How many moles of ‘He’ are contained in a 5- litre canister at 100 KPa and 25 ° C. Take R= 0.0821 L. atm/K. mol

Use PV = nRT

Remember T must be in kelvin. If you are to use 0.0821 L*atm/mol*K, then you must convert 100 kPa to atmospheres.
atm = 100/101.325 = ?
Post your work if you get stuck