hi
the question is meant to confuse you in the units
the question is not in SI units at all, instead of converting them, we need to cancel them out
PV=nRT so PV=n/Mw RT so Mw=(mRT)/PV
R is given in cm³ atm K⁻¹ mol⁻¹, and we need Mw to be in g/mol
we need to cancel the units cm³ atm K⁻¹ by not converting anything except temperture from celsius to kelvin (35+273=308°K)
Mw=(1,42 * 82,05 * 308)/(250 * 0,85)
Mw=(g cm³ atm K⁻¹ mol⁻¹ K)/(cm³ atm) = g * mol⁻¹ the units are now balanced