Question

Two identical gas containers are placed on the two pans of beam balance. One container is empty and open. Second tank is evacuated and then filled with helium until two containers balance. The pressure of Helium in atmospheres is nearly (mol. wt. of air is 28 and that of He is 4)

• A. $7$
• B. $15$
• C. $4$
• D. $1$

Answer 1

Answer: (A)

$7$

Two identical containers will contain same no of atoms of gas. So, we can replace volume with the molecular weight of the respective gases.
So, Boyle's law can be written as,
$P_1{M}_1$ $=$ $P_2{M}_2$, where $M_1$ and $M_2$ are the molecular weight for air and He respectively.
Putting $P_1$ $=$ 1 atm, we get $P_2$ $=$ $\frac{28}{4}$ $=$ 7 atm.