A strong acid ionises completely in water. A weak acid ionises only partly. A titration finds an unknown concentration from a known one.
The idea
Acids produce hydrogen ions in water. Hydrochloric acid is strong: the molecules become ions almost completely. Ethanoic acid is weak: only a small fraction of the molecules ionise at any moment. Both can have the same concentration and different pH values, because pH depends on the hydrogen ion concentration, not on how much acid was dissolved. A base that is soluble is an alkali. Neutralisation of an acid by an alkali produces a salt and water.
In a titration you run one solution from a burette into a measured volume of the other until an indicator changes. Moles are concentration times volume in cubic decimetres. Convert cubic centimetres by dividing by 1000. Use the equation to relate the moles, then divide the moles of the unknown by its volume. For hydrochloric acid and sodium hydroxide the ratio is 1 to 1. Write that ratio down. A 1 to 2 equation, such as with sulphuric acid and sodium hydroxide, is a different sum.
What earns the marks
Strong is not the same word as concentrated. Concentrated means a lot of substance in the volume. Strong means fully ionised. A dilute solution of a strong acid is still strong. In the calculation, volume must be in cubic decimetres before you multiply by concentration in mol per cubic decimetre. Forgetting to divide by 1000 makes the answer a thousand times too big. The indicator names the end of the titration. It is not a reactant in the equation.
Practice questions
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
1Question 1
4 marks
(a) Distinguish between a strong acid and a concentrated acid. (2 marks)
(b) Name one strong acid and one weak acid. (2 marks)
Model answer
(a) A strong acid ionises completely in water. A concentrated acid contains a large amount of acid in a given volume. Strength is about ionisation. Concentration is about how much is dissolved.
(b) Hydrochloric acid is strong. Ethanoic acid is weak.
Mark scheme
Strong: complete ionisation in water1 mark
Concentrated: a large amount of solute in the volume1 mark
A strong acid, such as hydrochloric, nitric or sulphuric acid1 mark
A weak acid, such as ethanoic acid or citric acid1 mark
2Question 2
4 marks
25.0 cm³ of sodium hydroxide of concentration 0.100 mol/dm³ is neutralised by 20.0 cm³ of hydrochloric acid. The reaction ratio is 1 to 1. Find the concentration of the acid. (4 marks)
Model answer
Moles of NaOH = 0.0250 × 0.100 = 0.00250 mol.
Moles of HCl = 0.00250 mol.
Volume of acid = 0.0200 dm³.
Concentration = 0.00250 / 0.0200 = 0.125 mol/dm³.
Mark scheme
Converts 25 cm³ to 0.0250 dm³, or an equivalent1 mark
Moles of alkali 0.002501 mark
Uses the 1 to 1 ratio so moles of acid equal moles of alkali1 mark
0.125 mol/dm³1 mark
3Question 3
1 mark
State the colour change of methyl orange at the end of this titration, from alkali to acid. (1 mark)
Model answer
From yellow to orange, or to red. The accepted end point colour moving into acid is red or pink.
Mark scheme
Yellow to red, or yellow to orange. Green is not the colour of methyl orange1 mark
Practise Chemistry with Anna
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Is a strong acid the same as a concentrated acid?+
No. Strong means the acid ionises completely, as hydrochloric acid does. Concentrated means there is a lot of it in the volume. A very dilute solution of hydrochloric acid is still a strong acid. Ethanoic acid is weak even when the bottle is concentrated.
How do you use a titration result?+
Convert the volumes to cubic decimetres, multiply the known concentration by its volume to get moles, use the ratio in the equation, then divide the unknown moles by the unknown volume. For hydrochloric acid against sodium hydroxide the ratio is 1 to 1. All WASSCE Chemistry past questions