Standard Grade Study Plan For Chemistry
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sundayherald
1 April 2007
Answer: Alloy
From page 17
mass (g) so 49g of H
= 49(g) = 0.5 moles
SO
2
4
(b) A 9-carat gold ring weighs 7.88g
g f m
98(g)
Calculate the number of moles of gold in
STANDARD GRADE STUDY PLAN
the ring.
e) Calculating the mass of a number of moles:
Show your working clearly.
Group
Times studied
number of moles x g f m
Answer: Mass of gold in ring = 37.5% of 7.88g
1
2 3 4 5 6 7 8 9
= 37.5
so 0.1 moles of H
SO
= 0.1 x 98g = 9.8g
x 7.88g = 2.96g
2
4
Reaction rates
100
Mass of 1 mole of gold = 197g
f) Calculating percentage composition
Periodic Table
Therefore, number of moles
Bonding (covalent, ionic)
% composition =
of gold in ring
= 2.96g
Hydrocarbons (including synthetic polymers)
197g
=0.015moles
Acids & alkalis
mass of element in compound(g)
x 100
Metals & corrosion
gram formula mass (g)
g) Calculating mass of a product or reactant,
Cells
so the percentage by mass of S in H 2 SO 4 =
knowing the balanced equation, for example:
Fertilisers
Cardohydrates
32(g) x 100 = 32.7%
Calculate the mass of magnesium chloride
produced when 3g of magnesium react with excess
98(g)
Calculations
hydrochloric acid according to the equation.
Many calculations are included under Knowledge
Sometimes the question will not be straightfor-
and Understanding because the method for
ward. For example, in the following question the
Mg + 2HCl
MgCl
+ H
2
2
carrying out the calculation should be learned
candidate has to realise that the pie chart shows
1 mole
1 mole
and practised.
that gold forms 37.5% of the total mass of the alloy.
24.3g
24.3 + (2 x 35.5)g
Chemists use the concept of the “mole” in
= 95.3g
almost all calculations. You should therefore
Standard Grade, 1997, Credit paper, Question 11
1g
95.3g
become familiar with the following procedures:
24.3g
3g
95.3g x 3
Pure gold is known as 24-carat gold. In
a) Writing formulae (using valencies)
24.3g
jewellery, it is often mixed with other metals
= 11.8g
to make it harder.
b) Using Relative Formula Masses from the data
One common mixture is 9-carat gold. Its
book to calculate formula masses. Formula mass
Note: The balanced equation tells us the
composition by mass is shown in the pie-chart.
is the sum of the RAMs of all of the atoms in the
relationship between the number of moles of
formula eg the formula mass for H
SO
is:
reactant and the number of moles of product. In
2
4
gold
the above example, one mole of magnesium gives
(1 x 2) + 32 + (16 x 4) = 98 amu
one mole of magnesium chloride. Only these two
silver
31%
31.5%
substances need be considered. Since the masses
copper
c) Finding one mole (the gram formula mass).
of both compounds are involved in the question,
This is the formula mass in grams, eg:
both numbers of moles are changed into
37.5%
grammes and then a proportion calculation is
One mole of H
SO
is 98g
carried out.
2
4
d) Finding the number of moles in a mass of a
(a)What name is given to a mixture of metals
h) calculating the number of moles in a solution
= C x V (litres)
substance:
such as 9-carat gold?
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