An ideal gas in a cylinder fitted with a piston expands at constant temperature from a pressure of 5 atm and a volume of $6.0 \mathrm{~L}$ to a final volume of $12 \mathrm{~L}$ against a constant opposing pressure of 2.5 atm. How much heat does the gas absorb, expressed in units of $L$ atm (liter $\times$ atm)? (Hint: See Exercise 18.121.)