Question

A sample of methane gas has a volume of 10 l at a pressure of 1.5 atm and a temperature of 20 °c. what volume does the gas occupy if the pressure is lowered to 0.9 atm at a temperature of 40 °c?

Answer 1

We can use combined gas laws to solve for the volume of the gas
 $\frac{PV}{T}$
where P - pressure, V - volume , T - temperature and k - constant
$\frac{P1V1}{T1} = \frac{P2V2}{T2}$
parameters for the first instance are on the left side and parameters for the second instance are on the right side of the equation 
T1 - temperature in Kelvin - 20 °C + 273 = 293 K
T2 - 40 °C + 273 = 313 K
substituting the values 
$\frac{1.5 atm*10L}{293 K} = \frac{0.9 atm*V}{313K}$
V = 17.8 L
volume of the gas is 17.8 L