## Learn the steps to take to solve this gas law problem

Avogadro's gas law states the volume of a gas is proportional to the number of moles of gas present when the temperature and pressure are held constant. This example problem demonstrates how to use Avogadro's law to determine the volume of a gas when more gas is added to the system.

Before you can solve any problem regarding Avogadro's gas law, it's important to review the equation for this law.

There are a few ways to write this gas law, which is a mathematical relation. It may be stated:

k = V/n

Here, k is a proportionality constant, V is the volume of a gas, and n is the number of moles of a gas. Avogadro's law also means the ideal gas constant is the same value for all gases, so:

constant = p1V1/T1n1 = P2V2/T2n2

V1/n1 = V2/n2

V​1n2 = V2n1

where p is pressure of a gas, V is volume, T is temperature, and n is number of moles.

A 6.0 L sample at 25°C and 2.00 atm of pressure contains 0.5 mole of a gas. If an additional 0.25 mole of gas at the same pressure and temperature are added, what is the final total volume of the gas?

### Solution

First, express Avogadro's law by its formula:

Vi/ni = Vf/nf

where
Vi = initial volume
ni = initial number of moles
Vf = final volume
nf = final number of moles

For this example, Vi = 6.0 L and ni = 0.5 mole. When 0.25 mole is added:

nf = ni + 0.25 mole
nf = 0.5 mole = 0.25 mole
nf = 0.75 mole

The only variable remaining is the final volume.

Vi/ni = Vf/nf

Solve for Vf

Vf ​= Vinf/ni

V​f = (6.0 L x 0.75 mole)/0.5 mole

Vf = 4.5 L/0.5 Vf = 9 L

Check to see if the answer makes sense. You would expect the volume to increase if more gas is added. Is the final volume greater than the initial volume? Yes.

Doing this check is useful because it is easy to put the initial number of moles in the numerator and the final number of moles in the denominator. If this had happened, the final volume answer would have been smaller than the initial volume.

Thus, the final volume of the gas is 9.0