Use the Ideal gas law: PV=nRT. Here n (number of moles of gas) is constant, as is R, so:
PV/T = nR = constant.
Thus:
P1V1/T1 = P2V2/T2
Where P1, V1, and T1 are the initial values of pressure, volume and temp (in kelvin) and P2, T2, and V2 are the final values. Solve for P2.
For the second part, use PV=nRT to solve for n. You can use either initial values of P, V and T or final values  it doesn't matter, just don't mix them up. Then from n calculate total mass of gas. Post back with your answers, and if still having issues with it or the third part we'll help you along.
