1 A solution contains 0.200 mol dm-3 of acetic acid, pKa= 4.7.

(a) Find the pH of this solution.

(b) What would be the pH if 0.200 mol dm-3 of NaOH were added to the solution?

(c) Find the pH if only 0.100 mol dm-3 of NaOH were added to the original acetic acid solution.

(d) What would be the new pH, and the percent change in pH, if 0.005 mol/dm3 of HCl were added to the acetic acid - NaOH solution in (c)?

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Class Reg Number

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Chemistry H2 9746Tutor TuteeRevision Exercise 7: Ionic Equilibria

2 Carbon dioxide, produced by oxidation of glucose in the tissues, is carried by the blood to the lungs. Part of it is in solution as carbonic acid, and nearly all the remainder is present as hydrogen carbonate ion, HCO3

-. If the pH of the blood is 7.4, find the fraction of carbon dioxide is carried by the ion. (Use K1 = 4:47 x 10-7)

3 An acid-base indicator has a pKa of 4.52. The acid form of the indicator is red and the basic form is blue. Over what range of pH will the color of this indicator change? Assume that a definite color change occurs when the relative concentrations of the two forms change from 75% of one form to 75% of the other.

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4(a) How would you prepare 1 dm3 of buffer solution having a pH of 8.50, starting with0.100 mol dm-3 KCN and the usual substances available in the laboratory? (For HCN, Ka =4.8 x 10-10).

(b) How much would the pH of this buffer solution change if 5.00 x 10-4 mol of HClO4 isadded to 100 cm3 of the solution? What would be the effect of adding the same quantityof NaOH to 100 cm3 of the buffer?

5 At 25oC, a 0.0100 mol dm-3 aqueous ammonia is 4.1% ionised.

(a)

Calculate

the concentration of OH- and NH4+ ions

(b) the concentration of ammonia

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(c) Kb of aqueous ammonia

(d) [OH-] after 0.0090 mol of NH4Cl is added to 1 dm3 of the above solution

(e) [OH-] of a solution prepared by dissolving 0.010 mol of NH3 and 0.0050 mol of HCl per dm3

5 Phenol, C6H5OH, is a powerful disinfectant and antiseptic. Phenol is a weak Brønsted-Lowry acid.

(a) Explain what is meant by the following terms;

(i) a Brønsted-Lowry acid

proton donor

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(ii) a weak acid

dissociates partially.................................................................................................................................................

(b) When phenol is mixed with aqueous sodium hydroxide, an acid-base reaction takes place.

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C6H5OH (aq) + OH– (aq) C6H5O– (aq) + H2O (l)Acid 1 Base 2 Base 1 Acid 2...................... ..................... ....................... .....................

In the spaces above,

• label one conjugate acid-base pair as acid 1 and base 1,• label the other conjugate acid-base pair as acid 2 and base 2.

(c) A solution of phenol in water has a concentration of 38 g dm–3.The acid dissociation constant, Ka, of phenol is 1.3 × 10–10 mol dm–3.

(i) Write an expression for the acid dissociation constant, Ka, of phenol.

(ii) Calculate the pH of this solution.

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6 A student carried out an investigation with aqueous solutions of nitric acid, sodium hydroxide, ethanoic acid and water.

(a) Nitric acid, HNO3, is a strong Brønsted-Lowry acid.

(i) Explain what is meant by a strong acid and a Brønsted-Lowry acid.

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(ii) What is the conjugate base formed from HNO3?

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(b) The student diluted 0.015mol dm–3 nitric acid with an equal volume of water and measured

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the pH of the diluted acid at 25 °C.

(i) Calculate the pH of 0.015mol dm–3 nitric acid.

(ii) Calculate the pH of the diluted acid.

(c) The student measured the pH of a solution of sodium hydroxide as 13.54 at 25°C.

Kw = 1.0 × 10–14 mol2dm–6 at 25 °C.

(i) Write down an expression for the ionic product, Kw, for water.

(ii) Calculate the concentration, in mol dm–3, of this solution of sodium hydroxide.

(d) The student prepared two solutions. Solution A was made by mixing together 25 cm3 0.010mol dm–3 aqueous sodium

hydroxide with 50 cm3 0.010mol dm–3 ethanoic acid, CH3COOH. Solution A is a buffer solution.

Solution B was made by mixing together 25 cm3 0.020mol dm–3 aqueous sodium hydroxide with 50 cm3 0.010mol dm–3 ethanoic acid, CH3COOH. Solution B is not a buffer solution.

(i) What is meant by a buffer solution?

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(ii) Explain why Solution A is a buffer solution whereas Solution B is not.

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(e) The student measured the pH of water as 7.0 at 25°C. The student then warmed the water to 40°C and measured the pH as 6.7.

What do these results tell you about the tendency of water to ionise as it gets warmer?Explain your reasoning in terms of equilibrium.

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