Question

$5$ litre of gas at STP weights $6.25g$. What is its gram molecular weight?

• A. $1.25$
• B. $14$
• C. $28$
• D. $56$

Answer 1

The correct option is C $28$

Given,

Volume occupied by gas, $V=5L$

Pressure $=1$atm (STP condition)

Temperature $=273K$(STP condition)

weight. of the gas $=6.25g$

Let the $M$ be the gram molecular weight and, the no. of moles of gas is , $n=\frac{\text{weight of the gas}}{\text{gram molecular weight}}$

$\Rightarrow n=\frac{6.25}{M}$

Now, using the ideal gas equation, i.e $PV=nRT$

$n=\frac{PV}{RT}$

$\Rightarrow \frac{6.25}{M}$ = $\frac{1\times 5}{0.0821\times 273}$

$\Rightarrow \frac{6.25}{M}$ = $\frac{5}{22.4}$

$\Rightarrow M=\frac{6.25\times 22.4}{5}=28$

Hence, the gram molecular weight of the gas is $28g/mol.$ Thus, Option C is correct.