HELP IB CHEMISTRY! PLZ HELP IDEAL GAS LAW?

You didn't supply the T, temperature in degrees K. When you have that, then use PV = nRT and solve for n, the number of moles. Then divide 0.4g by n to get molar mass. Don't forget to convert cm3 to liters, and use the correct R values for kPa i.e. 8.314

#1 You haven't specified the temperature (t) which should be in Kelvin (K)