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Chemistry (AQA)
Quantitative Chemistry
Relative Formula Mass
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Cards (6)
Compounds have a
relative formula mass
, Mr
Example
Find the relative formula mass of MgCl2
1)Look up the relative atomic masses of all the elements in one compound on the periodic table
> Ar of Mg =
24
and the Ar of Cl =
35.5
2) Add up all the relative atomic masses of the atoms in the compound
> Mg + (2 x Cl) =
24
+ (2 x 35.5) = 95. So Mr of MgCl2 =
95
You can find the relative
atomic mass
(Ar) of an element from the
periodic
table - it's the same as its
mass number
(Ar x number of atoms of that
element
/ Mr of the compound) x 100 =
Percentage mass of an
element
in a compound
Example
Find the percentage mass of sodium in sodium carbonate, Na2CO3
> Ar of sodium = 23, Ar of carbon =
12
, Ar of oxygen = 16
> Mr of Na2CO3 = (2 x 23) +
12
+ (3 x
16
) =
106
> Percentage mass of sodium = (
23
x 2) /
106
x
100
=
43
%
Example
A mixture contains 20% iron ions by mass. What mass of iron chloride (FeCl2) would you need to provide the iron ions in 50 g of the mixture?
1)Find the mass of iron in the mixture
> The mixture contains
20%
iron by mass, so in 50 g there will be
50
x (
20
/ 100) =
10
g of iron
2
) Calculate the % mass of iron in iron chloride
> % mass of iron =
56
/ [
56
+ (2 x
35.5
)] x
100
=
44.09...
%
3) Calculate the mass of iron chloride that contains 10 g of iron
> Iron chloride contains
44.09%
iron by mass, so there will be 10 g of iron in 10 / (44.09... / 100) =
23
g
= So you need
23
g
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