Chapter 2 Analysis by mass
Answers-Key questions on page 12
Q1.
a
b
Why was the soup sample in Worked Example 2.1 heated to 110°C?
Why was it necessary to weigh the sample four times?
A1.
a
b
The soup was heated above 100°C to evaporate water from the sample.
By repeatedly heating the sample until the mass remained unchanged, the analyst could
be sure that all the water had been removed.
Q2.
Some laboratories use microwave ovens in place of conventional ovens to dry samples. What
advantage could this have?
A2.
Microwave ovens dry samples more rapidly than conventional ovens.
Q3.
Soy sauce weighing 74.6 g was heated in an oven to constant mass. The final mass was 14.2
g. What percentage of water did the sauce contain?
A3.
Mass water = 74.6 – 14.2 = 60.4 g
mass of water
100
60.4 100
%Water =
×
=
×
= 80.97 = 81.0%
1
74.6
1
mass of sample
(three significant figures)
Q4.
Three brands of dog food were heated and dried to constant mass. The data recorded are
shown in Table 2.2.
Table 2.2 Determination of water content of dog food.
Dog food tested
Phydeaux Deluxe
K9 Budget
Fresh meat – buffalo mince
a
b
Mass of dog food sample (g)
19.8
7.4
15.0
Final mass (g)
3.9
1.9
3.8
Which brand contained the highest percentage of water?
Do you consider that water content is a good guide to the relative value of different dog
foods? What other factors might be important?
A4.
a
b
19.8  3.9 100
×
= 80%
19.8
1
7.4  1.9 100
K9 Budget
% water =
×
= 74%
7 .4
1
15.0  3.8 100
Fresh meat – buffalo mince % water =
×
= 75%
15.0
1
Water would not be a good guide to the nutritional value of the dog food. The amount of
protein, carbohydrates, fats vitamins and minerals need to be considered.
Phydeaux Deluxe
% water =
Q5.
A student determined the water content of a sample of jam. The following measurements
were obtained:
Mass of evaporating dish:
20.22 g
Mass of jam and evaporating dish
before heating:
30.95 g
after heating:
27.22 g
after more heating:
26.50 g
after more heating:
26.49 g
What was the percentage, by mass, of water in the jam?
A5.
Step 1 Find the mass of the moist jam by subtracting mass of evaporating dish.
Mass of moist jam
= 30.95 – 20.22 g
= 10.73 g
Step 2 Find the mass of water.
Mass of water = 30.95 – 26.49 g
= 4.46 g
Step 3 Find the percentage of water in moist jam.
4.46
% (H2O) =
 100
10.73
= 41.57%
Step 4 Express the answer with the correct number of significant figures.
% (H2O) = 41.6%