Question

A mixture of methane and ethene in a molar ratio of x : y has an average molecular mass of 20U. The mean molar mass when they are mixed in the molar ratio of y : x will be –

• A. 20 U
• B. 25 U
• C. 24 U
• D. 15 U

Answer 1

Answer: (C)

24 U

Answer 2

The mol. Wt of CH₄ = 16

The mol. Wt of C₂ H₄ = 28

\ 20 = $\frac{16x + 28y}{x + y}$

or 16x + 28 y = 20 x + 20 y

or 4x = 8y

or x = 2y

In the gas mix. when the mole ratio of CH₄ and C₂ H₄ is y : x

then avg. mol. wt = $\frac{16y + 28x}{x + y}$ = $\frac{16y + 56y}{3y}$ = $\frac{72y}{3y}$ = 24