Salts

Question 1

Naming of Salts

How does one tell which acid a salt is derived from?

Answer 1

A salt is derived from an acid by replacing its hydrogen ion with a metallic ion. E.g sodium chloride is dervied from Hydrochloric Acid by replacing H+ ions with Na+.

Sulfates are derived from sulfuric acid.
Nitrates are derived from nitric acid.
Phosphates are derived from phosphoric acid.
Carbonates are derived from carbonic acid.
Sufites are derived from sulfurous acid.

Question 2

Naming of Salts

What is the product formed when there is only partial replacement of the hydrogen ion in an acid?

Answer 2

An acid salt is formed.
E.g :
Acid is Sodium Carbonate, H2CO3.
Partial Replacement: Only 1 of the H+ ions are replaced, hence forming NaHCO3.

When full replacement, normal salts such as Na2CO3 are formed.

Question 3

Water of Crystallization

What are salts that combine with water molecules to form crystals called?

Answer 3

They are known as hydrated salts.

Examples include:
1. Na2CO3.10H2O
2. CuSO4. 5H2O
3. ZnSO4.7H2O
4. MgSO4.7H2O

Question 4

Precipitation Method of Preparing Salts

What is the precipitation method used to prepare? Insoluble or soluble salts

Answer 4

This method is used to prepare insoluble salts, as a precipitate is an insoluble chemical produced during a chemical reaction.

Question 5

Precipitation Method of Preparing Salts

What is the overview of the precipitation method of preparing salts?

Answer 5

Overview: To prepare an insoluble salt, mix a soluble salt (e.g nitrate salt) that contains its cations with a soluble salt (e.g sodium salt) that contains its anions.

1.

Question 6

Precipitation Method of Preparing Salts

What are the exact steps in preparing an insoluble salt?

Answer 6

1. Add an excess of one aqueous solution to the other aqueous solution.
2. Stir thoroughly until no more precipitate forms. (The reaction is complete)
3. Filter the mixture and collect the residue, which contains the precipitate. The filtrate may then be disposed of.
4. Wash the residue with deionised water to remove unwanted reactants.
5. Dry the residue between sheets of filter paper.

Question 7

Precipitation Method of Preparing Salts

Please provide some examples of the precipitation method.

Answer 7

Calcium Carbonate: Sodium Carbonate + Calcium Nitrate

Silver iodide: Silver nitrate + Sodium iodide

Lead(II) Chloride: Lead(II) Nitrate + Sodium Chloride

Question 8

Filtration Method

What salt does the filtration method prepare?

Answer 8

The filtration method prepares soluble salts, not containing Group 1 or Ammonium Salts.

Question 9

Filtration Method

What are the steps for the filtration method?

Answer 9

1. Add excess, moderately reactive metal/insoluble base/insoluble carbonate to warm dilute acid and stir until there is no further reaction.
2. Filter the mixture to obtain the filtrate.
3. Heat the filtrate to the point of saturation (do not evaporate to dryness as the salt particles will decompose upon heating)
4. Test for saturation by dipping a glass rod into the solution. If crystals form quickly on the surface of the rod, the solution is saturated.
5. Cool the filtrate to allow for crystals to form.
6. Filter to obtain the crystals and wash the crystals carefully with cold deionised water.
7. Dry the crystals between sheets of paper.

Question 10

Filtration Method

Provide some examples of instances when Filtration Method is used to produce a soluble salt.

Answer 10

ZnSO4.
Add excess Zn powder into warm dilute sufulric acid, and stir thoroughly until reaction completes. Filter the mixture to obtain the filtrate ZnSO4 to filter out any excess Zn powder. Heat the filtrate to obtain a concentrated solution of ZnSO4, and test for saturation. Do this by testing the solution with a glass rod. Should crystals form on the glass rod, the ZnSO4 solution is saturated. Cool the filtrate to obtain the crystals, and wash them carefully with cold deionised water to remove unwanted reactants. Dry them between sheets of paper.

Question 11

Filtration Method

What are some applications of the filtration method?

Answer 11

i) Acid and Metal Reactions: Salt+Hydrogen Gas
ii) Acid and Carbonate Reactions: Salt + Carbon Dioxide + water
iii) Acid and Base reactions: Salt + Water

Question 12

Filtration Method

What is the filtration method not suitable for?

Answer 12

1) Unreactive Metals such as Lead and Copper
2) Very Reactive Metals such as sodium, potassium and Calcium as the reaction is too vigorous.
3) Making insoluble salts

Question 13

Titration Method

What does the titration method prepare?

Answer 13

It prepares soluble salts which contain Group 1 or ammonium cations

Question 14

Titration Method

What are the steps to the Titration Method? E.g Preparing NaNO3.

Answer 14

Part 1:
1) Fill in the burette with dilute HNO3, and note the initial burette reading.
2) Pipette 25.0 cm3 of aqueous NaOH solution into a conical flask.
3) Add 2 to 3 drops of an indicator such as methyl orange or phenolpthalein to test the pH.
4) Add dilute HNO3 slowly from the burette to titrate the alkali with the acid, until the endpoint is reached. Deduce the later burette reading and derive the volume of HNO3 used to titrate NaOH.

Part 2:
1) Repeat the experiment without indicators as they would make the filtrate impure, by adding the same volume of acid to titrate the alkali.

2) Transfer the solution to a crucible and heat the solution until it is saturated.

3) Test for saturation by dipping glass rod into the solution.

4) Filter solution to obtain crystals, rinse with cold water, dry using sheets of filter paper.

Question 15

Types of Salt Preparation Methods

What is the structure of open-ended answer for precipitation method?
(e.g trying to produce CaCO3)

Answer 15

1) Add an excess of CaNO3 to Na2CO3 and stir thoroughly until no further precipitate is formed.

2) Filter the mixture to obtain the residue of CaCO3, hence the filtrate of NaNO3 may then be disposed.

3) Wash the residue thoroughly with deionised water to remove any unwanted reactants such as Na+ ions.

4) Dry the the residue between sheets of filter paper.

Question 16

Types of Salt Preparation Methods

What is the format of open-ended answers for filtration method?
e.g (Zinc Sulfate)

Answer 16

1) Add an excess of Zinc powder to warm dilute sulfuric acid, and stir thoroughly.
2) Filter the mixture to obtain the filtrate of Zinc Sulfate, and the residue may be disposed of.
3) Heat the filtrate to the point of saturation, and test for saturation by dipping a glass rod into the solution of ZnSO4. If crystals form on the glass rod, the solution is saturated and no longer requires heating.
4) Cool the solution to allow for the formation of crystals on the surface of the crucible.
5) Collect the crystals formed and gently wash them with cold deionised water.
6) Dry the crystals gently between sheets of filter paper.

Question 17

Types of Salt Preparation Methods

What is the answer format for open-ended answers of the titration method?

E.g Preparing Potassium Sulfate

Answer 17

1) Pipette 25Cm^3 of Sulfuric acid into a conical flask.
2) Add 2-3 drops of any indicator such as phenolpthalein/methyl orange into the solution.
3) Using a burette filled with dilute K(OH), tritrate K(OH) against sulfuric acid until the endpoint is reached, whereby an additional drop of indicator would result in a permanent color change.
4) Record the volume of K(OH) used and repeat the experiment without the indicator as it could make the product impure.
5) Transfer the solution to a crucible and heat the solution to the point of saturation.
6) Test for saturation by dipping a glass rod into the solution; if crystals form on the glass rod, no further heating is required.
7) Collect, wash and dry the crystals carefully.