# A gas occupies 15 liters at a pressure of 80.0 mm Hg. What is the volume when the pressure is increased to 100.0 mm Hg?

We use Boyle's law: ${P}_{1} {V}_{1} = {P}_{2} {V}_{2}$; temperature and moles are constant.
${V}_{2} = \frac{{P}_{1} {V}_{1}}{P} _ 2$ $=$ $80 \cdot m m \cdot H g$ $\times$ $15 \cdot L$ $\times$ $\frac{1}{100 \cdot m m \cdot H g}$
So the answer is $\frac{4}{5}$ $\times$ $15 \cdot L$ $=$ $12 \cdot L$.