Answer: 1.8x10^-4 M H+
Explanation: The number of moles of NaOH added to the 100 ml soultion will be equal to the number of moles of H+ present in that solution.
Moles NaOH in 7.2 ml of 2.5x10^-3 M NaOH
(2.5x10^-3 M NaOH) is the same as 2.5x10^-3 moles NaOH/liter
(2.5x10^-3 moles NaOH/liter)*(7.2 ml)*(1 liter/1000ml) = 1.8x10^-4 moles NaOH used to titrate 100 ml of the acid. That means there was 1.8x10^-4 moles of H^+ in 100 ml.
Concentration of H^+ = (1.8x10^-4 moles H)/0.100 l) = 1.8x10^-4 M H+
(pH of 3.74)
