if there is 10.0g of CO2 in a 20.0L cylinder @ pressure of 0.300 atm and im finding temperature of the CO2 sample does n=10.0g in the equation and should I set it up pv=nRT

Yes, use PV = nRT; no, n is not 10 g. n = mols = grams CO2/molar mass CO2 = 10.0/44 = ?