# A gas mixture contains each of the following gases at the indicated partial pressures: N2, 211 torr ; O2, 139 torr ; and He, 126 torr.?

A gas mixture contains each of the following gases at the indicated partial pressures: N2, 211 torr ; O2, 139 torr ; and He, 126 torr . What mass of each gas is present in a 1.50 −L sample of this mixture at 25.0 ∘C?

Relevance

For each gas, just use the ideal gas law to calculate moles and then masses of each one.

For N2:

PV = n RT

(211/760 atm) (1.50 L) = n (0.08206 Latm/mol K) (298 K)

n = 0.0170 moles

Mass N2 = 0.0170 mol X 28.0 g/mol = 0.477 grams N2

Repeat these calculations for each gas and you're there.

• Gas mixture ....

This question is asking you to compute the mass of a quantity of gas.  The fact that there are several gases will not affect the calculation for each gas.

PV = nRT ............... ideal gas equation

PV = mRT / M ........ n = m / M .... m=mass, M=molar mass

m = PVM / RT ........ solve for m

m = 211 torr x 1.50L x 28.0g/mol / 62.36 Ltorr/molK / 298K

m = 0.477g N2

Since you have several more calculations to do and V, R and T are the same, collapse those into a constant: k=8.07x10^-5, and store that in a memory in your calculator.  Then simply recall it for each calculation.

m(O2) = 139 torr x 32.0g/mol x k = 0.351g O2

m(He) = 126 torr x 4.0 g/mol x k = 0.0407g He

One other thing.  You don't have to convert the pressure to atmospheres. You can use the value of R which matches the pressure units in the question.

R = 0.08206 Latm/molK ........ atm

R = 8.314 LkPa/molK ............ kPa

R = 0.08314 Lbar/molK ......... bar

R = 62.36 Ltorr/molK ............. torr (aka mm Hg)