Question

(a) Calcium carbide is used for the artificial ripening of fruits. Actually the fruit ripens because of the heat evolved while calcium carbide reacts with moisture. During this reaction calcium hydroxide and acetylene gas is formed. If 200 cm³ of acetylene is formed from a certain mass of calcium carbide, find the volume of oxygen required and carbon dioxide formed during the complete combustion. The combustion reaction can be represented as below:
2C₂H_2(g) + 5O_2(g) → 4CO_2(g) + 2H₂O_(g)

(b) A gaseous compound of nitrogen and hydrogen contains 12.5% hydrogen by mass. Find the molecular formula of the compound if its relative molecular mass is 37.
[N = 14, H = 1].

Answer 1

(a) According to the balanced chemical reaction, for two unit volume of acetylene, five unit volume of oxygen is required.
Volume of oxygen required by 200 cm³ of acetylene = $\frac{5}{2}\times 200\times {10}^{-3}{\mathrm{dm}}^3=500\times {10}^{-3}{\mathrm{dm}}^3=500{\mathrm{cm}}^3$
Also, two units of acetylene produce four units of carbon dioxide.
Volume of carbon dioxide produced by 200 cm³ of acetylene = $\frac{4}{2}\times 200\times {10}^{-3}{\mathrm{dm}}^3=400\times {10}^{-3}{\mathrm{dm}}^3=400{\mathrm{cm}}^3$

(b)

Element	Atomic mass	Percentage	Relative ratio of atoms	Simplest ratio of atoms
H	1	12.5	12.5	$\frac{12.5}{6.25}=2$
N	14	=(100−12.5) = 87.5	$\frac{87.5}{14}=6.25$	$\frac{6.25}{6.25}=1$

Empirical formula of compound is NH₂.
Empirical formula mass = (14 + 2) g = 16 g
Relative molecular mass of the compound is 37.
Molecular formula of the compound = (Empirical formula of the compound)_n
$n=\frac{\mathrm{Molecular}\mathrm{formula}\mathrm{mass}}{\mathrm{Empirical}\mathrm{formula}\mathrm{mass}}=\frac{37}{16}=2.3\approx 2$
Molecular formula = (NH₂)₂ = N₂H₄