Question

250 ml of nitrogen at 720 mm of hg and 380 ml of oxygen at 650 mm of hg are put together in 1 litre flask. find the final pressure of mixture if temperature is constant?

Answer 1

P V = μ R T

720 mm * 250 ml = μ1 * R T  = P1 * 1000 ml       - Nitrogen
       P1 = 180 mm

650 mm * 380 ml = μ2 * R T  = P2 * 1000 ml    -  oxygen
           P2 = 247 mm

Pressure of mixture = P1 + P2 as per Dalton's law of partial pressures
       = 180 + 247 mm = 427 mm of Hg

Answer 2

This will be solved using Boyle's law and Dalton's law of partial pressures.

At constant temperature:
pV = p'V' (Boyle's law)

where p and V are partial pressure and volume of a gas respectively. p' is the partial pressure of the same gas at a different volume V'.
First we need to calculate the partial pressures of both the gases in the mixture which is 1 L (1000 mL).

For nitrogen:
 
pN2×1000 mL=720 mm Hg×250 mL⇒pN2=180 mm Hg

Similarly for oxygen:

pO2×1000 mL=650 mm Hg×380 mL⇒pO2=247 mm Hg

Now, we have the partial pressures of both the gases in the mixture. So, we can now easily find the total pressure of the mixture using Dalton's law of partial pressures, according to which it will be the sum of partial pressures of the individual gases. Thus,

pmixture=pN2+pO2⇒pmixture=180+247⇒pmixture=427 mm Hg