Chapter 2 - Salt Preparation

Question 1

What is a salt?

Answer 1

• an ionic compound

- formed when the hydrogen ion, H+ from an acid is replaced by a metallic ion or an ammonium ion (NH4+)

Question 2

What is water of crystallisation?

Answer 2

Water molecules that combine with salts to form crystals

Question 3

Hydrated salts are ________ to the touch

Answer 3

dry

Question 4

What are hydrated salts?

Answer 4

Salts that contain water of crystallisation

Question 5

What are anhydrous salts?

Answer 5

Salts that do not contain water of crystallisation

Question 6

Soluble salts are in ________ state at room temperature

Answer 6

aqueous (aq)

Question 7

Insoluble salts are in ________ state at room temperature

Answer 7

solid (s)

Question 8

Soluble salt, soluble reactants → ________

Answer 8

Titration

Question 9

Soluble salt, insoluble reactants → ________

Answer 9

Acid with excess insoluble metal/carbonate/base

Question 10

Insoluble salt, soluble reactants → ________

Answer 10

Ionic precipitation

Question 11

Method 1: Acid with excess insoluble metal/carbonate/base

Answer 11

1. React excess insoluble base/metal/carbonate with acid. Some heating may be required. Stir until no more solids dissolve (in excess).
2. Filter the mixture to remove excess solids.
3. Collect the filtrate.
4. Heat the filtrate until the solution is saturated.
5. Cool and crystallize.
6. Filter away excess solution.
7. Wash and dry.

Question 12

Why must the metal/carbonate/base be added in excess?

Answer 12

To ensure that all the acid has been reacted

Question 13

Why must the metal/carbonate/base be insoluble?

Answer 13

If the reactant used was soluble, one would not be able to tell if it has been added in excess, and hence would not be able to form a pure salt

Question 14

Acid with excess insoluble metal is suitable for ________ metals

Answer 14

moderately reactive

Question 15

Acid with excess insoluble metal is not suitable for ________ and ________

Answer 15

highly reactive metals: Group 1 metals, calcium

unreactive metals: copper, silver, gold

Question 16

Method 2: Titration (part 1)

Answer 16

1. Fill up a burette with dilute acid. Note the initial burette reading (V1cm3).
2. Pipette 25.0cm3 of alkali solution into a conical flask.
3. Add 1−2 drops of indicator to the alkali solution.
4. Add dilute acid slowly from the burette until the solution just turns (turning point colour of indicator).
5. Stop adding dilute acid. Record the final burette reading (V2cm3).

Question 17

Method 2: Titration (part 2)

Answer 17

1. Pipette 25.0cm3 of alkali solution into a beaker.
2. Add (V1-V2)cm3 of dilute acid from the burette.
3. Heat to saturate the solution.
4. Cool and crystallise.
5. Filter away excess solution
6. Wash and dry.

Question 18

Titration is suitable for ________

Answer 18

Sodium, potassium and ammonium salts

Question 19

What is a titrant?

Answer 19

A solution of known concentration (in the burette)

Question 20

What is an analyte?

Answer 20

A solution of unknown concentration (in conical flask)

Question 21

Method 3: Ionic precipitation

Answer 21

1. Mix aqueous solutions of 2 soluble salts.
2. Filter the mixture to obtain the precipitate.
3. Wash the precipitate (to remove impurities).
4. Dry the precipitate.

Question 22

What is solubility?

Answer 22

The maximum amount of solute that can dissolve in a given amount of solvent at a given temperature

Question 23

Solubility ________ as temperature increases (________ gradient)

Answer 23

increases

positive

Question 24

Solubility ________ as temperature decreases (________ gradient)

Answer 24

decreases

negative

Question 25

Unsaturated solution: solution which contains ________ the maximum amount of solute that can dissolve at a given temperature (________ the line)

Answer 25

less than

below

Question 26

Saturated solution: solution which contains the ________ amount of solute dissolved at a given temperature (________ the line)

Answer 26

maximum

on

Question 27

Supersaturated solution: solution which contains ________ dissolved solute than a saturated solution at a given temperature (________ the line)

Answer 27

more

above

Question 28

Supersaturated solution = solute is added in ________

Answer 28

excess

Question 29

Crystallisation: used for mixtures with ________ of solubility to temperature (________ gradient)
such that a ________ amount of salt will precipitate upon cooling

Answer 29

significant positive relationship
steep increasing
significant

Question 30

Evaporation to dryness: used for mixtures with ________ relationship of solubility to temperature (gradient is close to ________)

Answer 30

constant

zero

Question 31

Filtration: used to separate mixtures with ________ solubilities, e.g. insoluble salts (graph is close to ________)

Answer 31

very low

zero

Question 32

Titration: Why is a suitable indicator required?

Answer 32

l