Question

if hydrogen gas is collected over water at 20 °c and 763 mm hg, what is the partial pressure of the h₂? the vapor pressure of water at 20 °c is 17.5 mm hg.
○ 17.5 mm hg
○ 746 mm hg
○ 763 mm hg
○ 781 mm hg
○ 743 mm hg

Explanation:

Step1: Recall Dalton's Law

The total pressure ($P_{total}$) is the sum of the partial pressures of the gas ($P_{H_2}$) and water vapor ($P_{H_2O}$), so $P_{total} = P_{H_2} + P_{H_2O}$.

Step2: Rearrange the formula

To find $P_{H_2}$, we use $P_{H_2} = P_{total} - P_{H_2O}$.

Step3: Substitute values

Given $P_{total} = 763$ mm Hg and $P_{H_2O} = 17.5$ mm Hg, then $P_{H_2} = 763 - 17.5 = 745.5 \approx 746$ mm Hg.

Answer:

746 mm Hg