P- Acid-base Equilibria =)

Question 1

Suggest why chloroethanoic acid (ClCH2COOH) is a stronger acid than ethanoic acid. (2)

Answer 1

• Cl is ↑ electronegative –> withdraws e-
• weakens O-H bond

Question 2

bromothymol blue 6.0 – 7.6
thymol blue 8.0 – 9.6
‘
Why is thymol blue a good indicator for a titration between ethanoic acid and sodium hydroxide? (2)

Answer 2

• weak acid-strong base
• equivalent at pH>7

Question 3

Two solutions, one with a pH of 4.00 and the other with a pH of 9.00, were left open to the air.
The pH of the pH 9.00 solution changed more than that of the other solution.
Suggest what substance might be present in the air to cause the pH to change.
Explain how and why the pH of the pH 9.00 solution changes. (3)

Answer 3

• CO2
• pH ↓
• acidic gas/ reacts w/ OH-/ alkali

Question 4

State how a buffer solution can be made from solutions of potassium hydroxide and ethanoic acid.
State how this buffer solution resists changes in pH when a small amount of acid is added. (2)

Answer 4

• add excess ethoanoic acid to KOH
• CH3COO- (in salt) reacts w/ H+ in acid

Question 5

Explain why [H2O] is not shown in the Kw expression. (1)

Answer 5

[H2O] is (almost) constant

Question 6

In neutralisation reactions between sulfuric acid and ethanedioc acid (HOOCCOOH) with potassium hydroxide respectively, the eenthalpy of neutralisation between H2SO4 and KOH was found to be more negative.
Suggest an explaination for the difference. (2)

Answer 6

• HOOCCOOH = weaker aciid, x completely dissociate
• ↑ energy needed to break bonds in HOOCCOOH to dissociate completely (dissocaition is endothermic)

Question 7

Which statement about pH is correct?
A The pH of a weak base is independent of
temperature.
B At temperatures above 298 K, the pH of pure
water is less than 7.
C The pH of 2.0 mol dm–3 nitric acid is
approximately 0.30
D The pH of 0.10 mol dm–3 sulfuric acid is greater
than that of 0.10 mol dm–3 hydrochloric acid.

Answer 7

B

Question 8

A buffer solution has a constant pH even when diluted.
Use a mathematical expression to explain this. (1)

Answer 8

ratio [HA]/ [A-] remains (almost) constant

Question 9

State how you can tell from a pH curve that an indicator is suitable for that titration. (1)

Answer 9

indicator has pH range within steep part of curve

Question 10

The 0.0131 mol dm−3 calcium hydroxide solution at 10 °C was a saturated solution.
A student added 0.0131 mol of magnesium hydroxide to 1.00 dm3 of water at 10 °C and stirred the mixture until no more solid dissolved.
Predict whether the pH of the magnesium hydroxide solution formed at 10 °C is larger than, smaller than or the same as the pH of the calcium hydroxide solution at 10 °C
Explain your answer. (2)

Answer 10

• lower pH
• Mg(OH)2 ↓ soluble

Question 11

A student plans to titrate butanoic acid solution with a solution of ethylamine.
Explain why this titration could not be done using an indicator. (2)

Answer 11

• weak acid-weak base titration
• pH change too gradual (at equivalence point so colour change diff to judge)