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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 15.39

What is the pH of each of the following aqueous solutions?
(a) 0.039 M HCl (c) 0.65 M HBr
(b) 0.070 M KOH (d) 2.5×10–4 M Ca(OH)2





Answer:

Assume 25 oC in all cases. Write out the reactions in order to find the [H3O+] or [OH] concentration for each acid or base. Then use the definitions pH = –log[H3O+] or pOH = –log[OH] and the relation pH + pOH = 14.00 (only true at 25 oC). Note that these are all strong acids or bases.

(a) HCl(aq) + H2O(l) H3O+(aq) + Cl(aq)

[H3O+] = [HCl] = 0.039

pH = –log[0.039] = 1.41

(b) KOH(aq) K+(aq) + OH(aq)

[OH] = [KOH] = 0.070

pOH = –log[0.070] = 1.15

pH = 14.00 – pOH = 14.00 – 1.15 = 12.85

(c) HBr(aq) + H2O(l) H3O+(aq) + Br(aq)

[H3O+] = [HBr] = 0.65

pH = –log[0.65] = 0.19

(d) Ca(OH)2(aq) Ca2+(aq) + 2 OH(aq)

[OH] = 2×[Ca(OH)2] = 2×2.5×10–4 = 5.0×10–4

pOH = –log[5.0×10–4] = 3.30

pH = 14.00 – pOH = 14.00 – 3.30 = 10.70