Question

(a) (i) A compound has the following percentage composition by mass: carbon 14.4%, hydrogen 1.2% and chlorine 84.5%. Determine the empirical formula of this compound. Work correct to 1 decimal place. (H = 1; C = 12; Cl = 35.5)
(ii) The relative molecular mass of this compound is 168, so what is its molecular formula?
(iii) By what type of reaction could this compound be obtained from ethyne?

(b) From the equation:
C + 2H₂SO₄ $\stackrel{}{\to }$ CO₂ + 2H₂O + 2SO₂
Calculate:
(i) the mass of carbon oxidised by 49 g of sulphuric acid (C = 12; relative molecular mass of sulphuric acid = 98).
(ii) the volume of sulphur dioxide measured at STP, liberated at the same time.
(Volume occupied by 1 mole of a gas at STP is 22.4 dm³).

Answer 1

(a)
(i)

Element	Atomic mass	Percentage	Relative ratio of atoms	Simplest ratio
C	12	14.4	$\frac{14.4}{12}=1.2$	$\frac{1.2}{1.2}=1$
H	1	1.2	$\frac{1.2}{1}=1.2$	$\frac{1.2}{1.2}=1$
Cl	35.5	84.5	$\frac{84.5}{35.5}=2.4$	$\frac{2.4}{1.2}=2$

Empirical formula of compound is CHCl₂.

(ii) Empirical formula mass of compound = (12 + 1 + 71) = 84 g
Relative molecular mass of compound = 168 g
Molecular formula = (Empirical formula)_n
$n=\frac{\mathrm{Molecular}\mathrm{formula}\mathrm{mass}}{\mathrm{Empirical}\mathrm{formula}\mathrm{mass}}=\frac{168}{84}=2$
Molecular formula = (CHCl₂)₂ = C₂H₂Cl₄

(iii) Addition reaction is the type of reaction from which this compound is formed from ethyne.
C₂H₂ + Cl₂ → C₂H₂Cl₂ $\stackrel{{\mathrm{Cl}}_2}{\to }$ C₂H₂Cl₄

(b)
(i) According to the balanced chemical equation, one mole of carbon is oxidised by two moles of sulphuric acid.
Molecular mass of sulphuric acid = 98 u
Number of moles of sulphuric acid in 49 g = $\frac{49}{98}=0.5$
Number of moles of carbon oxidised by 0.5 mol of sulphuric acid = $\frac{0.5}{2}=0.25$
Mass of 0.25 moles of carbon = (Moles of carbon) (Atomic mass of C)
= (0.25) (12) = 3 g

(ii) According to the balanced chemical equation, one mole of sulphur dioxide gas is produced by one mole of sulphuric acid.
Volume occupied by 0.5 mol of sulphur dioxide gas = (Number of moles of sulphur dioxide gas) (Molar volume)
= (0.5) (22.4) = 11.2 dm³