# A sample of hydrogen gas is collected over water at 25°C. The total pressure of the hydrogen and water vapor is 116.0 kPa. The partial pressure of water vapor at this temperature is 3.2 kPa. What is the pressure of the hydrogen gas alone?

##### 1 Answer
May 18, 2017

This shouldn't feel too complicated...

We know:

• what the vapor pressure of water vapor plus hydrogen gas is.
• what the vapor pressure of just water vapor is.

Hence, by subtraction...

${P}_{{H}_{2} \left(g\right)} = \stackrel{{P}_{{H}_{2} \left(g\right)} + {P}_{{H}_{2} O \left(g\right)}}{\overbrace{{P}_{t o t}}} - {P}_{{H}_{2} O \left(g\right)}$

$= \text{116.0 kPa" - "3.2 kPa}$

= ???

How many sig figs are in this result?