Salts

Question 1

What is a salt?

Answer 1

A salt is an ionic compound which is formed by a metal or ammonium ion replacing one or more hydrogen ions of an acid.

Question 2

What is an acid salt?

Answer 2

An acid salt is a salt where not all the hydrogen ions of the acid has been replaced. This occurs from dibasic or tribasic acids.

Question 3

What three forms can salts exist in?

Answer 3

Aqueous (in solution), hydrated, or anhydrous.

Question 4

What are hydrated salts?

Answer 4

Hydrated salts are salts that appear in crystal form, and contain water molecules trapped in its structure called water of crystallisation.

Question 5

How are hydrated salts formed from aqueous solution?

Answer 5

Crystallisation

Question 6

How do we write the formula of hydrated salts?

Answer 6

Use the ratio of the salt to the water molecules trapped in it, e.g. CoCl2.6H2O –> 1 cobalt(II) chloride salt molecule : 6 water molecules

Question 7

What are anhydrous salts?

Answer 7

Anhydrous salts are salts in solid powder form. There are no water molecule trapped in their structure.

Question 8

How are anhydrous salts obtained from aqueous solution?

Answer 8

They can be obtained by evaporating to dryness in an evaporating dish, from an aqueous solution.

Question 9

How are anhydrous salts obtained from hydrated salts?

Answer 9

They are obtained by heating the hydrated salts gently so that the salt does not decompose under the heat.

Question 10

How do we get an aqueous solution from an hydrated/anhydrous salt?

Answer 10

Dissolve them in water lor

Question 11

What are salt nitrates?

Answer 11

Salts that contain nitrate anion(s).

Question 12

What are salt halides?

Answer 12

Salts that contain chloride or iodide anion(s) [Grp VII ions]

Question 13

What are salt sulfates?

Answer 13

Salts that contain sulfate anion(s)

Question 14

What are salt bases?

Answer 14

Salts that contain hydroxide or oxide anion(s)

Question 15

What are salt carbonates?

Answer 15

Salts that contain carbonate anion(s)

Question 16

Name the insoluble salt nitrates.

Answer 16

None (All nitrates are soluble)

Question 17

Name the insoluble halides.

Answer 17

Lead(II), and silver. (The rest are soluble)

Question 18

Name the insoluble sulfates

Answer 18

Calcium, Lead(II), Barium (the rest are all soluble)

Question 19

Name the insoluble bases

Answer 19

ALL except Group 1 ions, ammonium, barium, calcium (1ABC)

Question 20

Name the insoluble carbonates

Answer 20

ALL except Group I ions and ammonium (1A)

Question 21

What is a precipitate?

Answer 21

A precipitate is an insoluble salt.

Question 22

How are precipitates formed? Why does this happen?

Answer 22

By mixing two aqueous salts. Ions can move freely in aqeuous solutions.

Question 23

How do you determine whether a precipitate is formed by mixing two aqueous salts?

Answer 23

Find the products (AB + CD –> CB + AD) and then compare with solubility table.

Question 24

What salts can be prepared with PRECIPITATION?

Answer 24

All insoluble salts (precipitates).

Question 25

What reactants are used in precipitation?

Answer 25

Two aqueous solutions containing the required cation and anion of salt being
prepared.

Question 26

How do you perform precipitation?

Answer 26

1. Mix the two solutions in a beaker by stirring with a glass rod, forming the precipitate.
2. Filter out the precipitate using a filter funnel and filter paper. Your precipitate will be collected as residue on the filter paper.
3. Wash the residue with distilled water to remove impurities
4. Dry the salt between two sheets of filter paper.

Question 27

What salts can be prepared with REACTION OF ACID WITH EXCESS INSOLUBLE REACTANT?

Answer 27

Soluble salts other than group I metals and ammonium salts.

Question 28

Why is reacting acid with excess insoluble reactant unable to prepare group I metal and ammonium salts?

Answer 28

They are all soluble in water and hence any excess reactant cannot be removed via filtration.

Question 29

Why is reacting acid with insoluble reactant only suitable for preparing soluble salts?

Answer 29

If the method was used to prepare an insoluble salt, the insoluble salt produced would form a layer over the insoluble reactant, preventing any further reaction and resulting in a low yield.

Question 30

What are the reactants used in reacting an acid with excess insoluble reactant?

Answer 30

An acid containing the anion of the salt and an insoluble solid containing the cation of the salt.

Question 31

What are the possible reactions in reacting acid with excess insoluble reactant?

Answer 31

• Acid + Metal
• Acid + Insoluble base
• Acid + Insoluble carbonate

Question 32

What is the process for reaction of acid with excess insoluble reactant?

Answer 32

1. Add excess insoluble reactant to the acid in a beaker
2. Constantly stir the mixture with a glass rod to ensure all the acid is used up in the reaction and until no more salt dissolves, i.e. there are excess solid particles that are unreacted at the bottom of the beaker or effervescence has ceased (when applicable).
3. Filter the mixture using the filter funnel and filter paper to remove the excess solid. The solution of the desired salt will be collected as filtrate.
4. Heat the filtrate in an evaporating dish until it becomes a saturated solution.
5. Allow the solution to cool and crystallise
6. Filter the crystals and wash with a little cold distilled water to remove impurities
7. Dry the crystals between two sheets of filter paper.

Question 33

In reacting an acid with excess insoluble reactant, why do we stir the mixture until we see those excess solid particles?

Answer 33

This is to ensure that all the acid is used up in the reaction so that the acid will not contaminate the salt solution. The volume of acid is “fixed” and
is the limiting factor, so we can only control the amount of insoluble solid added.

Question 34

In reacting an acid with excess insoluble reactant, how do we know if the solution is saturated?

Answer 34

A clean glass rod can be dipped into the solution. If

crystals form on the glass rod, the solution is saturated.

Question 35

In reacting an acid with excess insoluble reactant, why do we only wash the crystals with ‘a little cold distilled water’?

Answer 35

So that the crystals don’t dissolve in the distilled water, and to remove any impurities.

Question 36

What salts can be prepared using titration?

Answer 36

Group I metals and ammonium salts.

Ammonium and Group I carbonates and bases are soluble and will form a colourless solution though.

Question 37

Why must we not add any reagents in excess in preparing salt through titration?

Answer 37

If any of the reagents (acid or alkali) is added in excess, the excess reagent cannot be removed from the salt solution and thus the salt will be impure. This is why a suitable indicator, such as methyl orange or phenolphthalein, is used to determine the exact volumes of reactants needed to form the salt.

Question 38

What reactants are used in preparing a salt through titration?

Answer 38

An acid containing the anion of the salt and an alkali/carbonate containing the cation of the salt are used.

• Acid + Group I / Ammonium Carbonate
• Acid + Group I / Ammonium Hydroxide

Question 39

How is preparing a salt through titration performed?

Answer 39

1. Add 25.0 cm3 of acid into a conical flask using a pipette. Then, add a few drops of methyl orange, the solution will turn red
2. While swirling the conical flask, add the alkali/carbonate solution from a burette into
   the conical flask dropwise until the solution turns orange permanently
3. Record the volume of alkali/carbonate solution required for complete neutralisation of the acid.
4. Repeat steps by adding the exact volume of alkali/carbonate solution needed to neutralise 25.0cm3 of acid into a clean conical flask with 25.0cm3 of acid. But this time, do not use an indicator so as to not contaminate the salt solution that is produced after complete neutralisation.
5. Heat the salt solution in an evaporating dish until saturated
6. Let the saturated salt solution cool down and crystalise
7. Filter the crystals and wash with a little cold distilled water to remove impurities.
8. Dry the crystals between two sheets of filter paper