Thermodynamics Question?

0.1m3 of gas is compressed from 120kN/m2 and temperature 25C to 1.2 MN/m2 according to the law PV^1.2=C. Find:a)work transferred during compression b)change in internal energy c)heat transferred during compression. Take Cv=0.72KJ/KgK and R=0.285 kJ/kgK. a) using WD=PV-PV/n-1 I got -28KJ b)using u=m(cv)deltaT i got 14.18 KJ c) using Q=deltaU+W (14.18-28) i got -13.86KJ My question is why do I get a different answer for C if I use the formula Q=m(cp)deltaT? when using that formula I get .1413*1.005*139.404=19.796KJ