Chemistry Reference
In-Depth Information
4
Stoichiometry - chemical calculations
Relative atomic mass
Reacting masses
Calculating formulae
Finding the formula
Moles and chemical equations
Calculating moles
Moles and elements
Moles and compounds
Moles and gases
Moles and solutions
Checklist
Additional questions
O
2
has a relative formula mass of 2 × 16 = 32
CO
2
has a relative formula mass of 12 + (2 × 16) = 44
So we can now use the equation to answer the
question asked earlier.
C
12
●
Relative atomic mass
There are at present 118 different elements known.
The atoms of these elements differ in mass because
of the different numbers of protons, neutrons and
electrons they contain. The actual mass of one atom
is very small. For example, the mass of a single atom
of sulfur is around:
0.000 000 000 000 000 000 000 053 16 g
This small quantity is not easy to work with so, as you
saw in Chapter 3, a scale called the
relative atomic
mass
scale is used. In this scale an atom of carbon is
given a relative atomic mass,
A
r
, of 12.00. All other
atoms of the other elements are given a relative
atomic mass compared to that of carbon.
+
→
O
2
CO
2
32
44
Converting these relative masses to actual masses by
adding mass units, g, would give:
+
C
O
2
32
→
CO
2
44
12
12 g
32 g
44 g
The above calculation shows that if 12 g of carbon
were burned completely then 44 g of carbon dioxide
gas would be formed. So 6 g of carbon burning
would result in the formation of 22 g of carbon
dioxide gas.
Let us look at another example. What mass of
hydrogen gas would be produced if 46 g of sodium
was reacted with water? First of all write down the
balanced chemical equation:
2Na + 2H
2
O → 2NaOH + H
2
Next find the relative atomic mass of sodium from
the Periodic Table (p. 136) and work out the relative
formula masses of water, sodium hydroxide and
hydrogen gas.
Relative atomic mass of sodium is 23.
Relative formula mass of water, H
2
O, is
(2 × 1) + 16 = 18
Relative formula mass of sodium hydroxide is
23 + 16 + 1 = 40
Relative formula mass of hydrogen gas, H
2
, is 2 × 1 = 2
Now you can write these masses under the balanced
chemical equation taking into account the numbers
used to balance the equation.
+
An H atom is
the mass of a C atom
C
1
12
An Mg atom is twice
the mass of a C atom
H
Mg
S
Ca
1
12
fixed
24
32
40
Figure 4.1
The relative atomic masses of the elements H, C, Mg, S and Ca.
●
Reacting masses
Chemists often need to be able to show the relative
masses of the atoms involved in a chemical process.
For example, what mass of carbon dioxide would be
produced if 6 g of carbon was completely combusted?
C
+
O
2
→ CO
2
Instead of using the actual masses of atoms we use
the relative atomic mass to help us answer this type of
question.
In the example above we can work out the
relative
formula mass
of molecules such as O
2
and CO
2
using the relative atomic masses of the atoms they
are made from. The RFM is the sum of the relative
atomic masses of all those elements shown in the
formula of the substance.
2Na
2 × 23 = 46
2H
2
O
2 × 18 = 36
→
2NaOH
2 × 40 = 80
+
H
2
2
