C3.4 Further Analysis and Quantitative Chemistry

Question 1

What is a titration?

Answer 1

An experiment that lets you see what volume of a reactant is need to react completely with a certain volume of another reactant

Question 2

How do you carry out a titration? (10 steps)

Answer 2

1. Use a pipette to measure out a volume of alkali
2. Put the alkali in a flask along with some indicator
3. Put the acid in the burette. Run a small amount through the tap, then turn the tap off. Set the burette up above the flask containing the alkali
4. Take a reading of the volume of acid in the burette. Read off the value where the bottom of the meniscus touches the scale
5. Add the acid to the alkali a bit at a time, giving the flask a regular swirl. Go especially slowly when you think the alkali is almost neutralised.
6. The indicator changes colour when the alkali has been neutralised - this is the end of the reaction
7. Record the volume of acid left
8. Calculate the amount of acid used (initial vol. of acid - final vol. of acid)
9. Repeat the process twice more
10. Calculate the mean volume of acid needed to neutralise the alkali

Question 3

Why can’t universal indicator be used in an acid-alkali titration? What do you need then?

Answer 3

Because it changes colour gradually - you need an indicator that will give you a definite colour change so you can see the exact point of neutralisation

Question 4

What are commonly used indicators in acid-alkali titrations? (2)

Answer 4

Phenolphthalein and methyl orange

Question 5

What colour is Phenolphthalein in acids/alkalis?

Answer 5

Pink in alkalis and colourless in acids

Question 6

What colour is methyl orange in acids/alkalis?

Answer 6

Yellow in alkalis and red in acids

Question 7

What would you include in the risk assessment of a acid-alkali titration? (2) How do you prevent these hazards?

Answer 7

Acids are often irritants and corrosive, and alkalis are often caustic - wearing safety googles will protect your eyes from these substances

Question 8

What is a mole?

Answer 8

A word used to describe a number (6.023 x 10²³)

Question 9

What does a mole present of an element/compound?

Answer 9

One mole of an atom of any element/compound weighs exactly the same number of grams as the relative formula mass of that element/compound

Question 10

How is are litres, cubic decimetres and cubic centimetres related?

Answer 10

1 litre = 1 dm³ = 1000 cm³

Question 11

What is concentration?

Answer 11

The amount of a particular substance contained within a solution or mixture or in a particular volume of space.

Question 12

What is concentration measured in? (2)

Answer 12

moles per dm³ or grams per dm³

Question 13

What does it mean when concentration is measure in moles per dm³?

Answer 13

A solution with a concentration of 1 mole per dm³ of substance x, means the solution has 1 mole of x in 1 dm³

Question 14

What does it mean when concentration is measure in grams per dm³?

Answer 14

A solution with a concentration of 56 grams per dm³ of substance x, means the solution has 56 grams of x in 1 dm³

Question 15

What is the formula for finding the number of moles in a substance?

Answer 15

Number of moles = concentration (mol/dm³) x volume (dm³) [n = c x v]

Question 16

What are the steps for finding the concentration of a substance in moles per dm³? (3 steps)

Answer 16

1. Work out the moles of the ‘known’ substance with n = c x v
2. Write down the balanced equation of the reaction and work out how many moles of the ‘unknown’ substances you must have had
3. Work out the concentration of the ‘unknown’ stuff with c = n/v
   [use the grid method for this]

Question 17

What are the steps for finding the concentration of a substance in grams per dm³? (3 steps)

Answer 17

1. Convert concentration into mol/dm³ by using n = c x v
2. Work out the relative formula mass for the acid (Mr - all the atomic masses added up)
3. Convert the concentration from step 1 into grams by using this equation: Mass in grams = moles x Mr

Question 18

What are ionic compounds made up of?

Answer 18

Positive and negative ions

Question 19

What is the flame test, a test for? How is it carried out?

Answer 19

It is a test for positive ions - a substance is put over a flame to see what colour the flame goes

Question 20

Flame test, flame of lithium?

Answer 20

Li+, gives a crimson flame

Question 21

Flame test, flame of sodium?

Answer 21

Na+, gives a yellow flame

Question 22

Flame test, flame of potassium?

Answer 22

K+, gives a lilac flame

Question 23

Flame test, flame of calcium?

Answer 23

Ca2+, fives a red flame

Question 24

Flame test, flame of barium?

Answer 24

Ba2+, gives a green flame

Question 25

What is needed to be done after a flame test if the wire loop used is to be reused?

Answer 25

It needs to be cleaned thoroughly, by dipping it into hydrochloric acid and then into distilled water

Question 26

How are precipitation reactions carried out?

Answer 26

A few drops of sodium hydroxide solution is added to a solution, the colour of the precipitate formed tells you what metal ions are present

Question 27

Precipitate test, colour of calcium precipitate?

Answer 27

Ca2+, forms a white precipitate

Question 28

Precipitate test, colour of copper (II) precipitate?

Answer 28

Cu2+, forms a blue precipitate

Question 29

Precipitate test, colour of iron (II) precipitate?

Answer 29

Fe2+, forms a green precipitate

Question 30

Precipitate test, colour of iron (III) precipitate?

Answer 30

Fe3+, form a brown precipitate

Question 31

Precipitate test, colour of aluminium precipitate?

Answer 31

Al3+, forms a white a precipitate at first, then redissolves in excess NAOH to form a colourless solution

Question 32

Precipitate test, colour of magnesium precipitate?

Answer 32

Mg2+, forms a white precipitate

Question 33

What are the tests for positive ions? (2)

Answer 33

Flame test and the precipitate test

Question 34

What is the test for carbonate ions?

Answer 34

A substances is reacted with a dilute acid, the gas produced is piped through limewater - if the limewater turns cloudy due to a white precipitate forming, carbon dioxide is present, so carbonate ions were present in the solution

Question 35

What is produced when a carbonate reacts with an acid?

Answer 35

Water, salt and carbon dioxide

Question 36

What are the halide ions? (3)

Answer 36

Chloride (Cl-), bromide (Br-), and iodide (I-)

Question 37

How is a test for halide ions carried out?

Answer 37

Dilute nitric acid, followed by silver nitrate solution, added to a substance containing halide ions, will produce different coloured precipitates

Question 38

Testing for halide ions, colour of chloride precipitate?

Answer 38

Cl-, produces a white precipitate

Question 39

Testing for halide ions, colour of bromide precipitate?

Answer 39

Br-, produces a cream precipitate

Question 40

Testing for halide ions, colour of iodide precipitate?

Answer 40

I-, produces a yellow precipitate

Question 41

Why can’t hydrochloric acid be used in a halide ions test?

Answer 41

Because then the precipitate formed will always be white as chloride ions are in HCl

Question 42

What is the test for sulfate ions?

Answer 42

Dilute hydrochloric acid, followed by chloride solution is added to a substance - a white precipitate of barium sulfate means the original substance was a sulfate

Question 43

What is the ionic equation for the test for sulfate ions?

Answer 43

Ba2+(aq) + SO42-(aq) ==> BaSO4(s)