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

##### 1 Answer

Feb 5, 2017

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

#P_i = chi_iP#

#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/(n_1+n_2+...+n_N)#

Mole fractions must be a ratio of mols for component ** 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.