# A sample of nitrogen gas is collected over water at 20°C. The vapor pressure of water at 20°C is 18 mmHg. What is the partial pressure of the nitrogen if the total pressure is 765 mmHg?

${P}_{\text{measured"=P_"SVP"+P_"Gas}}$
${P}_{\text{SVP}}$ is the saturated vapour pressure, which is the vapour pressure of water, and is extensively tabulated across a range of temperatures. Of course, at $100$ ""^@C the saturated vapour pressure is $1$ $a t m$. Why?
So if we want ${P}_{\text{Gas}}$ (which we do), all we have to do is subtract ${P}_{\text{SVP}}$ from ${P}_{\text{measured}}$
${P}_{\text{measured"-P_"SVP}}$ $=$ ${P}_{\text{Gas}}$ $=$ $\left(765 - 18\right) \cdot m m \cdot H g$