NCERT Solutions
Class 11 Chemistry
Some Basic Concepts of Chemistry

Q.1.34
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 (i) empirical formula, (ii) molar mass of the gas, and (iii) molecular formula.
(i) 1 mole (44 g) of CO2 contains 12 g of carbon.
Therefore, 3.38 g of CO2 will contain carbon = ((12 g/44g) x 3.38g)
= 0.9217 g
18 g of water contains 2 g of hydrogen.
Therefore, 0.690 g of water will contain hydrogen = ((2g/18g) x 0.690)
= 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) x 100
= 92.32%
Percent of H in the compound = (0.0767 g/0.9984 g) x 100
= 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) x 22.4L
= 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)
Therefore, Molecular formula of gas = (CH)n
= C2H2