Question

A compound contains 4% hydrogen, 24% carbon and 71% chlorine. Its molar mass is 99 g. Its molecular formula is:
1. CH₂Cl
2. C₂H₂Cl₂
3. C₂H₄Cl₂
4. CH₂Cl₂

Answer 1

Correct Answer - Option 3 : C₂H₄Cl₂

Concept:

• The molecular formula is the formula of a compound that shows its constituent atoms and their number per molecule denoted by subscripts.
• Molecular formula if known can be used to calculate:
  • Molecular mass or molar mass.
  • Percentage of elements in the compound.
• ​If the percentage composition and molar mass is known, then it can be used to calculate the empirical formula which represents the formula in the simplest whole-number ratio.
• The empirical formula gives us the Molecular formula as:

\(Molecular\;formula = {\left( {Empirical\;formula} \right)_n}\) where n = largest ratio.

Calculation:

Given:

• The compound contains:
  • 4% hydrogen
  • 24% carbon
  • 71% chlorine
• The molar mass of the compound = 99 g.

Elements	Atomic mass	% composition	Atomic ratio	Simplest ratio
H	1	4	\({4 \over 1} =4\)	\({4 \over 2} =2\)
C	12	24	\({24 \over 12} =2\)	\({2 \over 2} =1\)
Cl	35.5	71	\({71 \over 35.5} =2\)	\({2 \over 2} =1\)

• The empirical formula comes as  CH₂Cl.
• We know,

\(Molecular\;formula = {\left( {Empirical\;formula} \right)_n}\)

• The molecular formula is given by (CH2Cl)₂ which becomes C2H4Cl2.

Hence, the compound which contains 4% hydrogen, 24% carbon, and 71% chlorine, and a molar mass is 99g is  C2H4Cl2.