A290 ml gas sample contains argon and nitrogen at a temperature of 65 ? c. the total pressure of the sample is 725 mmhg and the partial pressure of argon is 231 mmhg. what mass of nitrogen is present

The partial pressure of nitrogen in the problem is 725 - 231 = 494 mmHg. 
494 / 760 = 0.650 atm. To continue, we will use the ideal gas equation to find the mole of N2 and from that, we can find the mass of N2.

PV = nRT > n= PV/RT

Find the mole of Nitrogen
Given P=0.650 atm

n= (0.650)x(255/1000) / (0.082 x 338)
  = (0.650) (0.255) / 27.716
  =  0.16575 / 27.716
  = 0.0060 moles 

Find the mass of nitrogen 
G= n x Mr (N2)
   = 0.0060 x 28
   = 0.168 grams 

So the mass of nitrogen is 0.168 

The answer is d.potassium