Question

What are some common mistakes students make with partial pressure?

Answer 1

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)`

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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.