Titrations

Question 1

Reaction in a titration.

Answer 1

acid + alkali → salt + water

Question 2

Describe the pH curve

Answer 2

• Only alkali is present at the beginning of the experiment so the pH is high.
• As acid is added it starts to neutralise the alkali and the pH drops slowly at first.
• The point where all the acid reacts with all the alkali is called the endpoint.
• At the end point, pH changes suddenly.

Question 3

It is often useful to sketch a pH titration curve. For many acid and alkali titrations the shape is always the same, but you make the curve the right size it is useful to know:

Answer 3

• The starting pH
• The end pH
• The endpoint (the most vertical section of the curve)
• The pH range over which the sudden change occurs.

Question 4

Why are indicators chosen?

Answer 4

• Indicators are chosen as they give sharp colour changes at specific pHs.

Question 5

Different types of indicators.

Answer 5

• Indicators such as phenolphthalein, screened methyl orange and litmus give a sudden colour change.
• Mixed indicators such as universal Indicator (UI) give a range of colours.

Question 6

Why are single indicators used in titration?

Answer 6

To show the end point, a single indicator needs to be used. Mixed indicators like universal indicator give continuous colour changes which are harder to see.

Question 7

Colour of litmus in acid and alkali.

Answer 7

• Acid: Red
• Alkali: Blue

Question 8

Colour of phenolphthalein in acid and alkali.

Answer 8

• Acid: Colourless
• Alkali: Pink

Question 9

Colour of screened methyl orange in acid and alkali.

Answer 9

• Acid: Pink
• Alkali: Green

Question 10

Why are titrations repeated?

Answer 10

So an average reading can be calculated to make the technique reliable.

Question 11

What are titrations used to find?

Answer 11

The concentration of an alkali from a from a known concentration of acid or vice versa.

Question 12

Concentration Eq.

Answer 12

concentration = number of moles/volume in dm³.

Question 13

Number of moles Eq.

Answer 13

number of moles = concentration x volume in dm³.

Question 14

Volume in dm³ Eq.

Answer 14

volume in dm³ = number of moles/concentration

Question 15

Working out the concentration of an acid

Answer 15

1. Work out how many moles of acid were used. Using: number of acid moles = acid concentration x volume x volume in dm³
2. Work out how many moles of alkali were used. Using amount of alkali moles = alkali concentration x volume in dm³
3. Link the acid and alkali. number of acid moles = Number of alkali moles