Calculate the relative molecular mass [molecular weight] of 290 ml. of a gas ‘A’ at 17 °C and 1520 mm pressure which weighs 2.73 g at s.t.p. [1 litre of hydrogen at s.t.p. weighs 0.09 g.]

Convert the volume to s.t.p. using gas equation

Using the gas equation,

⇒ P_1V_1\over T_1=P_2V_2\over T_2

Substituting the values we get,

⇒ 1520× 290\over 290=760× x\over 273

⇒ x = 1520 × 273\over 760

$x=\; 546\; ml$

546 ml of gas at s.t.p. weighs 2.73 g

∴ Wt. of 1000 ml of gas = 2.73\over 546 × 1000 = 5 g

Vapour density of gas X

=  Wt\ of\ 1\ lit\ of\ gas\ X \over Wt\ of\ 1\ lit\ of\ gas\ H_2

= 5\over 0.09

= 55.55

Hence, vapour density of gas is 55.55

Molecular weight = 2 × Vapour Density

= 2 × 55.555

= 111.11 g

Hence, molecular weight of gas is 111.11 g