3.2 Practice problem answers

Keya Jani

1. Solutions

   a) 2 mol gas

   b) 1 mol gas

2. 5.5 mol of gas X, 1.0 mol of gas B and 3.5 mol of gas A

   a) 5.5 L of X, 1.0 L of B and 2.5 L of A

   b) 5.5 kPa of X, 1.0 kPa of gas B, 2.5 kPa of gas A

3. Using Boyle’s Law, we can calculate the partial pressure of each gas in the new container, which has a combined volume of 5 L.

   $\ce{P1V1 = P2V2 → P2 = P1V1/V2}$. For $\ce{He}$, $P_2 = (250)(3.5)/(5) = 175 \ce{\ torr}$.

   For $\ce{Ar}$, $P_2 = (150)(1.5)/(5) = 45 \ce{\ torr}$.

   By Dalton’s Law, the total pressure is the sum of the partial pressures, or 220 torr. By comparing the ratio of the pressure of $\ce{Ar}$ to the total pressure, we can obtain the mole fraction of $\ce{Ar}$: $45/220 = 0.205$.