Getting Ready How many equations can you use to calculate number of mole that involve volume and concentration What are the conventional units for Concentration Volume What other units might we measure concentration in ID: 625367
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Slide1
Volumetric AnalysisSlide2
Getting Ready
How many equations can you use to calculate number of
mole that involve volume and concentration?
What
are the conventional units for:
Concentration
Volume
What
other units might we measure concentration in?Slide3
Key Terms
Concentration
Volume
Redox reaction
Acid-Base reaction
pH
Equivalence Point
Primary Standard
Standard Solution
Titration
Aliquot
Titre
End PointSlide4
Volumetric Analysis
Uses solutions and volumes.
Requires the use of standard
solutions (solutions with
accurately known
concentrations).Slide5
Standard Solutions
A standard solution can be made from a primary standard.
A primary standard is a soluble solid that can be weighed accurately then dissolved in a solvent.Slide6
Standard Solutions
A primary standard must:
Have a high state of purity.
Have an accurately known formula.
Must not react with the atmosphere or degrade over time.
Be cheap and readily available.
Should also have a high molar mass to reduce weighing errors.Slide7
Titrations - Review
What is the difference between a strong acid and a concentrated acid?
Can you remember the difference between the end point and the equivalence point?
How would you identify the equivalence point in a practical situation using an indicator?Slide8
pH Indicators
There is a vast number of substances that can be used as indicators.
Each changes colour at different pH ranges.Slide9
Equivalence Points
When titrating, the equivalence point will not always be 7.
Chemists need to select an appropriate indicator for the type of acid and base being used.
Study the graphs on the following page, then make a decision about what would be a suitable indicator for each titration.Slide10
Equivalence Points
CH
3
COO
Na
H
HH
O
H
O
N
H
H
H
H
Cl
H
H
OSlide11
Example Calculation
A solution of sodium hydroxide is reacted with a 0.30M solution of hydrochloric acid. An average of 23.5mL of hydrochloric acid is required to neutralise a 20mL aliquot of the sodium hydroxide solution. Calculate the concentration of the sodium hydroxide solution.Slide12
Example Calculation
0.30M
HCl
23.5mL of
HCl
required20mL aliquot of NaOH
What is the concentration of NaOH?Slide13
Example Calculation
At around what pH would you expect the equivalence point to be in this titration?
Calculate the pH of the sodium hydroxide.Slide14
Example Calculation
A student tested the citric acid content of a sample of vinegar by diluting 20.0mL of the sample to 250.0mL. The student then titrated 20mL aliquots of the diluted juice with 0.100M sodium hydroxide. The average titre was 12.52mL. Calculate the concentration of acetic acid in the vinegar in grams per litre.Slide15
Example Calculation
20mL of sample, diluted to 250mL
20mL aliquots
Concentration of
NaOH
= 0.100MAverage titre = 12.52Answer in grams per litreSlide16
Example Calculation
20mL of sample, diluted to 250mL
20mL aliquots
Concentration of
NaOH
= 0.100MAverage titre = 12.52Answer in grams per litreSlide17
Back Titrations
Done by adding a known amount of a third reagent, then determining the amount of excess.
For example:
Unknown amount of CaCO
3
React it with 3 mol of HCl
Titrate with NaOHEquivalence point at 1 molHow many mole of CaCO3?Slide18
Back Titrations
Can be done when the substance being analysed is:
Insoluble.
A weak acid or base and you can’t get a sharp end point.
Volatile.Slide19
Example Calculation
The amount of SO
2
in polluted air can be determined by passing a measured volume of gas through a solution containing excess KMnO
4
.SO
2 + MnO4- + SO4 + Mn2+ + The excess permanganate can then be determined by titration with a standard solution of Fe2+.
Fe
2+
+ MnO
4
-
+
Fe
3+
+
Mn
2+
+Slide20
Example Calculation
10.0m
3
of polluted air was passed through 100mL of 0.0201M KMnO
4
. The KMnO4 that was left after the reaction reacted with 26.95mL of 0.158M Fe
2+ solution.How many mole of MnO4- was present before the SO2 was bubbled through it?Slide21
Example Calculation
How many mole of MnO
4
-
reacted with the Fe2
+?
How many mole of MnO4- reacted with the SO2?Slide22
Example Calculation
Calculate the mass of SO
2
in the sample of air
.
Calculate the concentration of SO2 in grams per cubic metre.Slide23
Poor Laboratory Technique
The book calls it ‘sources of error’ which I disagree with.
What would happen if:
You left rinsing wate
r in the burette?
There was some water in the titration flask?
You chose an indicator that changed after the equivalence point?Slide24
Significant Figures
ALL
non-zero numbers (1,2,3,4,5,6,7,8,9) are ALWAYS significant.
ALL
zeroes between non-zero numbers are ALWAYS significant.
ALL
zeroes which are SIMULTANEOUSLY to the right of the decimal point AND at the end of the number are ALWAYS significant. ALL zeroes which are to the left of a written decimal point and are in a number >= 10 are ALWAYS significant.Slide25
Image Credits
Nuno
Nogueira
[CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons