Transcript Titrations

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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:
concentration =
mass dissolved (g)
volume of solution (dm3)
If 1.0 g of solid sodium hydroxide is dissolved in 250 cm3 of
solution, what is the concentration in g/dm3?
mass of solid = 1 g
volume of solution = 250 cm3 = 0.25 dm3
concentration = 1/0.25 = 4 g/dm3
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Standard solutions
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Calculating concentrations in g/dm3
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Calculating concentrations in mol/dm3
To calculate concentration in mol/dm3:
concentration =
mass dissolved (mol)
volume of solution (dm3)
The equation for
concentration can be
put into a formula
triangle:


m
c=
v
x
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Calculating concentration
If 1.0 g of solid sodium hydroxide are dissolved in 250 cm3 of
solution, what is the concentration in mol/dm3?
 Convert mass of solid into moles of solid:
RAM of sodium hydroxide = 40
moles = mass/RAM = 1/40 = 0.025 mol
 Calculate concentration:
volume of solution = 250 cm3 = 0.25 dm3
concentration = moles/volume = 0.025/0.25 = 0.1 mol/dm3
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Calculating concentrations in mol/dm3
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Titration
Titration is a technique that can be used to accurately analyse
the concentrations of substances in solution.
safety filler
burette
stand
pipette
conical
flask
beaker
Titrations are often carried out by using a neutralization
reaction between an acid and an alkali.
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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.
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How does titration work?
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Titration calculations
How are the results of a titration used to calculate the
concentration of an unknown acid solution?
Titration
Attempt 1 Attempt 2 Attempt 3
initial burette
reading (cm3)
0.0
0.0
19.9
final burette
reading (cm3)
20.0
19.9
40.0
volume of NaOH
added (cm3)
20.0
19.9
20.1
Average volume of NaOH = (20.0 + 19.9 + 20.1)/3
= 20.0 cm3
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Titration calculations
Result: 20 cm3 of NaOH neutralizes 25 cm3 of HCl of unknown
concentration.
Working:
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
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Titration calculations
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:
concentration =
moles
volume (dm3)
= 0.002 / (25 / 1000)
= 0.08 mol/dm3
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Titration calculations
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Titration apparatus
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Making accurate measurements
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Choosing suitable measuring apparatus
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Plotting a pH curve
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pH curve for strong alkali and strong acid
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?
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The pH starts off
high and steadily
decreases as the
acid is added. The
endpoint is at pH7.
pH 7
0
0
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5
10
15 20 25 30 35 40
volume acid added (cm3)
45
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Producing different pH curves
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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.
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Acid
Alkali
strong
strong
7
strong
weak
less than 7
weak
strong
more than 7
weak
weak
7
pH at endpoint
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pH ranges of indicators
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Indicators and pH ranges
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.
 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.
Why is litmus not a
particularly good indicator
for a titration?
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Choosing the right indicator
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True or false?
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Glossary
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Anagrams
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Multiple-choice quiz
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