Cambridge International AS & A Level Marine Science Workbook

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Exercises
Exercise 1.1 Comparing the ionic composition of seawater by
calculating percentages and drawing bar charts
This exercise will help you to compare the solute composition of seawater and represent this as a
bar chart.
Seawater has many different solutes dissolved in it. The proportions and masses of each of these
solutes can differ in different bodies of water. Table 1.1 shows the masses of solutes found in 1 dm3of
seawater taken from three different bodies of water. Notice these two important features of the data:
• The different seawaters have different overall solute concentrations as the total amount of solute
varies.
• The proportions of each individual solute are different.
Solute
Mass of solute dissolved in 1 dm3
/ mg
Atlantic Ocean Mediterranean Sea Sea off coast of
Kuwait
chloride 18 980 21 200 23 000
sodium 10 556 11 800 15 850
sulfate 2 649 2 950 3 200
magnesium 1 262 1 403 1 765
calcium 400 423 500
potassium 380 463 460
bicarbonate 140 143 142
others 98 229 92
Total 34 465 38 611 45 009
Table 1.1: The solute composition of different bodies of seawater.
If you want to compare the proportions of the solutes of different samples of seawater, you could
directly compare the masses of the solutes. This is valid if the overall volumes of the samples are the
same, but not a fair comparison if they are different. The mass of chloride ions dissolved in one cubic
decimetre of seawater will obviously be higher than in one cubic centimetre, simply because there is
more seawater. Also, the overall concentrations (salinities) of the seawaters are different and we are
trying to compare the proportion of total solutes that each individual solute takes up. A more valid
way of comparing the proportions of the solutes would be to calculate the percentage of each solute
compared to the total solutes.
To calculate a percentage, divide the number by the total and then multiply by one hundred.
For example, the total mass of solutes dissolved in Atlantic Ocean seawater in Table 1.1 is 34 465 mg.
To calculate the percentage of dissolved solute that consists of chloride ions, divide the mass of
chloride by the total mass and then multiply by 100:
percentage of chloride ions=1898034465×100%=55.1%
1 a Calculate the percentages of all the solutes for the three different bodies of water shown in
Table 1.1. Copy and complete Table 1.2.
b Compare and contrast the percentages of the solutes by identifying clear similarities and
differences.
Solute
Percentage of each solute / %
Atlantic Ocean Mediterranean Sea Sea off coast of
Kuwait
chloride 55.1
sodium
sulfate
magnesium

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