2K +Cl₂ --> 2KCl
First calculate how many moles we have of potassium.
15.5 g * (mol/39.0983 g) = 0.396 mol K
2 moles of K reacts with 1 mole of Cl₂, so we know we'll need 0.198 moles of Cl₂.
Standard temperature and pressure is 273.15 degrees K and 1 atm.
Using the gas law PV = nRT, with R = 0.0821 L*atm*K^-1 *mol^-1, you should be able to solve for V, volume, in Liters.