2

Solution 2

Question 3

Find the total percentage of oxygen in magnesium nitrate crystal Mg(NO₃)₂.6H₂O.
[H = 1, N = 14, O = 16, Mg = 24]

Solution 3

Question 4

A compound contains 87.5% by mass of nitrogen and 12.5% by mass of hydrogen. Determine the empirical formula of this compound.

Solution 4

Question 5

(a) Is it possible to change the temperature and pressure of a fixed mass of a gas without changing its volume? Explain your answer.
(b) Define or explain the meaning of term 'molar volume'.

Solution 5

Question 6

What is the mass of nitrogen in 1000 Kg of urea [CO(NH2)2] ?
[H = 1, C= 12, N= 14, O = 16]

Solution 6

Question 7

(a) Calculate the empirical formula of the compound having 37.6% sodium, 23.1% silicon and 39.3% oxygen.(Answer correct to two decimal places) (O = 16, Na = 23, Si = 28)
(b) The empirical formula of a compound is C₂H₅. Its vapour density is 29. Determine the relative molecular mass of the compound and hence its molecular formula.

Solution 7

Question 8

Solution 8

Question 9

(a) Calculate the percentage of boron (B) in borax (Na₂B₄O₇.10H₂O). [H = 1, B = 11, O = 16, Na = 23] , answer correct to 1 decimal place).
(b) (i) The compound A has the following percentage composition by mass:
C =26.7%, O = 71.1%, H = 2.2%. Determine the empirical formula of A.(Answer to one decimal place) (H=1,C=12,O=16)
(ii) If the relative molecular mass of A is 90, what is the molecular formula of A?

Solution 9

Chapter 5 - Mole Concept And Stoichiometry Exercise 118

Question 1

Solution 1

Question 2

Solution 2

Question 3

If a crop of wheat removes 20 Kg of nitrogen per hectare of soil, what mass of the fertilizer calcium nitrate, Ca(NO₃)₂ would be required to replace nitrogen in 10 hectare field? (N = 14, O = 16, Ca = 40)

Solution 3

Question 4

(a) A vessel contains N molecules of oxygen at a certain temperature and pressure. How many molecules of sulphur dioxide can the vessel accommodate at the same temperature and pressure?
(b) Each of the two flasks contains 2.0 g of gas at the same temperature and pressure. One flask contains oxygen and the other hydrogen.
(i) Which sample contains the greater number of molecules?
(ii) If the hydrogen sample contains N molecules, how many molecules are present in oxygen sample?

Solution 4

Question 5

Solution 5

Question 6

Determine the empirical formula of the compound whose composition by mass is 42% nitrogen, 48% oxygen and 9% hydrogen.[N=14,O=16,H=1]

Solution 6

Chapter 5 - Mole Concept And Stoichiometry Exercise 119

Question 1

When gases react their volumes bear a simple ratio to each other, under the same conditions of temperature and pressure. Who proposed this gas law?

Solution 1

Gay - Lussac proposed this law.

Question 2

Solution 2

Molecular mass of ethane = 30
According to Gay-Lussac's law:
2 vol. of C₂H₆ requires= 7 vol. of oxygen
Vol. of C₂H₆ = 2 vol. = 100 L
Vol. of oxygen required = 7 vol. =350 L

Question 3

Solution 3

Question 4

Mention the term defined by the following sentence:
The mass of a given volume of gas compared to the mass of an equal volume of hydrogen.

Solution 4

The term is vapour density.

Question 5

Solution 5

Question 6

Calculate the percentage of phosphorous in the fertilizer superphosphate, Ca(H₂PO₄)₂. [Ca = 40, H =1, P =31, O = 16] (Correct to 1 decimal place)

Solution 6

Question 7

A metal M, forms a volatile chloride containing 65.5% Chlorine. If the density of the chloride relative to hydrogen is 162.5, find the molecular formula of the chloride. [M = 56, Cl = 35.5]

Solution 7

Question 8

Samples of the gases O₂, N₂, CO₂ and CO under the same conditions of temperature and pressure contain the same number of molecules represented by X. The molecules of Oxygen, occupy V litres and have a mass of 8 g. Under the same conditions of temperature and pressure:
(a) What is the volume occupied by:
(i) X molecules of N₂,
(ii) 3X molecules of CO? [C=12,N=14,O=16]
(b)  What is the mass of CO₂ in g?
(c) In answering the above questions, which law have you used?

Solution 8

Question 9

Calculate the percentage of platinum in ammonium chloroplatinate (NH₄)₂PtCl₆. [N = 14, H = 1, Pt = 195, Cl =35.5] (Give your answer correct to the nearest whole number)

Solution 9

Question 10

The percentage composition of sodium phosphate, as determined by analysis is: 42.1% Na, 18.9% P, 39% of O. Find the empirical formula of the compound. [H =1, N =14, Na = 23, P = 31, Cl = 35.5, Pt = 195]

Solution 10

Chapter 5 - Mole Concept And Stoichiometry Exercise 120

Question 1

Solution 1

Question 2

Solution 2

Question 3

Solution 3

Question 4

Solution 4

Chapter 5 - Mole Concept And Stoichiometry Exercise 121

Question 1

(a) The volumes of gases A, B, C and D are in the ratio, 1:2:2:4 under the same conditions of temperature and pressure.
(i) Which sample of gas contains the maximum number of molecules?
(ii) If the temperature and the pressure of gas A are kept constant then what will happen to the volume of A when the number of molecules is doubled?
(iii) If this ratio of gas volumes refers to the reactants and products of a reaction, which gas law is being observed?
(iv) If the volume of A is actually 5.6 dm3 at STP, calculate the number of molecules in the actual volume of D at STP.
(v) Using your answer from (iv), state, the mass of D if the gas is dinitrogen oxide (N₂O)
(b) Calculate the percentage of nitrogen in aluminium nitride. [Al = 27, N = 14]

Solution 1

Question 2

Solution 2

Question 3

(a) Calculate the number of moles and number of molecules present in 1.4 g of ethylene gas. What is the volume occupied by the same amount of ethylene?
(b) What is the vapour density of ethylene? (Avogadro's number = 6x10²³; Atomic weight of C = 12, H = 1; Molar volume = 22.4 litres at STP)

Solution 3

Question 4

Solution 4

Question 5

Solution 5

Chapter 5 - Mole Concept And Stoichiometry Exercise 122

Question 1

A compound 'X' consists of 4.8% of C and 95.2% of Br by mass.
(i) Determine the empirical formula of this compound. [C = 12, Br = 80]
(ii) If the vapour density of the compound is 252, what is the molecular formula of the compound?
(iii) Name the type of chemical reaction by which X can be prepared from ethane.

Solution 1

Question 2

Choose the correct alternative:
The gas laws which relates the volume of a gas to the number of molecules of the gas is:
(a) Avogadro's  law
(b) Gay-Lussac's law
(c) Boyle's law
(d) Charle's law

Solution 2

The gas laws which relates the volume of a gas to the number of molecules of the gas is avogadro's law

Question 3

Solution 3

Question 4

Solution 4

Chapter 5 - Mole Concept And Stoichiometry Exercise 123

Question 1

Choose the correct answer:
A gas cylinder of capacity of 20 dm³ is filled with gas X the mass of which is 10 g. When the same cylinder is filled with hydrogen gas at the same temperature and pressure the mass of the hydrogen is 2 g, hence the relative molecular mass of the gas is:
(a) 5
(b) 10
(c) 15
(d) 20

Solution 1

The relative molecular mass of the gas is 10.

Question 2

Solution 2

Question 3

Correct the following statement:
Equal masses of all gases under identical conditions contain the same number of molecules.

Solution 3

The correct statement is that equal volumes of all gases under identical conditions contain the same number of molecules.

Question 4

Solution 4

Question 5

(i) LPG stands for liquefied petroleum gas. Varieties of LPG are marketed including a mixture of propane (60%) and butane (40%). If 10 litre of this mixture is burnt, find the total volume of carbon dioxide gas added to the atmosphere. Combustion reaction can be presented as :

(ii) Calculate the percentage of nitrogen and oxygen in ammonium nitrate [Relative molecular mass of ammonium nitrate is 80, H=1, N=14, O = 16].

Solution 5

(ii) Molecular mass of NH₄(NO₃) = 80

H = 1, N = 14, O = 16

% of nitrogen

As 80 g of NH₄ (NO₃) contains 28 g of nitrogen,

 

% of nitrogen

As80 g of NH₄(NO₃) contains48 g of oxygen

 

Question 6

4.5 moles of calcium carbonate are reacted with dilute hydrochloric acid.

(i) Write the equation for the reaction.

(ii) What is the mass of 4.5 moles of calcium carbonate? Relative molecular mass of calcium carbonate is 100)

(iii) What is the volume of carbon dioxide liberated at STP?

(iv) What mass of calcium chloride is formed? (Relative molecular mass of calcium chloride is 111)

(v) How many moles of HCl are used in this reaction

Solution 6

(i) Equation for the reaction of calcium carbonate with dilute hydrochloric acid:

 

(ii) Relative molecular mass of calcium carbonate=100

 Mass of 4.5 moles of calcium carbonate

 = No. of moles× Relative molecular mass

 = 4.5×100

 = 450g

(iii)  

 As100g of calcium carbonate gives 22.4dm³ of CO₂,

 

(iv) Molecular mass of calcium carbonate =100

 Relative molecular mass of calcium chloride =111

 As 100 g of calcium carbonate gives 111g of calcium chloride,

 

(v) Molecular mass of HCl=36.5

 Molecular mass of calcium carbonate =100

 As 100 g of calcium carbonate gives (2×36.5)= 73g of HCl,

 

Chapter 5 - Mole Concept And Stoichiometry Exercise 124

Question 1

(i) Calculate the volume of 320 g of SO₂ at STP (Atomic mass : S = 32 and O = 16)

(ii) State Gay-Lussac's Law of combining volumes.

(iii) Calculate the volume of oxygen required for the complete combustion of 8.8 of propane (C₃H₅). (Atomic mass : C = 14, O = 16, H = 1, Molar Volume = 22.4 dm³ at STP)

Solution 1

(i) Atomic mass: S = 32 and O = 16

 Molecular mass of SO₂=32+(2×16)=64g

 As 64 g of SO2 = 22.4dm³,

 

(ii) Gay-Lussac's law: When gases react, they do so in volumes which bear a simple ratio to one another and to the volume of the gaseous product, if all the volumes are measured at the same temperature and pressure.

(iii) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

 Molar mass of propane = 44

 44 g of propane requires 5 × 22.4 litres of oxygen at STP.

 

Question 2

An organic compound with vapour density = 94 contains C = 12.67%, H = 2.13% and Br = 85.11%. Find the molecule formula.

[Atomic mass C = 12, H = 1, Br = 80]

Solution 2

Element	Relative atomic mass	% Compound	Atomic ratio	Simplest ratio
H	1	2.13	2.13/1 = 2.13	2
C	12	12.67	12.67/12 = 1.055	2
Br	80	85.11	85.11/80 = 1	1

Empirical formula = CH₂Br

n(Empirical formula mass of CH₂Br) = Molecular mass (2 × VD)

n(12 + 2 + 80) = 94 × 2

n = 2

Molecular formula = Empirical formula × 2

= (CH₂Br) × 2

= C₂H₄Br₂

Question 3

Calculate the mass of

(i) 10²² atoms of sulphur

(ii) 0.1 mole of carbon dioxide

 [Atomic mass : S = 32,C=12, and O = 16 and Avogadro's number = 6 × 10²³]

Solution 3

(i) 10²² atoms of sulphur

 6.022 × 10²³ atoms of sulphur will have mass = 32 g

 

(ii) 0.1 mole of carbon dioxide

 1 mole of carbon dioxide will have mass = 44 g

 0.1 mole of carbon dioxide will have mass = 4.4 g

Question 4

O₂is evolved by heating KCIO₃ using MnO₂ as a catalyst.

(i) Calculate the mass of KClO₃ required to produce 6.72 litre of O₂at STP

[Atomic masses : K = 39, Cl = 35.5, O = 16]

(ii) Calculate the number of moles of oxygen present in the above volume and also the number of molecules

(iii) Calculate the volume occupied by 0.01 mole of CO₂at STP

Solution 4

Question 5

Solve the following :

(i) What volume of oxygen is required to burn completely 90dm³ of butane under similar conditions of temperature and pressure? 

(ii) The vapour density of a gas is 8. What would be the volume occupied by 24.0 of the gas at STP?

(iii) A vessel contains X number of molecules of hydrogen gas at a certain temperature and pressure. How many molecules of nitrogen gas would be present in the same vessel under the same conditions of temperature and pressure?

Solution 5

Question 6

(i) Oxygen oxidase ethyne to carbon dioxide and water as shown by the equation :

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O

What volume of ethylene gas at STP is required to produce 8.4 dm³of carbon dioxide at STP?

(ii) A compound made up of two elements X and Y has an empirical formula X₂Y. if the atomic weight of X is 10 and that Y is 5 and the compound has a vapour density 25, find its molecular formula.

Solution 6

Chapter 5 - Mole Concept And Stoichiometry Exercise 125

Question 1

Give one word or phrase for the following:

(i) The ratio of the mass of a certain volume of gas to the mass of an equal volume of hydrogen under the same conditions of temperature and pressure.

(ii) Formation of ions from molecules

Solution 1

(i) Vapour density

(ii) Ionisation 

Question 2

(i) State Avogadro's Law

(ii) A cylinder contains 68g of ammonia gas at STP

(1) What is the volume occupied by this gas?

(2) How many moles of ammonia are present in the cylinder?

(3) How many molecules of ammonia are present in the cylinder? [N-14,H-1]

Solution 2

(i) Avogadro's law:Equal volumes of all gases under similar conditions of temperature and pressure contain the same number of molecules.

(ii)  

Question 3

Which of the following would weigh the least?

(A) 2 gram atoms of Nitrogen

(B) 1 mole of Silver

(C) 22.4 litres of oxygen gas at 1 atmospheric pressure and 273K

(D) 6.02 × 10²³ atoms of carbon

[Atomic masses : Ag = 108, N - 14, O = 16,C = 12]

Solution 3

6.02 × 10²³ atoms of carbon

Question 4

Complete the following calculations. Show working for complete credit:

(i) Calculate the mass of calcium that will contain the same number of atoms as are present in 3.2 gm of Sulphur. [Atomic masses : S = 32, Ca = 40]

(ii) If 6 litres of hydrogen and 4 litres of chlorine are mixed and exploded and if water is added to the gases formed, find the volume of the residual gas.

(iii) If the empirical formula of a compound is CH and it has a vapour density of 13, find the molecular formula of the compound.

Solution 4

(i) 3.2 g of S has number of atoms = 6.023 × 10²³ x 3.2 /32

 = 0.6023 × 10²³

 So, 0.6023 × 10²³ atoms of Ca has mass=40 × 0.6023 ×10²³/6.023 × 10²³ = 4g

(ii) 6 litres of hydrogen and 4 litres of chlorine when mixed result in the formation of 8 litres of HCl gas.

 When water is added to it, it results in the formation of hydrochloric acid. Chlorine acts as a limiting agent leaving behind only 2 litres of hydrogen gas.

 Therefore, the volume of the residual gas will be 2 litres.

(iii)  

Question 5

Consider the following reaction and based on the reaction answer the questions that follow :

Calculate :

(i) The quantity in moles of (NH₄)₂Cr₂O₇ if 63 gm of (NH₄)₂Cr₂O₇ is heated.

(ii) The quantity in moles of nitrogen formed.

(iii) The volume in litres or dm³ of N₂evolved at STP

(iv) The mass in grams of Cr₂O₃ formed at the same time

 [Atomic masses : H = 1, Cr=52, N=14]

Solution 5

(i)  

(ii) 252 g of ammonium dichromate gives 22.4 dm³of N₂

(iii) 

(iv) 252 g of ammonium dichromate gives 152 g of CrO₃

Question 6

A gas cylinder contain 12 × 10²⁴ molecules of oxygen gas

If Avogadro's number is 6 × 10²³, calculate :

(i) The mass of oxygen present in the cylinder

(ii) The volume of oxygen at STP present in the Cylinder [O = 16]

Solution 6

Question 7

A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapour density is 29, find its molecular formula.

[C = 12, H = 1]

Solution 7

% of carbon = 82.76%

% of hydrogen = 100 - 82.76 = 17.24%

Element	% Weight	Atomic Weight	Relative No. of moles	Simplest Ratio
C	82.76	12	82.76/12 = 6.89	6.89/6.8 = 1 × 2 = 2
H	17.24	1	17.24/1 = 17.24	17.24/6.89 = 2.5 × 2 = 5

Empirical formula = C₂H₅

Empirical formula weight = 2 × 12 + 1 × 5 = 24 + 5 = 29

Vapour density = 29

Relative molecular mass = 29 × 2 = 58

Chapter 5 - Mole Concept And Stoichiometry Exercise 126

Question 1

The equation 4NH³5O₂ → 4NO + 6H₂O represents the catalytic oxidation of ammonia. If 100 cm³of ammonia is used, calculate the volume of oxygen required to oxidise the ammonia completely.

Solution 1

Question 2

A gas of 32 g mass has a volume of 20 litres at STP. Calculate the gram molecular weight of the gas.

Solution 2

Mass of gas = 32 g

Volume occupied by 32g of gas = 20litres

Question 3

How much calcium oxide is formed when 82 g of calcium nitrate is heated? Also find the volume of nitrogen dioxide evolved:

2Ca(NO₃)₂→ 2CaO + 4NO₂ + O₂ (Ca = 40,N = 14, O = 16)

Solution 3

2Ca(NO₃)₂→ 2CaO + 4NO₂ + O₂

Molecular weight of 2Ca(NO₃)₂ = 2[40+2(14+48) = 32g

Molecular weight of CaO = 2(40 + 16) = 112g

328 g of Ca(No₃]₂ liberates 4 moles of NO₂

328 g of Ca (NO₃)₂ liberates 4 × 22.4 L of NO₂

Question 4

The ratio between the number of molecules in 2g of hydrogen and 32 g of oxygen is

(A) 1:2

(B) 1:0:01

(C) 1:1

(D) 0.01:1

[Given that H=1,O=16]

Solution 4

Question 5

(a) Calculate the number of gram atoms in 4.6 grams of sodium (Na = 23)

(b) Calculate the percentage of water of crystallization in CuSO_4. 5H₂O

(H = 1, O = 16, S = 32, Cu = 64)

(c) A compound of X and Y has the empirical formula XY₂. Its vapour density is equal to its empirical formula weight. Determine its molecular formula.

Solution 5

(a) 

(b) 

(c)  

Question 6

(a) Propane burns in air according to the following equation :

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

What volume of propane is consumed on using 1000 cm³of air, considering only 20% of air contains oxygen?

(b) The mass of 11.2 litres of a certain gas at s.t.p is 24 g. Find gram molecular mass of the gas

Solution 6

(a)  

 Given:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Volume of air = 1000 cm³

Percentage of oxygen in air = 20%

From the given information,

 

According to Gay-Lussac's law,

1 vol. of propane consumes 5 vol. of oxygen.

Volume of oxygen = 1000 cm³ × 20% = 200 cm³

Therefore,

Volume of propane burnt for every 200 cm³ of oxygen,

40 cm³of propane is burnt. 

(b)  

(i) Given:

 Volume of gas at STP = 11.2 litres

Mass of gas at STP = 24 g

Gram molecular mass = ?

Mass of 22.4 L of a gas at STP is equal to its gram

molecular mass.

11.2 L of the gas at STP weighs 24 g

So,

22.4 L of the gas will weigh

Gram molecular mass = 48 g 

Question 7

A gas cylinder can hold 1 kg of hydrogen at room temperature and pressure.

(a) Find the number of moles of hydrogen present

(b) What weight of CO₂can the cylinder hold under similar conditions of temperature and pressure? (H = 1 C = 12, O = 16)

(c) If the number of molecules of hydrogen in the cylinder is X, calculate the number of CO₂ molecules in the cylinder with the same conditions of temperature and pressure.

(d) State the law that helped you to arrive at the above result

Solution 7

(a)  

 Given:

Mass of hydrogen = 1 kg at 298 K and 1 atm pressure

 Moles of hydrogen =?

 

(b) Molecular mass of CO₂ = 12 + 2 × 16 = 44g

So, vapour density (VD) = mol. Mass/2 = 44/2 = 22

(c) According to Avogadro's law, equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules.

(d) So, number of molecules of carbon dioxide in the cylinder = number of molecules of hydrogen in the cylinder = X