Question

Two balloons are filled, one with pure $He$ gas and the other by air, respectively. If the pressure and temperature of these balloons are same then the number of molecules per unit volume is :

• A. more in the $ He $ filled balloon
• B. same in both balloons
• C. more in air filled balloons
• D. in the ratio of $ 1 : 4 $

Answer 1

Answer: (B)

Ideal gas equation can be written as
$pV = nRT \quad ...(i)$
In this equation,
$n =$ number of moles of the gas
$p =$ pressure of the gas
$V =$ volume of the gas
$R =$ universal gas constant
and $T =$ temperature of the gas
From Eq. $(i)$, we have
$\frac{n}{V}= \frac{p}{RT} =$ constant
So, at constant pressure and temperature, all gases will contain equal number of molecules per unit volume.
Note: This result is nothing but Avogadro’s law.