Acid and Base Past Year

Why is ethanoic acid describes as a weak acid?

• It is only slightly ionised in water

• It is a poor conductor of electricity

• It is an organic acid

• It reacts only with very reactive metals

Answer explanation

Main point about weak acid is that they only SLIGHTLY dissociate in water (ionise).

Which statement does not describe a property of a weak acid in solution?

• It forms a salt with sodium hydroxide

• It has a pH of between 8 and 9

• It is only partly dissociated into ions

• It reacts with sodium carbonate to give off carbon dioxide

Answer explanation

Acids have a pH of less than 7!

Higher than pH 7 would mean that it is an alkali.

Which element will burn in oxygen to form an acidic oxide?

• Calcium

• Carbon

• Iron

• Magnesium

Answer explanation

Acidic oxides are non-metal oxides.

The only non-metal provided in the choices is Carbon!

Carbon dioxide

The table gives information about three indicators.

Which colours would be obtained when each indicator was added separately to PURE WATER?

• A

• B

• C

• D

Answer explanation

pH of water is pH 7.

Check what colour the pH indicators are at pH 7. Correct answer: D

Example:

- Phenolphthalein is colourless until pH 10. So at pH 7, it is still colourless!

Which pair of substances reacts to form a salt and water only?

• Sodium chloride solution and silver nitrate solution

• Sodium hydroxide solution and dilute ethanoic acid

• Sodium carbonate solution and dilute sulfuric acid

• zinc and hydrochloric acid

Answer explanation

There are 4 equations you need to remember for reactions of acids.

One of them is neutralization.

Acid + Base -> Salt + Water

(ethanoic acid + sodium hydroxide solution)

Which ionic equation represents the neutralisation of aqueous sodium hydroxide with dilute nitric acid?

• H⁺ + OH^- -> H₂O

• Na⁺ + NO₃^- -> NaNO₃

• Na⁺ + HNO₃ --> NaNO₃ + H⁺

• NaOH + H⁺ -> Na⁺ + H₂O

Answer explanation

Ionic equation of neutralisation always consists of reaction between Hydrogen ions (from acid) and hydroxide ions (from base).

H⁺ + OH^- -> H₂O

The table shows the solubility of some salts of metal Y in cold water.

What is metal Y?

• Barium

• Lead

• Magnesium

• Sodium

Answer explanation

Check solubility of salts!

Barium is the only one which is a:

- soluble chloride

- insoluble sulfate

- insoluble carbonate

Salts are made by reacting acids with bases.

For which combination of acids and bases is the titration method of preparation suitable?

• An insoluble acid with an insoluble base.

• An insoluble acid with a soluble base.

• A soluble acid with an insoluble base.

• A soluble acid with a soluble base.

Answer explanation

Preparation of soluble salt [Titration]

Is made from:

Soluble acid + Soluble base -> Soluble salt

**Notice how we do not use solids and gases in titration!

Which reactants could be used safely to prepare potassium chloride?

• Aqueous potassium hydroxide and dilute hydrochloric acid

• Aqueous potassium sulfate and aqueous sodium chloride

• Potassium and aqueous sodium chloride

• Potassium and dilute hydrochloric acid

Answer explanation

Steps:

a) Identify the solubility of salt. [Potassium chloride is soluble].

b) There is 4 method to prepare soluble salt. How do we know which one to use?

- Check your Step to success Paper!

- Potassium is group I and TITRATION (acid + base) is the preferred method to prepare Group I salts.

Which reagent is added to aqueous potassium chloride to prepare lead chloride?

(Hint: Please check if the salt to be prepared (lead chloride) is soluble or insoluble first!)

• Aqueous lead nitrate

• Lead

• Lead carbonate

• Lead sulfate

Answer explanation

Step:

1. Check the solubility of the salt prepared (lead chloride is insoluble)

2. Insoluble salt only have ONE method of preparation [PRECIPITATION]

- Precipitation is made using soluble salt + soluble salt -> insoluble salt

- The only soluble salt given in the choice is aqueous lead nitrate.