Question

Find H⁺ , OH^- , and the pH of the following solutions.

(a) 30.0 mL of a 0.216 M solution of HCL diluted with enough water to make 125 mL of solution.

(b) A solution made by dissolving 275 mL of HBr gas at 25^oC and 1.00atm in enough water to make 475 mL of solution. Assume that all the HBr dissolves in water.

Answer 1

a) for dilution formula is M1V1 = M2V2   where V2 is final vol = 125 ml

0.216 x 30 = M2 x 125 ,    M2 = 0.05184 = [H+]

pH = -log [H+] = - log ( 0.05184) = 1.285 ,   [OH-] = 10^ -14 / [H+] = 10^ -14 / ( 0.05184) = 1.93 x 10^ -13 M

b) we fidn moles of gas HBr by using PV = nRT

1 x 0.275 = n x 0.08206 x 298                ( T = 25C = 25+273 = 298 K)

n = 0.01124566

[H+] = [HBr] = ,oles/vol of solution = 0.01124566 /0.475 = 0.023675

pH = -log ( 0.023675) = 1.63 ,    [OH-] = 10^ -14 / ( 0.023675) = 4.2 x 10^ -13 M