Question

A certain organic substance used as a solvent in many reactions contains carbon, hydrogen, oxygen and sulphur. Weight % of hydrogen in the compound is 7.7. The weight ratio C : O : S = 3 : 2 : 4. What is the least possible molar mass of the compound ?

• A. 86
• B. 63
• C. 94
• D. 78

Answer 1

Answer: (D)

78

Answer 2

(4)

C : O : S = 3 : 2 : 4

Hydrogen is = 7.7%

$\therefore$ 100 – 7.7 = 92.3 % contains C,O & S

$\% C = \left( \frac{3}{3 + 2 + 4} \right)92.3; \% O = \frac{2}{9} \times 92.3; \% S = \frac{4}{9} \times 92.3$

\begin{tabular} { | c | c | c | c | c | } \hline Elements & % & %/Atomic mass & Simple ratio & Simplest whole no. \ \hline H & 7.7 & 7.7 & 6 & 6 \ \hline C & 30.76 & $30.78 / 12 = 2.56$ & 2 & 2 \ \hline O & 20.51 & $20.15 / 16 = 1.28$ & 1 & 1 \ \hline S & 41.02 & $41.02 / 32 = 1.28$ & 1 & 1 \ \hline \end{tabular}

$\therefore$ empirical formula C2H6OS

minimum molar mass = 24 + 6 + 16 + 32 = 78