Unit 8 Worksheets 4 Strong & weak acids & bases
1. (a) Explain why 25.0 cm3 of 1.00 x 10-2 mol dm-3 hydrochloric acid solution, HCl(aq), has a lower pH
value than 25.0 cm3 of 1.00 x 10-2 mol dm-3 ethanoic acid solution, CH3COOH(aq).
(b) When an acid is neutralized by a base the ionic equation for the reaction taking place is:
H+(aq) + OH–(aq) → H2O(l)
Explain why both of the above solutions react completely with exactly 25.0 cm3 of 1.00 x 10-2 mol dm3
sodium hydroxide solution, NaOH(aq).
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
2. The equilibrium constant for the dissociation of ammonia, NH3, in water is 1.8 x 10-5.
(a) Write the equation for the dissociation of ammonia in water.
(b) Write the equilibrium expression for the dissociation of ammonia in water.
(c) Describe how the electrical conductivity of an aqueous solution of ammonia differs to an aqueous
solution of sodium hydroxide with the same concentration.
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
3. The equilibrium constant for the dissociation of ethanoic acid, CH3COOH, in water is 1.8 x 10-5. The
equilibrium constant for the dissociation of chloroethanoic acid, CH2ClCOOH, in water is 1.4 x 10-3.
(a) Write the equation for the dissociation of ethanoic acid in water.
(b) Write the equilibrium expression for the dissociation of ethanoic acid in water.
(c) Deduce which of the two acids given above is weaker and explain your reasoning.
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
4. The ethoxide ion, C2H5O−, is a stronger base than the hydroxide ion, OH−. Deduce how the equilibrium
constant for the dissociation of ethanol will differ to the dissociation of water at the same temperature.
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
5. Sulfuric acid is often assumed to be a strong diprotic acid. In fact the equilibrium constant for the
dissociation of H2SO4(aq) to form HSO4–(aq) and H+(aq) is 2.4 x 106 and the equilibrium constant for the
dissociation of HSO4–(aq) to form SO42–(aq) and H+(aq) is 1.0 x 10-2.
Use this information to properly describe the strength of sulfuric acid.
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
6. (a) Distinguish between the words “concentrated”, “corrosive” and “strong” when applied to an acid.
(b) Distinguish between the words “dilute” and “weak” when applied to an alkali.
(c) Explain why a dilute aqueous solution of a strong acid might be a better conductor of electricity than
a concentrated aqueous solution of a weak acid.
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
Answers
1. (a) Hydrochloric acid is a strong acid and so is completely dissociated into its ions. Ethanoic acid is a
weak acid so is only slightly dissociated with fewer ions in solution to conduct electricity.
(b) Both acids have the same amount (in mol) of acid so once hydrogen ions from ethanoic acid are
neutralised more of the acid dissociates to restore equilibrium until all have reacted.
2. (a) NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH–(aq)
(b)
(c) Ammonia would be a poorer conductor as there are less ions present as it is a weak base and only
slightly dissociated into its ions whereas sodium hydroxide being a strong base is completely dissociated
into its ions.
3. (a) CH3COOH(aq) + H2O(l) ⇄ CH3COO–(aq) + H+(aq)
(b)
(c) Although both are weak acids, ethanoic acid is weaker as the value for its equilibrium constant is
lower so it is even less dissociated than chloroethanoic acid.
4. Ethanol, the conjugate acid of the ethoxide ion, will be less dissociated than water as it will be a
weaker acid than water, the conjugate base of the hydroxide ion so the equilibrium constant will be
smaller. This is because the ethoxide ion is the stronger base so will have the weaker conjugate acid.
5. Sulfuric acid a strong monoprotic acid as it is almost completely dissociated to form the HSO4– ion and
H+ but not a strong diprotic acid as HSO4– behaves as a weak acid when it forms the SO42– ion.
6. (a) “Concentrated” means a considerable amount (in mol) of acid dissolved in one litre of aqueous
solution, “corrosive” means it is very chemically reactive and “strong” means the acid is completely
dissociated in aqueous solution.
(b) “Dilute” means a small amount (in mol) of alkali dissolved in one litre of aqueous solution, and
“weak” means the alkali is only slightly dissociated in aqueous solution.
(c) Although there will be less of the strong acid dissolved in a dilute solution there may be more ions
present than in the concentrated solution of the weak acid as the strong acid is completely dissociated
into its ions and hence it will be a better conductor.