1 of 31© Boardworks Ltd 20091 of 31© Boardworks Ltd 2009.

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volume of solution (dm3)

Calculating concentrations

The concentration of a solution can be measured in grams per dm3 (g/dm3) or moles per dm3 (mol/dm3).

The following equation gives concentration in g/dm3:

If 1.0 g of solid sodium hydroxide is dissolved in 250 cm3 of solution, what is the concentration in g/dm3?

concentration = 1/0.25 = 4 g/dm3

concentration =mass dissolved (g)

mass of solid = 1 g

volume of solution = 250 cm3 = 0.25 dm3


Standard solutions


Calculating concentrations in g/dm3


Calculating concentrations in mol/dm3

To calculate concentration in mol/dm3:

x

The equation for concentration can be put into a formula triangle:

volume of solution (dm3)concentration =

mass dissolved (mol)

c =mv


Calculating concentration

If 1.0 g of solid sodium hydroxide are dissolved in 250 cm3 of solution, what is the concentration in mol/dm3?

moles = mass/RAM = 1/40 = 0.025 mol

Calculate concentration:

concentration = moles/volume = 0.025/0.25 = 0.1 mol/dm3

Convert mass of solid into moles of solid:

RAM of sodium hydroxide = 40

volume of solution = 250 cm3 = 0.25 dm3


Calculating concentrations in mol/dm3


Titration

Titration is a technique that can be used to accurately analyse the concentrations of substances in solution.

Titrations are often carried out by using a neutralization reaction between an acid and an alkali.

stand

beaker

burette

pipette

conical

flask

safety filler


How does titration work?

Imagine you have a sample of hydrochloric acid and you need to know its concentration.

First, measure out a specific amount of the acid, and neutralize it with a standard solution of an alkali. Measuring the amount of alkali that is needed to neutralize the acid will allow you to work out the concentration of the acid.

If you measured out 25 cm3 of the unknown acid, and found that it was neutralized by 20 cm3 of 0.1 mol/dm3 alkali, is the acid more or less concentrated than the alkali?

The acid is less concentrated than the alkali.


How does titration work?


Attempt 3

Titration calculations

0.0

20.0

20.0

initial burette reading (cm3)

volume of NaOH added (cm3)

final burette reading (cm3)

Titration Attempt 1 Attempt 2

0.0

19.9

19.9

19.9

20.1

40.0

How are the results of a titration used to calculate the concentration of an unknown acid solution?

Average volume of NaOH = (20.0 + 19.9 + 20.1)/3

= 20.0 cm3



Result: 20 cm3 of NaOH neutralizes 25 cm3 of HCl of unknown concentration.

1. Write a balanced equation for the reaction:

HCl + NaOH → NaCl + H2O

2. Calculate the number of moles of alkali:

moles = concentration (mol/dm-3) × volume (dm3)

= 0.1 × (20.00 / 1000)

= 0.002 moles NaOH

Working:



3. The balanced equation shows that one mole of HCl reacts with one mole of NaOH, so 0.002 moles of NaOH will react with 0.002 moles of HCl.

4. Calculate the concentration of the HCl solution in mol/dm3:

= 0.002 / (25 / 1000)

= 0.08 mol/dm3

concentration =moles

volume (dm3)


Titration apparatus


Making accurate measurements


Choosing suitable measuring apparatus


Plotting a pH curve


How would the pH curve look if you started with a strong alkali in the conical flask and added the strong acid to it from the burette?

pH curve for strong alkali and strong acid

0 5 10 15 20 25 30 35 40 45 500

7

volume acid added (cm3)

pH

14

The pH starts off high and steadily decreases as the acid is added. The endpoint is at pH7.


Producing different pH curves


End points

The endpoint of a titration is not always at pH 7. Titrations involving a weak acid or alkali can cause the indicator to change colour at a different pH.

strong

strong

weak

strong

weak

strong

Acid Alkali pH at endpoint

7

more than 7

less than 7

weakweak 7


pH ranges of indicators


When choosing an indicator for a titration, it is important to match its pH range (the pH values where it changes colour) to the endpoint of the titration.

Why is litmus not a particularly good indicator for a titration?

Indicators and pH ranges

Phenolphthalein has a pH range of 8–9.

Methyl orange has a pH range of 3–4.

Litmus has a pH range of 5–8.

Bromothymol blue has a pH range of 6–7.


Choosing the right indicator


True or false?


Glossary


Anagrams


Multiple-choice quiz

1 of 31© Boardworks Ltd 20091 of 31© Boardworks Ltd 2009.

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