# The pressure on 30mL of a gas at constant temperature Is changed from 4 atm to 2 atm. What Is the new volume of a gas?

At constant pressure, and constant amount of gas, $P \propto \frac{1}{V} \mathmr{and} P V = k$, Boyle's law.
Since $P V = k$, under these condtions, ${P}_{1} {V}_{1} = {P}_{2} {V}_{2}$, so ${V}_{2} = \frac{{P}_{1} {V}_{1}}{{P}_{2}}$; here $\frac{30 \cdot m L \times 4 \cdot \cancel{a t m}}{2 \cdot \cancel{a t m}}$ $=$ ?? mL