A mass of 0.4 g of hydrogen (H2) and 3.2 g of helium (He) are mixed in a rigid 500 mL container.

Question

What is the partial pressure of helium when the container is cooled to 77 K ?

Answer 1

Use PV = nRT

P = unknown to be calculated.
V = 500 mL but convert to L.
n = grams He/molar mass He.
R = 0.08206 L*atm/mol*K
T = 77 K.
Note this calculation does not involve H2. Dalton's Law tells us that EACH gas exerts its own partial pressure regardless of other gases present. You could calculate the partial pressure of the H2 the same way and you could calculate the TOTAL pressure by added the partial pressure of He to partial pressure H2.