Required practical 2 - Neutralisation

Question 1

How could you find out the volume of sulfuric acid hat will exactly neutralise 25cm³ of sodium hydroxide solution?

Answer 1

• Place exactly 25cm³ of sodium hydroxide in a conica flask on a white tile, using a pipette and pipette filler.
• Clamp the burette above the conical flask and fill it with sulfuric acid using a funnel, filling the burette to the 0.00cm³ line.
• Add 5-10 drops of phenolphthalein to the conical flask.
• Slowly add the sulfuric acid to the flask, swirling the flask at the same time.
• Close the burette tap as soon as the mixture turns from pink to colourless.
• Record the final volume of the sulfuric acid added.

Question 2

List the apparatus required to carry out a titration

Answer 2

• Burette.
• Conical flask.
• Pipette and pipette filler.
• Funnel.
• Clamp and stand.
• White tile.

Question 3

Why should a white tile be used in a titration experiment?

Answer 3

The white tile is placed under the reacting mixture, making the colour change easier to see.

Question 4

Why might a volumetric pipette and pipette filler be used to measure 25cm³ rather than measuring cylinder?

Answer 4

A volumetric pipette will measure 25cm³ more accurately so it is usually used for titrations.

Question 5

What is the purpose of repeating a titration?

Answer 5

To allow a mean titre to be calculated, reducing the effect of random error.

Question 6

What colour is phenolphthalein in acid and alkali?

Answer 6

Acid - colourless.

Alkali - pink.

Question 7

Why is the first trial in a titration often called a ‘rough’ trial?

Answer 7

The first trial gives an approximate idea of where the end point is so then for further titres, you can be more precise as you know approximately what volume will be required for neutralisation.

Question 8

What safety precautions should be undertaken when carrying out a titration?

Answer 8

• Wear eyewear and be aware of the dangers of the chemicals used. For example, H₂SO₄ is corrosive, NaOH is an irritant.
• The burette should be filled below the eye level so there’s no risk of it spilling and splashing on your face.
• Any broken glassware needs to be cleared immediately.

Question 9

Why are burettes used for titrations?

Answer 9

Burettes allow the solution to be added drop by drop in very small quantities.

Question 10

Apart from phenolphthalein, what other indicator could be used in a titration? What is the colour change?

Answer 10

Methyl orange: red (acid) to yellow (alkali).

Litmus blue: red (acid) to blue (alkali).

Question 11

if you know the volume of acid required to neutralise an alkali, how could you calculate the concentration of the acid, given the alkali concentration and volume?

Answer 11

• Calculate the number of moles of the alkali using the known volume and concentration.
• Use the chemical equation to work out the ratio of acid an alkali that react and hence work out how many moles of acid have reacted.
• Divide the moles of acid by the volume used in neutralisation.

Question 12

If you know the concentration of an acid in mol/dm³, how could you convert this to g/dm³?

Answer 12

Multiply the concentration in mol/dm³ by the molar mass (Mr) of the acid to give the value in g/dm³.