Solutions Note: The significant figures in the concentration of [H+] or [OH -] is equal to the number of decimal places in the pH or pOH and vice versa. 1) What is the pH of a 0.0235 M HCl solution? pH = -log[H+] = -log(0.0235) = 1.629 2) What is the pOH of a 0.0235 M HCl solution? pH = -log[H+] = -log(0.0235) = 1.629 pOH = 14.000 – pH = 14.000 – 1.629 = 12.371 3) What is the pH of a 6.50 x 10-3 M KOH solution? pOH = -log[OH-] = -log(6.50 x 10-3) = 2.187 pH = 14.000 – pOH = 14.000 – 2.187 = 11.813 4) A solution is created by measuring 3.60 x 10-3 moles of NaOH and 5.95 x 10-4 moles of HCl into a container and then water is added until the final volume is 1.00 L. What is the pH of this solution? Since there is both acid and base we will assume a 1 mole acid:1 mole base ratio of neutralization. There is more base than acid so the leftover base is what will affect the pH of the solution. 3.60 x 10-3 moles - 5.95 x 10-4 moles = 3.01 x 10-3 moles NaOH 3.01 x 10-3 moles NaOH = 3.01 x 10-3 M NaOH 1.00 L soln pOH = -log[OH-] = -log(3.01 x 10-3) = 2.521 pH = 14.000 – pOH = 14.000 – 2.521 = 11.479 5) What is the pH of a 6.2 x 10-5 M NaOH solution? pOH = -log[OH-] = -log(6.2 x 10-5) = 4.21 pH = 14.00 – pOH = 14.00 – 4.21 = 9.79 6) A solution with a H+ concentration of 1.00 x 10-7 M is said to be neutral. Why? pH = -log[H+] = -log(1.00 x 10-7) = 7.000 pOH = 14.000 – pH = 14.000 – 7.000 = 7.000 pOH = -log[OH-] = -log(OH-) = 7.000 we can use this to find the OH- concentration -log[OH-] = 7.000 log[OH-]-1 = 7.000 log[OH-]-1 10 7.000 = 10 [OH-]-1 = 107.000 1 = 107.000 [OH-] [OH-] = 1.00 x 10-7 M The concentrations of H+ and OH- are equal, as are the pH and pOH, so the solution must be neutral.