We can use the combined gas law to solve this problem. Assuming that the pressure and temperature remain constant, we can use the following formula:
(P1 x V1) / n1 = (P2 x V2) / n2
where P1, V1, and n1 are the initial pressure, volume, and number of moles of the gas, and P2, V2, and n2 are the final pressure, volume, and number of moles of the gas.
Substituting the given values, we get:
(1 atm x 25.5 L) / (4.0 mol + 2.5 mol) = (1 atm x 32.1 L) / n2
Simplifying this equation, we get:
n2 = (1 atm x 32.1 L x (4.0 mol + 2.5 mol)) / (1 atm x 25.5 L) = 6.34 mol
Therefore, we need to add 6.34 - (4.0 mol + 2.5 mol) = 0.84 moles of gas to the tank to achieve a volume of 32.1 liters.