[Science geek]

Could you please help me solve this problem:
EMF of a compound on 25 celsius degree Pt,H2(p) | HCl(a=x) || 1 moldm-3 KCl, Hg2Cl2(s) | Hg is 0,5164. THe outside pressure is 100,538 kPa and the vapour pressure of water is 3,172 kPa . Determine the pH value of solution which is in contact with hydrogen's electrode.
The result is :  pH=4
I am so confused by the outside pressure and the vapour pressure of water.