1. Volumetric Analysis
Chapter 13

2. Learning Objectives

3. Learning Objectives

4. Concentrations of solutions
A solution is a mixture of a solute and a solvent
In a homogeneous solution the solute is uniformly (evenly) distributed throughout the solvent
The concentration of a solution is the amount solute that is dissolved in a given volume of solution
There are three main ways of expressing the concentration of a solution.
Percentages
Parts per million (p.p.m.)
Molarity

5. The darker the colour of the solution the more concentrated it is (more solute present).

6. 1. Percentage of solute
Used for many household substances as well as in the areas of biochemistry and medicine
Three different ways in which the % of solute in a solution is expressed:
Weight per weight (w/w)
Weight per volume (w/v)
Volume per volume (v/v)

7. (i) Weight per weight (w/w)
A solution labeled 10% w/w NaCl = 10g of NaCl in 100g of solution This solution is made by weighing out 10g of NaCl, dissolving it in water and adding water until the solution weights 100g

8. (ii) Weight per volume (w/v)
A solution labeled 10% w/v NaCl = 10g of NaCl in 100cm3 of solution This solution is made by weighing out 10g of NaCl, dissolving it in water and adding water until the total volume of the solution is 100cm3

9. (iii) Volume per volume (v/v)
This type of solution is made of two liquids A solution labeled 10% v/v ethanol = 10cm3 of ethanol in 100cm3 of solution This solution is made by measuring out 10cm3 of ethanoland adding water until the total volume of the solution is 100cm3

10. Questions:
A salt solution that is administered by a doctor is marked 0.85% w/v. What mass of sodium chloride is needed to make up 250cm3 of the solution?

11. Question
A sample of sea water has a mass of 300g. On evaporating the water, 50g of salt were recovered from it. Express the concentration of the salt as % w/w.

12. 2. Part per million
Only used for very dilute solutions
Used in water analysis
1ppm = 1mg/L
E.g.
A 100cm3 sample of water contains 0.005g of calcium carbonate. What is the concentration of the calcium carbonate in ppm?

14. Convert the following concentrations to ppm
0.75 g/L
0.06 g/L
0.017 g/100cm3
g/100cm3

15. 3. Moles per Litre (Molarity)
The molarity of a solution is the number of moles of solute per litre of solution
Often expressed as M or mol/L

16. 1000cm3
500cm3
500cm3
1 mole in 1 L
1 mol/L 1M
1 mole in 500cm3
2 mol/L 2M
2 mole in 500cm3
4 mol/L 4M

17. Convert to moles per L 2 moles in 250cm3 3 moles in 2L

18. Molarity to number of moles
How many moles of a 2M HCL solution in 300cm3?
How many moles of a 1.5M solution in 270cm3?
How many moles of a 0.1M solution in 25cm3?
How many moles of a 0.5M solution in 50cm3?

19. Calculations involving solutions
A 2M solution of NaOH contains 2 moles of NaOH in 1 Litre of solution
What mass of NaOH would this solution contain?

20. (a) Mol/L (Molarity)  g/L
How many grams of HCl per litre are present in a solution marked 2M HCl? (H = 1, Cl – 35.5) How many grams of NaCl per litre are present in a solution marked 0.25M NaCl?

21. Calculate the concentration in grams per litre of bench dilute sulfuric acid whose concentration is 1.5 mol/L

22. (b) g/L  Mol/L (Molarity)
What is the concentration of a solution in mol/L that contains 3.68g of NaOH per litre of solution? Calculate the concentration in moles per litre of a solution containing 45g of sulfuric acid per 250cm3 of solution.

23. (c) Calculating the number of moles given the molarity and the volume
How many moles of NaOH present in 25cm3 of 0.55M NaOH
Number of moles = volume (in cm3) x molarity
1000

24. Reactions between a solution and a solid
What mass of magnesium will react with 50cm3 of 0.5M H2SO4? The balanced equation for the reaction is: Mg + H2SO4  MgSO4 + H2

25. Dilutions
What volume of 12M HCl solution is required to make up 500cm3 of 3M HCl solution?

26. Titrations

27. Standard Solutions
A standard solution is a solution whose concentration is accurately known.
A Primary Standard is a substance which meets the following criteria:
Is available in a highly pure state
Is stable in air (e.g. not lose or gain water from the air)
Must dissolve easily in water
Should have a fairly high rmm (so that it can be weighted out as accurately as possible)
When used in volumetric analysis it should undergo complete and rapid reaction
It must not be hydrated (have no water of crystallisation)

28. A Primary Standard
A substance that can be obtained in a stable, pure and soluble solid form so that it can be weighed out and dissolved in water to give a solution of accurately known concentration

29. Titrations
Titration is a lab procedure where a measured volume of one solution is added to a known volume of another solution until the reaction is complete.
Following equipment is needed:
Volumetric flask
Burette
Pipette
Conical Flask
Graduated cylinder

30. Volumetric Flask Contains a definite volume of solution.
Calibration mark on narrow neck ensures accuracy.
Rinsed with deionised water only before use not the solution it is to contain.
Flask is filled with deionised water until the bottom of the meniscus is on the calibration mark.
Contents must be mixed by stoppering flask and inverting 20 times (ensures homogeneous mixture).

31. Burette
Vertical to allow meniscus to be properly lined up with graduation mark.
Read meniscus at eye level and make sure no air bubbles.
Rinse first with deionised water (to remove impurities) and then with the solution it is to contain (this removes any water which would dilute the solution).
The tip of the burette must be full as the graduation on the burette includes this space.
Remove the funnel before adjusting to zero, so that no drops can fall into the burette.

32. Pipette Rinse with deionised water (to remove impurities)
Then with the solution it is to contain.
Use a pipette filler, read meniscus at eye-level and make sure the bottom of the meniscus is on the graduation mark.
Make sure there are no air bubbles trapped and hold pipette vertically.
Take solution from beaker not volumetric flask. Impurities in the pipette might contaminate the solution, or drops of water might dilute it.
Touch tip of pipette against side of conical flask to release last drop. Another drop will remain but should not be blown out.

33. Conical Flask
Shaped so as to prevent spillage when it is being swirled.
Rinse with deionised water only.
It should not be rinsed with the solution as remaining drops will mean that the total volume in the flask will be unknown.

34. Graduated Cylinder
Not very accurate and is only used for measuring out approximate volumes of liquids (neck is too wide).

35. Equipment Summary Name Usually holds Washed with Precaution
Volumetric flask
Acid or base
Deionised water
Fill to bottom of meniscus on grad. Mark.
Lid on and invert 20 times.
Burette
Acid (Base blocks tap)
Deionised water & then solution it is to contain.
Remove funnel
Space below tap must be full.
Pipette
Base
Use pipette filler, no air bubbles.
Don’t force out extra liquid at end.
Conical flask
Sides can be rinsed down during titration.
Graduated cylinder
Either acid or base
Not very accurate
Used for approx. amounts.

36. Acid-Base Titrations
Using a standard solution of sodium carbonate to standardise hydrochloric acid solution.
Using a standard solution of hydrochloric acid to standardise sodium hydroxide.
To determine the concentration of ethanoic acid in vinegar.
To determine the amount of water of crystallisation in hydrated sodium carbonate.

37. Preparing a Standard Solution of Sodium Carbonate.
*Common exam q: describe how sodium carbonate was transferred into volumetric flask and made up to 250cm3 Transfer: 1. Accurately weigh on a clock glass 2. Wash (rinse) into beaker of deionised (distilled, pure) water AND stir to dissolve MADE-UP: 3. Pour through funnel (down glass rod) into volumetric flask adding rinsings of beaker 4. Add last few drops of deionised water drop by drop (using dropper) 5. Bring bottom of meniscus level with (up to, on, at) mark 6. Read at eye level 7. Stopper and invert
Learn these steps!

38. Preparing a Standard Solution of Sodium Carbonate.

39. Equation: Na2CO3 + 2HCl → NaCl + H2O + CO2
Titration 1: Using a Standard solution of sodium carbonate to standardise hydrochloric acid solution.
Equation: Na2CO HCl → NaCl + H2O + CO2
Burette: Dilute solution of acid of unknown molarity
Conical: 25cm3 of standardised sodium carbonate
Indicator: Methyl orange
Colour change: yellow – red/pink
Acid is usually in the burette as common bases stick to or attacks glass and can block the tip of the burette.

40. Calculation: nA nB MA = molarity of acid MB = molarity of base
Titration 1
Calculation:
MA x VA = MB x VB
nA nB
MA = molarity of acid MB = molarity of base
VA = volume needed to react VB = volume needed to react
nA = ratio of moles reacting nB = ratio of moles reacting

41. Titration 1
Example:
Sodium carbonate reacts with 26cm3 hydrochloric acid according to the equation:
Na2CO HCl → 2NaCl H2O + CO2
What Molarity of HCl will react with 25cm3 of 0.05M Na2CO3 ?
Now convert it to grams/litre.

42. Titration 1

43. Titration 1

44. Titration 1

45. Titration 1
1. A student determined the concentration of a hydrochloric acid solution by titration with 25.0 cm3
portions of a 0.05 M primary standard solution of anhydrous sodium carbonate. The portions of
sodium carbonate solution were measured into a conical flask using a 25 cm3 pipette. The
hydrochloric acid solution was added from a burette. The mean titre was 20.8 cm3.
The balanced equation for the titration reaction was:
2HCl + Na2CO3 → 2NaCl + H2O + CO2
(a) Explain the underlined term (5)
(b) Describe how the student should have prepared 500 cm3 of the 0.05 M primary standard solution from a known mass of pure anhydrous sodium carbonate, supplied on a clock glass. (12)
Calculate the exact mass of anhydrous sodium carbonate (Na2CO3) required to prepare this
solution (6)
(c) (i) Describe how the liquid level in the burette was adjusted to the zero mark.
(ii) Why was a pipette filler used to fill the pipette with 25.0 cm3 of the sodium carbonate
solution? (6)
(d) Name a suitable indicator for this titration.
State the colour change observed at the end point (9)
(e) Calculate, correct to two decimal places, the concentration of the hydrochloric acid solution in
(i) moles per litre,
(ii) grams per litre (12)

46. Titration 1

47. Titration 2: Using a Standard solution of hydrochloric acid to standardise sodium hydroxide.
Equation: NaOH + HCl → NaCl + H2O
Burette: Dilute solution of acid of known molarity from last experiment.
Conical: 25cm3 of sodium hydroxide of unknown molarity
Indicator: Methyl orange
Colour change: yellow - pink
Calculation: Calculate the Molarity and then g/l of the NaOH.

48. Titration 2
Example:
Sodium hydroxide reacts with 0.25M hydrochloric acid according to the equation:
NaOH HCl → NaCl H2O
The volume of NaOH is 18cm3. The average titre of 0.25M HCl is 27cm3. Calculate the Molarity (moles/litre) of NaOH
Now convert it to grams/litre.

49. Titration 2

50. Titration 2

51. Titration 2

52. Percentage Purity of a chemical
Titration 2
Percentage Purity of a chemical
We can use a titration to find out how pure a certain chemical is.
Percentage purity =
Mass of pure substance x
Mass of impure substance

53. Titration 2
Example:
A student was asked to determine the percentage purity of a sample of sodium hydroxide.
10g of the impure sample were dissolved in water and made up to 1000cm3 in a volumetric flask.
25 cm3 portions of the base were titrated against 0.15M H2SO4 solution. The average titration figure was 16.75cm3.
The equation for the reaction is:
H2SO NaOH → Na2SO H2O

54. Titration 2

55. Titration 2
Example:
A student was asked to determine the percentage purity of a sample of potassium hydroxide.
4.3g of the impure sample were dissolved in water and made up to 250cm3 in a volumetric flask.
25 cm3 portions of the base were titrated against 0.125M HCl solution.
The average titration figure was 21.2cm3.
The equation for the reaction is:
HCl KOH → KCl H2O

56. Titration 2

57. Titration 2
Example:
A student was asked to determine the percentage purity of a sample of potassium hydroxide.
2.2g of the impure sample were dissolved in water and made up to 250cm3 in a volumetric flask.
25 cm3 portions of the base were titrated against 0.125M HCl solution.
The average titration figure was 19.2cm3.
The equation for the reaction is:
HCl KOH → KCl H2O

58. Titration 2

59. Titration 3: To determine the concentration of ethanoic acid in vinegar.
NaOH + CH3COOH → CH3COONa + H2O
CH3COOH – ethanoic acid is found in vinegar and it is a weak acid.
Different indicator will be needed: Phenolphthalein which is pink in base and colourless when neutralised.
The concentration of vinegar is always expressed as %w/v (The number of grams of solute per 100cm3 of solution.)

60. Titration 3
Vinegar:
Concentration of vinegar will be too strong compared with the standardised NaOH so it must be diluted first.
If it was not diluted only a few drops of vinegar would neutralise the base so it would not give accurate titration.
When the vinegar is diluted the dilution factor must be noted and used in calculation later.

61. Titration 3
Vinegar Questions
A 20cm3 sample of vinegar was diluted to 200cm3 in a volumetric flask. This diluted solution was then titrated against 25cm3 of 0.15M NaOH solution. The balanced equation for the reaction is:
CH3COOH NaOH → CH3COONa + H2O
The average titration figure was 15.8cm3.
Calculate:
the concentration of dilute ethanoic acid in mol/l
the concentration of conc. ethanoic acid in mol/l
Work out the g/l of conc vinegar and the %w/v

62. Titration 3

63. Titration 3
Vinegar Questions
A 25cm3 sample of vinegar was diluted to 500cm3 in a volumetric flask. This diluted solution was then titrated against 25cm3 of 0.112M NaOH solution. The balanced equation for the reaction is:
CH3COOH NaOH → CH3COONa + H2O
The average titration figure was 16.2cm3.
Calculate the concentration of original ethanoic acid in moles/l, g/l and % w/v.

64. Titration 3

65. Titration 3

66. Titration 3

67. Titration 4
Titration 4: To determine the amount of water of crystallisation in hydrated sodium carbonate.
Na2CO HCl → 2NaCl H2O CO2
Source of Na2CO3 is hydrated crystals – formula: Na2CO3. xH2O
In order to find the amount of water in the crystals and hence to value for x , Na2CO3 is titrated against HCl as usual and the amount of Na2CO3 is calculated in moles and grams.
The indicator used is methyl orange as the titration is a strong acid against a strong base. Colour change yellow – pink.

68. Experiment Calculation:
Titration 4
Experiment Calculation:
To find the mass of water in the hydrated crystal the mass of Na2CO3 is subtracted from the original mass of the hydrated crystal.
The percentage of water of crystallisation:
Mass of water x
Mass of hydrated crystal
To calculate the value of x the formula:
Moles = Mass (gr)
Relative Molecular Mass

69. and the value of x in the above formula.
Titration 4
A student was asked to analyse a sample of washing soda, Na2CO3.xH2O, in order to find the percentage of water in the sample.
The student weighs out 25g of the washing soda and dissolves this amount in water and makes it up to 1000cm3. 25cm3 of this solution required an average of 27.8cm3 of 0.21M HCl for neutralisation.
Calculate the percentage of water of crystallisation in the washing soda crystals
and the value of x in the above formula.
Na2CO HCl → 2NaCl H2O CO2

70. Titration 4

71. and the value of x in the above formula.
Titration 4
Na2CO HCl → 2NaCl H2O CO2
A student was asked to analyse a sample of washing soda, Na2CO3.xH2O, in order to find the percentage of water in the sample.
The student weighs out 8.61g of the washing soda and dissolves this amount in water and makes it up to 250cm3. 25cm3 of this solution required an average of 26 cm3 of 0.2M HCl for neutralisation.
Calculate the percentage of water of crystallisation in the washing soda crystals
and the value of x in the above formula.

72. Titration 4

73. Titration 4
2006 Question 1 Higher

74. Titration 4

75. Titration 4

76. Titration 4

77. Titration 4
2014 Question 1 Higher

78. Titration 4

79. Procedural Steps to Know:
Common exam q
Procedural Steps to Know:
Steps for making solution from solid/crystals/powder
e.g: dissolving and making up to 250cm3
wash (rinse) into beaker of deionised (distilled, pure) water
AND stir to dissolve
MADE-UP: pour through funnel (down glass rod) into volumetric flask adding rinsings of beaker
add last few drops of deionised water drop by drop (using dropper) to bring bottom of meniscus level with (up to, on, at) mark reading at eye level
stopper and invert

80. Procedural Steps to Know:
Common exam q
Procedural Steps to Know:
Steps for measuring solution using pipette
e.g: transferring 25cm3 of solution to conical flask:
pour some solution into a clean, dry beaker
use pipette
previously rinsed with deionised (distilled) water
rinsed with solution it is to contain
fill using pipette filler until bottom of meniscus is on mark
read at eye-level
allow drainage time /last drop to remain (not to be shaken out, blown out) / drain under gravity / touch (tip, tap) pipette against wall of conical flask

81. Procedural Steps to Know:
Common exam q
Procedural Steps to Know:
Steps for dilution
pour some solution into a clean, dry beaker
pipette into 500 cm3 volumetric flask
add deionised (distilled, pure) water until near mark
add dropwise (by dropper, by wash bottle, other suitable method)
until bottom of meniscus up to (on, at) mark / flask at eye- level (vertical)
stopper and invert several times / mix thoroughly / solution homogeneous (even concentration, same concentration throughout)

82. Procedural Steps to Know:
Common exam q
Procedural Steps to Know:
Preparing the burette:
rinse with deionised (distilled) water
rinse with a little of the solution
add solution to burette and clamp vertically / ensure burette is vertical
use a funnel to fill / remove funnel before adjusting to mark / mark at eye-level
open tap to fill jet (part below tap)
use tap (or dropper) to set bottom of meniscus on zero mark

83. Procedural Steps to Know:
Common exam q
Procedural Steps to Know:
During Titration:
swirl conical flask to mix contents
allow time after addition from burette for reaction to occur
wash down sides of conical flask with deionised water
use white tile

84. Chapter Checklist: Definition of Solution
Concentration using percentages
Concentration using p.p.m
Concentration using Molarity
Definition of standard solution
Definition of primary standard
Definition of titration
Titration equipment
Volumetric flask
Burette
Conical
Pipette
Graduated cylinder

85. Chapter Checklist:
Formula for finding moles using molarity and volume.
Formula for calculating molarity in titration experiment.
Indicators used and colour changes
Calculating the percentage purity
Vinegar titration – diluted (reason why)
Calculating the %w/v of vinegar
Calculating the % of water in Na2CO3
Calculating the value of x in Na2CO3xH2O