Question

Calculate the H+ concentration (M), the OH- concentration and the number of H+ and OH- per litre in solutions having pH value of 2.73

Answer 1

pH = -log(H^+)

Substitute and solve for (H^+).
Then (H^+)(OH^-)= Kw = 1E-14. You have Kw and H^+, solve for OH^-?. You need to clarify what you mean by number. If that is mols/L, that is the same the M above.