SECTION 5:
HUMAN ACTIVITY ALSO IMPACTS ON WATERWAYS.
CHEMICAL MONITORING AND MANAGEMENT ASSISTS IN
PROVIDING SAFE WATER FOR HUMAN USE AND TO PROTECT
THE HABITATS OF OTHER ORGANISMS.
Many factors affect water quality. In this section we shall study the following
points:

Concentrations of common ions – AAS is used to measure the concentration of
common ions in water, such as Fe3+, Na+, and F-. Gravimetric analysis (by
precipitating ions out of the sample) can also be used to determine ion
concentration.

Total dissolved solids (TDS) – A conductivity meter can be used to measure the
total dissolved solids because most solutes are ionic. In the lab TDS can be
measured by slowly evaporating a known volume of filtered water. TDS measures
the total salinity of water.

Hardness – water hardness is caused by dissolved calcium and magnesium ions.
Water hardness is measured in mol/L of CaCO3. A titration using EDTA is used to
determine the hardness of a water sample. However, hardness can also be
assessed qualitatively using ‘height of lather head’ technique.

Turbidity – turbidity can be measured by pouring water in to a turbidity tube which
allows turbidity to be measured in NTU. Potable water needs to have a reading
lower than 3 NTU.

Acidity – Acidity can be measured using a pH probe or universal indicator. Potable
water needs to be in the pH range of 6.5-8.5.

Dissolved oxygen (DO) – dissolved oxygen levels are important because aquatic
organism require oxygen to survive. DO can be measured using a calibrated
oxygen sensor electrode, or special tablets that dissolve in water and change
colour depending on the DO level.

Biochemical oxygen demand (BOD) – Biochemical oxygen demand is a measure
of the rate of oxygen use of microscopic organisms. It is also an indirect measure
of organic waste present in the water. One sample is measured for DO as soon as
possible while the other sample is kept in a dark place for 5 days and then tested
for DO. The BOD is calculated by subtracting the DO value after 5 days from the
initial DO value.
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 54
WATER HARDNESS CALCULATIONS
EXAMPLE
Calculate the hardness of a sample of water containing 1.5x10-4 mol/L Mg2+ and
2.0x10-4 Ca2+.
Solution
Step 1:
1.0x10-4 + 2.5x10-4 = 3.5x10-4 mol/L of total hardness.
Step 2:
3.5x10-4 x 100.1 (MM of CaCO3 ) = 3.5x10-2g g/L
Step 3:
3.5x10-2g g/L = 3.5 mg/L = 35 ppm
QUESTION 49
Calculate the hardness of a sample containing 3.8x10-4 mol/L Mg2+ and 0.8x10-4 mol/L Ca2+.
Solution
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 55
QUESTION 50 (5 marks [a:2 marks] [b:3 marks]) (HSC 2002:26)
Water can be described as either ‘hard’ or ‘soft’.
(a)
Describe a test you have used to determine whether a given sample of water is ‘hard’
or ‘soft’.
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(b)
A sample of hard water contains 6 x 10-4 mol L-1 of magnesium carbonate.
Calculate the mass, in mg, of magnesium carbonate in 150 mL of this sample.
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© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 56
QUESTION 51 (4 marks [a:1 mark] [b:2 marks] [c:1 mark]) (HSC 2006:27)
One of the most common methods for determining the concentration of metal ions in water
samples involves titration with a reagent called EDTA. In alkaline solution EDTA is present
as an anion with a 4- charge. In this form it reacts with metal ions such as calcium and
magnesium in a 1 : 1 ratio:
Ca 2  EDTA4  Ca ( EDTA) 2
When the reaction between the metal ions and EDTA4- is complete, and indicator also
present in the solution changes colour.
A student used the following procedure to determine the concentration of calcium in a
sample of water.

50.0 mL of water sample was pipetted into a conical flask.

5.0 mL of ammonia/ammonium ion buffer and two drops of indicator were added.

Sample was titrated with 0.0200 mol L-1 EDTA4- until indicator changed colour.

The above procedure was repeated a further three times.

The average volume of EDTA4- used in the four titrations was 24.0 mL.
(a)
What is the average number of moles of EDTA4- added to reach the end point?
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(b)
The student used the answer to part (a) to calculate the concentration of Ca2+ in the
water sample in mg L-1.
What concentration was obtained?
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(c)
The concentration of Ca2+ in the water sample was also determined by atomic
absorption spectroscopy and found to be 16% lower than the value obtained by titration
with EDTA4-.
Suggest a reason why the concentration of Ca2+ determined by EDTA titration was
higher.
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© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 57
QUESTION 52 (HSC 2005:15)
The Winkler method is used to determine the amount of dissolved oxygen in a sample. In
this procedure, oxygen reacts with Mn2+ under the alkaline conditions to produce a
precipitate of MnO (OH ) 2 .
2 Mn(2aq )  O2( aq )  4OH (aq )  2 MnO (OH ) 2( s )
The precipitate is then dissolved in acid and reacted with iodide, forming iodine and Mn2+.
MnO (OH ) 2( s )  2 I (aq )  4 H (aq )  I 2( aq )  Mn(2aq )  3H 2O( aq )
Finally, the amount of iodine produced is determined by reaction with thiosulfate.
I 2( aq )  2 S 2O3(2aq )  2 I (aq )  S 4O6(2aq )
When a sample of water was analysed using the Winkler method, a total of 0.60 mol of
thiosulfate was used in the reaction.
How many moles of oxygen were present in the original sample?
A
B
C
D
0.15
0.30
0.60
1.20
QUESTION 53 (5 marks) (HSC 2010:25)
What is the relationship between dissolved oxygen and biochemical oxygen demand and
why is it important to monitor both in natural waterways?
Solution
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 58

Identify factors that affect the concentrations of a range of ions in solution in
natural bodies of water such as rivers and oceans.

Agricultural run-off (eg. increasing phosphate and nitrates).

Industrial wastes and runoff (eg. releasing heavy metals).

Sewage discharge into water (eg. increasing phosphates and nitrates).

Rainfall in (increasing minerals washed from soils and rocks, can become very
significant in land clearing/building/mining).

pH of rain – acidic rain is able to better leach certain ions from soils.
EXAMPLE (2009 HSC Q25) (7 marks)
An analytical chemist determined the phosphate concentration of water samples from three
local streams.
(a)
Using the absorbance values in the table and graph, determine the mean absorbance
and mean phosphate concentration for each stream and complete the table.
Stream
Absorbances measured
1
0.090, 0.092, 0.088
2
0.513, 0.511, 0.514
3
0.234, 0.237, 0.234
© The School For Excellence 2011
Mean absorbance
Mean phosphate
concentration (mg/L)
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 59
(b)
The recommended maximum level of phosphate in streams is 0.100 mg/L
With reference to the recommended level of phosphate for stream water, explain why
there are differences between the three streams. (3 marks)
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(a)
Why is phosphate concentration a water quality issue? (2 marks)
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© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 60

Monitor possible eutrophication of waterways.
Eutrophication is when the presence of nutrients enriches waterways so that aquatic
plants grow abundantly. Eutrophication can lead to algal blooms. To monitor
eutrophication the concentrations of nitrogen and phosphorus needs to be determined.
To avoid eutrophication nutrients must be diverted from waterways or absorbed by
plants before they reach waterways. Also, fertilisers should not be applied before heavy
rain.
QUESTION 54 (4 marks) (HSC 2003:26)
Describe the process of eutrophication and assess the suitability of water quality tests used
to monitor it.
Solution
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 61

Describe and assess the effectiveness of methods used to purify and sanitise
mass water supplies.
Water is purified and sanitised in three steps: screening, flocculation, filtration and
chlorination.
1.
Flocculation is the addition of a flocculent such as Iron chloride to coagulate
suspended particulates allowing them to settle in tanks.
2.
Filtration is then performed through sand and anthracite (a type of coal) filters to
remove all undissolved particulates.
3.
Water is then chlorinated to kill any microorganisms. NB It is Cl2 that is added not
Cl-. Also, some authorities add Fluoride (F- not F2!)
The water treatment is very effective but not perfect. Chlorination is highly. An example
of failure, however, was when Cryptosporidium and Giardia entered Sydney’s drinking
water in the late 1990’s. Careful, regular monitoring is essential.
QUESTION 55 (HSC 2001:11)
Why is chlorine used to treat local water supplies?
A
B
C
D
To make water suitable for swimming.
To kill micro-organisms living in the water.
To promote sedimentation of finely suspended solids.
To precipitate heavy metal ions such as lead and mercury.

Describe the design and composition of microscopic membrane filters and
explain how they purify contaminated water.
A membrane filter is a thin film of a synthetic polymer through which there are
microscopic pores. With the aid of high pressure on the ‘unclean’ side of the
membrane, water can be forced through the membrane leaving impurities behind.
This process is thus termed ‘reverse osmosis’ as it involves water moving high to low
concentration (as opposed to low to high concentration as is the definition of osmosis).
These membranes can be used in desalination.
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 62
QUESTION 56 (HSC 2001:14)
Which diagram represents the most effective design for a microscopic membrane filter to
purify contaminated water?
QUESTION 57 (HSC 2006:13)
Why are microscopic membrane filters useful for water purification?
A
B
C
D
They can kill bacteria.
They adjust the pH of water to 7.
They are composed of biodegradable polymers.
They can remove very small particles from water.
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 63
QUESTION 58 (7 marks) (HSC 2005:26)
The map shows the catchment for a town water supply.
Describe TWO possible sources of contamination in this catchment, and assess methods
that could be used for purifying the water before it reaches the town.
Solution
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 64
OTHER WAYS OF EXPRESSING CONCENTRATIONS
This section is added to these notes as expressing concentration in units such as %w/v,
5%w/w and %v/v is an expectation of the NSW HSC course. However, it is not clearly noted
in the syllabus and is not covered effectively. Hopefully these examples may fill the gap.
PERCENTAGES
Percentage by mass (w/w) describes the mass of solute (g) in 100 g of solution.
For example:
0.8% w/w indicates that there are 0.8 g of solute dissolved in 100 g
of solution.
Concentrat ion ( w / w) 
mass of solute ( g )
 100
mass of solution ( g )
QUESTION 59
Twenty grams of a salt solution contains 4.0 grams of salt. What is the concentration (w/w)
of salt in this solution?
Solution
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 65
QUESTION 60
How much salt is required to prepare 500 g of a 10% w/w salt solution?
Solution
Percentage by volume (v/v) describes the volume of solute (ml) in 100 ml of solution.
For example:
11% alcohol v/v indicates that there are 11 ml of alcohol dissolved in
100 ml of solution.
Concentrat ion (v / v ) 
volume of solute (ml )
 100
volume of solution (ml )
QUESTION 61
A 170 ml glass of fruit drink contains 15% (v/v) of pure orange juice. What volume of pure
orange juice is present in this solution?
Solution
Percentage mass per volume (w/v) describes the mass of solute (g) dissolved in
100 ml of solution.
For example:
2% (w/v) indicates that there are 2% of solute dissolved in 100 ml
of solution.
Concentrat ion ( w / v) 
mass of solute ( g )
 100
volume of solution (ml )
QUESTION 62
The concentration of Mg2+ in water is 10% (w/v). Calculate the mass of Mg2+ in 2.00 L.
Solution
© The School For Excellence 2011
Trial Exam Preparation Lectures – Chemistry – Book 3
Page 66