# What are some common mistakes students make with partial pressure?

Feb 5, 2017

Partial pressure is the pressure exerted by a single gas in a mixture of many gases:

${P}_{i} = {\chi}_{i} P$

• ${P}_{i}$ is the partial pressure.
• $P$ is total pressure.
• ${\chi}_{i}$ is the mol fraction of $i$ in the mixture. ${\chi}_{i} = {n}_{i} / \left({n}_{1} + {n}_{2} + \ldots + {n}_{N}\right)$

Mole fractions must be a ratio of mols for component $i$ with the mols for all components.

Common mistakes:

• Taking the ratio of the mols of component $i$ with mols of $j$, instead of mols of $i$ with total mols. That's the mol ratio, not mol fraction.
• Taking a given mass and dividing by the molar mass. That's the mols, not mol fraction...
• Taking given masses and finding the ratio of the mass of $i$ with the total mass. That's the mass fraction, not mol fraction.