9.7 Dalton's Law of Partial Pressures
Notes
In a mixture of gases, each gas pushes as if it were alone. Its pressure is its partial pressure.
- Dalton's law: $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$
- Partial pressure is proportional to moles: $\displaystyle P_A = \frac{n_A}{n_{\text{total}}} \times P_{\text{total}}$.
- Gas collected over water includes water vapor: $P_{\text{gas}} = P_{\text{total}} - P_{\text{water}}$.
- Example: $\text{H}_2$ collected over water at 25 °C, where water vapor pressure is 24 mmHg, with total 760 mmHg: $P_{\text{H}_2} = 760 - 24 = 736$ mmHg.
Practice
A mixture contains $\text{N}_2$ at 0.60 atm, $\text{O}_2$ at 0.25 atm and Ar at 0.15 atm. What is the total pressure?
$0.60 + 0.25 + 0.15 = 1.00$ atm.
A container holds 2.0 mol of He and 3.0 mol of Ne at a total pressure of 5.0 atm. What is the partial pressure of He?
$\displaystyle P_{\text{He}} = \frac{2.0}{5.0} \times 5.0 = 2.0$ atm.
Oxygen is collected over water at a total pressure of 750 mmHg. The water vapor pressure is 18 mmHg. What is the pressure of the oxygen?
$750 - 18 = 732$ mmHg.
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