Calculate total pressure of mixtures?

Question

What is the total pressure exerted by a mixture of 4.85g N2, 3.75h H2, and 7.50g He when the gas mixture is confined to a 5.50L container at 25 C?

I'm getting confused as to calculating the partial pressure and total pressure in combination with the moles. Help!!!

Answer 1

General rule:  total moles are proportional to pressure and molar fractions are proportional to partial pressures.

As for your case:

N2 moles --> 4.85 g / 28 g/mol = 0.173 mol;

H2 moles --> 3.75 / 2.02 = 1.86 mol;

He moles --> 7.50 / 4.00 = 1.88 mol;

Therefore, total moles --> 3.91 mol.

Now we can apply the relation PV=nRT, where P is unknown atm, V=5.50 L, n=3.91 mol; R=0.082; T=273+25=298 K.

Well, P comes out as 17.4 atm (total pressure).

To calculate partial pressures, we first get molar fractions: N2 is 0.173 mol / 3.91 mol = 0.0442 or 4.42 %;

then, similarly, you will get H2= 47.6% and He= 48.1%.

The percentages above must be the same, if you refer them to pressures. Therefore, eg, N2 part. press. is 4.42% of 17.4 atm, and so on.