Volumetric Analysis

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?

Key TermsoConcentrationoVolumeoRedox reactionoAcid-Base reactionopHoEquivalence Point

oPrimary StandardoStandard SolutionoTitrationoAliquotoTitreoEnd Point

Volumetric Analysis

•Uses solutions and volumes.

•Requires the use of standardsolutions (solutions withaccurately knownconcentrations).

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.

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.

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?

pH Indicators

•There is a vast number of substances that can be used as indicators.

•Each changes colour at different pH ranges.

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.

Equivalence Points

CH3COONa

H H

HOH

O

NH

HH HCl

H HO

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.

Example Calculation• 0.30M HCl• 23.5mL of HCl required• 20mL aliquot of NaOH• What is the concentration of NaOH?

Example Calculation

•At around what pH would you expect the equivalence point to be in this titration?

•Calculate the pH of the sodium hydroxide.

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.

Example Calculation• 20mL of sample, diluted to 250mL• 20mL aliquots• Concentration of NaOH = 0.100M• Average titre = 12.52• Answer in grams per litre

Example Calculation• 20mL of sample, diluted to 250mL• 20mL aliquots• Concentration of NaOH = 0.100M• Average titre = 12.52• Answer in grams per litre

Back Titrations

•Done by adding a known amount of a third reagent, then determining the amount of excess.

•For example:▫Unknown amount of CaCO3

▫React it with 3 mol of HCl▫Titrate with NaOH▫Equivalence point at 1 mol▫How many mole of CaCO3?

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.

Example Calculation

•The amount of SO2 in polluted air can be determined by passing a measured volume of gas through a solution containing excess KMnO4.▫SO2 + MnO4

- + SO4 + Mn2+ +

•The excess permanganate can then be determined by titration with a standard solution of Fe2+.▫Fe2+ + MnO4

- + Fe3+ + Mn2+ +

Example Calculation

•10.0m3 of polluted air was passed through 100mL of 0.0201M KMnO4. The KMnO4 that was left after the reaction reacted with 26.95mL of 0.158M Fe2+ solution.▫How many mole of MnO4

- was present before the SO2 was bubbled through it?

Example Calculation

▫How many mole of MnO4- reacted with the

Fe2+?

▫How many mole of MnO4- reacted with the

SO2?

Example Calculation

▫Calculate the mass of SO2 in the sample of air.

▫Calculate the concentration of SO2 in grams per cubic metre.

Poor Laboratory Technique

•The book calls it ‘sources of error’ which I disagree with.

•What would happen if:▫You left rinsing water in the

burette?▫There was some water in the

titration flask?▫You chose an indicator that

changed after the equivalence point?

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.

Image Credits• Nuno Nogueira [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-

sa/2.5)], via Wikimedia Commons