Question

A mixture of $O_2$ and gas "Y" (mol.wt.80) in the mole ratio $a:b$ has a mean molecular weight 40. What would be mean molecular weight, if the gases are mixed in the ratio $b:a$ under identical conditions? (gases are non-reacting):

• A. 40
• B. 48
• C. 62
• D. 72

Answer 1

The correct option is D 72

Let mole fraction of $O_2$ is $x$
$40=32\times x+80(1-x)$
Or $x=\frac{5}{6}$
$a:b=x:(1-x)=\frac{5}{6}:\frac{1}{6}$
When ratio is changed
$M_{mixture}=32\times \frac{1}{6}+80\times \frac{5}{6}=72$