Suppose 2500 J of heat are added to 4.1 mol of argon gas at a constant pressure of 120 kPa. (Assume that the argon can be treated as an ideal monatomic gas.) Find the change in internal energy.

Question

Suppose 2500 J of heat are added to 4.1 mol of argon gas at a constant pressure of 120 kPa. (Assume that the argon can be treated as an ideal monatomic gas.) Find the change in internal energy. 
I started using U = N (3/2 kB T). But I have too many unknowns. The only other equation that our teacher gave us is Won + Qon = deltaU (Work on system + Heat on system = Change in internal energy). Please help!

Qon = deltaU - Won =

deltaU + Wsystem,

where Wsystem is the work performed by the system (in this case the argon gas). Because the pressure is constant, the work performed is:

P deltaV = Delta (PV) = (using ideal gas law) Delta (N kB T)

This means that:

Qon = deltaU + Wsystem =

5/2 N kB delta T =

(5/2)(2/3)(3/2)N kB delta T =

(5/2)(2/3) delta U = 5/3 delta U

And it follows that:

delta U = 3/5 Qon = 1500 J