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 litre (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate molecular formula of gas.

C_{2} H_{4}

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C_{4} H_{8}

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C_{2} H_{2}

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None of the above

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Solution

The correct option is C $C_{2} H_{2}$
Since, 3.38 g of carbon dioxide are obtained, the mass of C present in the sample of welding fuel gas is
$\frac{12 \times 3.38}{44} = 0.92 g$
Since, 0.690 g of water are obtained, the mass of hydrogen present in the welding fuel gas sample is
$\frac{2 \times 0.690}{18} = 0.077 g$

The percentage of C is $\frac{0.92 \times 100}{0.0770 + .92} = 92.30$ %

The percentage of H is $\frac{.077 \times 100}{0.0770 + .92} = 7.70$ %

(i) The number of moles of carbon = $\frac{92.30}{12} = 7.7$ .

The number of moles of hydrogen = $\frac{7.70}{1} = 7.7$ .

The mole ratio $C : H = 7.7 : 7.7 = 1 : 1$ .

Hence, the empirical formula of the welding fuel gas is $C H$ .

The empirical formula of welding gas is $C H$ and empirical formula mass of welding gas $= 13$ .
Molecular mass $=$ (empirical formula mass) $\times n$
$∴ n = \frac{25.98}{13} ≃ 2$
$∴$ Molecular formula $= 2 \times$ empirical formula
$\Rightarrow 2 \times (C H) = C_{2} H_{2}$

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Q. 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. Choose the correct molecular formula of the gas.

Q. 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 litre (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate molecular formula of gas.

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.

Q. 1) A welding fuel gas contain carbon and hydrogen only. Burning a small sample of it in oxygen gives