Unit 8 Worksheet 3 The pH scale
1. 10.0 cm3 of a solution of 0.100 mol dm-3 hydrochloric acid, HCl(aq), has a pH of 1.
(a) Deduce the pH of the resulting solution if 90.0 cm3 of distilled are mixed with the above solution.
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(b) Deduce the pH of the mixture if a further 900 cm3 of distilled water are added to the solution.
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(c) Deduce the pH of the resulting solution if 10.0 cm3 of 0.100 mol dm-3 sodium hydroxide solution,
NaOH(aq), are added to 10.0 cm3of 0.100 mol dm-3 hydrochloric acid, solution.
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2. 10.0 cm3 of a solution of 0.100 mol dm-3 sodium hydroxide, NaOH(aq), has a pH of 13.
(a) Deduce the pH of the resulting mixture if the total volume is made up to 1.00 dm3 with distilled
water.
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(b) Deduce the pH if 9.990 dm3 of water is added to 10.0 cm3 of a solution of 0.100 mol dm-3 sodium
hydroxide, NaOH(aq).
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(c) Deduce the pH of the resulting solution if 20.0 cm3 of 0.100 mol dm-3 sodium hydroxide solution,
NaOH(aq), is added to 10.0 cm3of a solution of 0.100 mol dm-3 hydrochloric acid, HCl(aq), followed by
the addition of 70.0 cm3 of distilled water.
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3. Calculate the pH of (a) 0.200 mol dm-3 hydrochloric acid solution, HCl(aq) and (b) 0.200 mol dm-3
sulfuric acid solution, H2SO4(aq), (assume sulfuric acid is a strong diprotic acid which is not strictly true).
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4. Determine the pH of an alkaline solution with a hydroxide concentration of 3.00 x 10−3 mol dm−3.
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5. The pH of a solution of 0.100 mol dm-3 ethanoic acid, CH3COOH(aq), is about 3 and the pH of a
solution of 0.100 mol dm-3 nitric acid, HNO3(aq), is 1.
(a) Explain why the solution of ethanoic acid has a higher pH value even though both acids have the
same concentration.
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(b) Approximately how many times stronger is the nitric acid solution compared to the ethanoic acid
solution?
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(c) Approximately how much distilled water would need to be added to 25 cm3 of the nitric acid solution
so that the resulting mixture has the same pH as the ethanoic acid solution?
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6. The pH of 0.100 mol dm-3 potassium hydroxide solution, KOH(aq), is 13 and the pH of 0.100 mol dm-3
ammonia solution, NH3(aq), is about 11. Explain why the pH values of the two solutions are different
even though their concentrations are the same.
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Answers
1. (a) The solution has been diluted 10 times so the new pH is 2 ( an increase in one pH unit).
(b) The solution has now been diluted 100 times so the new pH is 3 (an increase in two pH units).
(c) The solution is now neutral so the pH will be 7.
2. 10.0 cm3 of a solution of 0.100 mol dm-3 sodium hydroxide, NaOH(aq) has a pH of 13.
(a) The solution has been diluted 100 times so the new pH is 11 (a decrease in two pH units).
(b) The solution has been diluted 1000 times so the new pH is 10 (a decrease in three pH units).
(c) The excess amount of NaOH is equivalent to 10 cm3 of 0.100 mol dm-3 sodium hydroxide
solution, NaOH(aq) and as the total volume has been made up to 100 cm3 this has been diluted
ten times so the new pH will be 12.
3. (a) pH = ﹣log10[H+] = ﹣log10 0.200 = 0.70
(b) pH = ﹣log10[H+] = ﹣log10 0.400 = 0.40
4. [H+] x [OH﹣] = 1.00 x 10﹣14
[H+] = 1.00 x 10﹣14 ÷ 3.00 x 10−3 = 3.33 x 10−12 mol dm-3
pH = ﹣log10[H+] = ﹣log10 3.33 x 10−12 = 11.5
5. (a) Ethanoic acid is a weak acid so is only slightly dissociated into its ions so the concentration of
hydrogen ions will be lower than for nitric acid which being a strong acid is fully dissociated.
(b) As the difference in pH is two units, nitric acid is approximately one hundred times stronger.
(c) It needs to be diluted 100 times so 2475 cm3 of distilled water needs to be added to bring the total
volume to 2500 cm3.
6. Potassium hydroxide is a strong alkali so is completely dissociated into potassium ions and hydroxide
ions in aqueous solution. Ammonia is a weak base and only slightly reacts with water to form
ammonium ions and hydroxide ions.
NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH–(aq)
As the concentration of hydroxide ions in ammonia solution is lower than the concentration of
hydroxide ions in potassium hydroxide solution the concentration of hydrogen ions will be higher so the
pH will be lower.