# A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. - Chemistry

Numerical

A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate

1. empirical formula,
2. molar mass of the gas, and
3. molecular formula.

#### Solution

(i) 1 mole (44 g) of CO2 contains 12 g of carbon.

∴ 3.38 g of CO2 will contain carbon =(12  "g")/(44  "g")xx3.38  "g"

= 0.9217 g

18 g of water contains 2 g of hydrogen.

∴ 0.690 g of water will contain hydrogen =(2  "g")/(18  "g")xx0.690

= 0.0767 g

Since carbon and hydrogen are the only constituents of the compound, the total mass of the compound is:

= 0.9217 g + 0.0767 g

= 0.9984 g

∴ Percent of C in the compound (0.9217  g)/(0.9984  g)xx100

= 92.32%

Percent of H in the compound =(0.0767  "g")/(0.9984  "g")xx100

= 7.68%

Moles of carbon in the compound =92.32/12.00

= 7.69

Moles of hydrogen in the compound =7.68/1

= 7.68

∴ Ratio of carbon to hydrogen in the compound = 7.69 : 7.68

= 1 : 1

Hence, the empirical formula of the gas is CH.

(ii) Given,

Weight of 10.0L of the gas (at S.T.P) = 11.6 g

∴ Weight of 22.4 L of gas at STP =(11.6  "g")/(10.0  "L")xx22.4  "L"

= 25.984 g

≈ 26 g

Hence, the molar mass of the gas is 26 g.

(iii) Empirical formula mass of CH = 12 + 1 = 13 g

"n" ="molar mass of gas"/"Empirical formula mass of gas"

=(26  "g")/(13  "g")

n = 2

∴ Molecular formula of gas = (CH)n

= C2H2

Concept: Mole Concept and Molar Masses
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Chapter 1: Some Basic Concepts of Chemistry - EXERCISES [Page 28]

#### APPEARS IN

NCERT Chemistry Part 1 and 2 Class 11
Chapter 1 Some Basic Concepts of Chemistry
EXERCISES | Q 1.34 | Page 28

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