1.34 A welding fuel gas contains carbon and hydrogen only. Burning a small sama
in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other
volume of 10.0 L (measured at STP) of this welding gas is found to weigh the
Calculate (1) empirical formula, (ii) molar mass of the gas. and (iii) molen
formula.
Answers
Since, 3.38 g of carbon dioxide are obtained, the mass of C present in the sample of welding fuel gas is 12/44 × 3.38 = 0.92 g.Since, 0.690 g of water ...
(i) percentage of C can be calculated as follows:
CO2 = C
i.e 44 parts of CO2= 12 parts of C
OR
44g of CO2 = 12 g of C
Therefore according to question
3.38 g of CO2 contains C = 12/44 * 3.38 = 0.921 g
18 g of water contains hydrogen = 2g
Therefore 0.690 g of water contains hydrogen = 2/18 * 0.690 = 0.0767g
0.690 g of water will contain hydrogen
= 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
Now percentage of carbon = weight of carbon/weight of compound * 100
=0.921 /0.998 * 100= 92.32 %
Also percentage of hydrogen = weight of hydrogen/weight of compound *100
=0.0766/0.998 * 100 = 7.68 %
(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
= 25.984 g
≈ 26 g
Hence, the molar mass of the gas is 26 g.
(iii) empirical formula
Element Percentage Atomic mass Atomic ratio Simplest ratio Simplest whole no ratio
C 92.32 12 92.32/12 = 7.69 7.69/7.65 = 1.00 1
H 7.65 1 7.65/1 = 7.65 7.65/7.65 = 1 1
Empirical formula of the compound = CH
Now molecular formula calculation
Empirical formula mass = 12 + 1 = 13 amu
Also molecular mass = 26 g (calculated in previous step)
Therefore n = molecular mass/empirical formula mass = 26/13 = 2