What is the volume of 0.25 mol of a gas at STP?

Answer 1

Hi Mary,

So for this type of question they are looking for you to use the "Ideal Gas Law" or PV = nRT.

The question gives you the n value of .25 mol of gas. STP is standard temperature and pressure which T = 273.15K and P = 101,250 Pa. So they have actually given you 3 of the 4 variables in the equation. Now you just need to pick a gas constant (R) and this will become an algebra problem.

There are many different values for the gas constant, R, so you have to pick the one with the same units that are given to you in the problem. Our units are Kelvin, K, for T, Pascals, Pa, for P, and moles, mol, for n. Using the table on wikipedia, https://en.wikipedia.org/wiki/Gas_constant, 8.314 x 10³ matches our units of K, Pa, and mol. This gas constant also gives us the volume unit Liters (L).

So PV = nRT becomes

101250 (Pa) x (V) = .25 (mol) x 8.314x10³ x 273.15 (K)

Solve for V and you will get Liters of gas.

If you do not get one of the given answer choices on a test, always check your units!

Hope this helps.