Question

The vapour density of a gas A is twice that of gas B. If the molecular weight of B is M , then the molecular weight of A

Answer 1

molecular weight = 2× vapour density
Let vapour density of A = v
so v.d. of B = v/2
molecular weight of B = M = 2×v/2= v
so v= M

Now,
molecular weight of A = 2×v =2×M= 2M

Answer 2

"Molecular mass of A is 2 times the molecular mass of B.

Solution:

From the given,

Molecular weight of B = M

The vapour density of A is two times greater than that of vapour density of B.

$Vapour\quad density\quad of\quad A\quad =\quad 2\quad \times \quad { Vapour\quad density\quad of }\quad B\quad \rightarrow \quad (1)$

Mass can be calculated by the following formula.

$Mass\quad =\quad 2\quad \times \quad Vapour\quad density$

Rearrange the formula is as follows.

$Vapour\quad density\quad =\quad \frac { { Mass } }{ 2 }$

$Vapour\quad density\quad of\quad gas\quad B\quad =\quad \frac { { Mass\quad of\quad gas\quad  }B }{ 2 }$

$\Rightarrow \quad { Vapour\quad density\quad of\quad}B\quad =\quad \frac { M }{ 2 }$

On substituting vapour density of B in equation (1), we get

$Vapour\quad density\quad of\quad A\quad =\quad 2\quad \times \quad \frac { M }{ 2} \quad =\quad M$

$Molecular\quad mass\quad of\quad A\quad =\quad 2\quad \times \quad Vapour\quad density$

$\Rightarrow \quad { Molecular\quad mass\quad of\quad}A\quad =\quad 2\quad \times \quad M\quad =\quad 2M$"