3b.1 - Titrations

Question 1

What is concentration measured in?

Answer 1

Moles per dm3 or grams per dm3

Question 2

The higher the concentration…

Answer 2

… the more solute you dissolve in a given volume

… the more crowded the solute molecules are

Question 3

What does a titration tell you?

Answer 3

How much acid or alkali it takes to neutralise the other

Question 4

How do you carry out a titration? (6)

Answer 4

• Put some acid or alkali in a flask with an indicator (phenolphthalein or methyl orange)
• Add the one you don’t use (acid or alkali) to a burette and add it a bit at a time giving the flask a regular swirl
• The indicator changes colour when it has been neutralised
• Record how much it took to neutralise
• Repeat and average

Question 5

Why do you not use universal indicator when carrying out a titration?

Answer 5

The colour change needs to be definite

Question 6

You have 25cm3 of sodium hydroxide in your flask with a concentration of 0.1 moles/dm3. From a titration, it takes 30cm3 of sulfuric acid to neutralise the sodium hydroxide.
Work out the concentration of the acid in moles/dm3.

Answer 6

1) Work out moles in known substance:
Number of moles = concentration x volume
=0.1 mol/dm3 x (25/1000) dm3
= 0.0025 moles of NaOH

2) Balanced equation:
2NaOH + H2SO4 -> Na2SO4 + 2H2O
There is 2 NaOH’s but 1 Na2SO4
so 0.0025 % 2 = 0.00125 moles sulfuric acid

3) Work out concentration:
Concentration = number of moles % volume
= 0.00125 mol % (30/1000) dm3
= 0.0417 mol/dm3

Question 7

How do find the concentration if it asks for the answer in mol/dm3? (3)

Answer 7

• Work out moles in known substance
• Write a balanced equation and make same if necessary
• Work out concentration

Question 8

What is the triangle for finding the concentration if it asks for the answer in mol/dm3?

Answer 8

n on top - number of moles
c on left - concentration
v on right - volume

Question 9

You have 25cm3 of sodium hydroxide in your flask with a concentration of 0.1 moles/dm3. From a titration, it takes 30cm3 of sulfuric acid to neutralise the sodium hydroxide.
Work out the concentration of the acid in grams/dm3
H - 1
S - 32
O - 16

Answer 9

1) Work out RFM of the acid:
H2SO4 = (1 x 2) + 32 + (16 x 4) = 98

2) Work out moles in known substance:
Number of moles = concentration x volume
=0.1 mol/dm3 x (25/1000) dm3
= 0.0025 moles of NaOH

3) Balanced equation:
2NaOH + H2SO4 -> Na2SO4 + 2H2O
There is 2 NaOH’s but 1 Na2SO4
so 0.0025 % 2 = 0.00125 moles sulfuric acid

4) Work out concentration:
Concentration = number of moles % volume
= 0.00125 mol % (30/1000) dm3
= 0.041666… mol/dm3

5) Convert concentration to concentration in grams:
Mass in grams = moles x RFM
= 0.041666… x 98 = 4.8333…g
= 4.08 g/dm3

Question 10

How do you find the concentration if it asks for the answer in g/dm3? (5)

Answer 10

• Calculate RFM of missing solution
• Moles in known substance
• Balanced equation
• Work out concentration
• Convert from mol/dm3 to gdm3

Question 11

What is the triangle for converting mol/dm3 g/dm3?

Answer 11

m on top - mass g
n on left - number of moles
mr on right - RFM