# Given the following, what is the pressure of dry hydrogen gas if the vapor pressure of water at 25°C is 3.8 mm Hg?

## Hydrogen gas produced in the laboratory by the reaction of zinc and hydrochloric acid as collected over water at 25°C. The barometric pressure at the time was 742.5 mm Hg.

Nov 6, 2016

${P}_{\text{Total"=P_"Gas"+P_"SVP"="742 mm Hg}}$

#### Explanation:

And ${P}_{\text{SVP}}$ is the so called $\text{saturated vapour pressure}$, which for this experiment $=$ $\text{3.8 mm Hg}$

So ${P}_{\text{dihydrogen}}$ $=$ $\text{742 mm Hg "-" 3.8 mm Hg = 738.2 mm Hg}$.

Note that here the quoted value of ${P}_{\text{SVP}}$ is wrong. I suspect that they got confused with respect to a pressure measurement in $\text{mm Hg}$ and $\text{Pascal}$.

The vapour pressure of water at $298 \cdot K$ is $\text{24 mm Hg}$.