Chemistry Reference
In-Depth Information
Unknown compound 2
Calculate the empirical formula of an organic
compound containing 92.3% carbon and 7.7%
hydrogen by mass. The
M
r
of the organic compound
is 78. What is its molecular formula? (
A
r
: H = 1;
C = 12)
This shows that 2 moles of magnesium react with
1 mole of oxygen to give 2 moles of magnesium
oxide.
Using the ideas of moles and masses we can use
this information to calculate the quantities of the
different chemicals involved.
2Mg(
s
)
2 moles
2 × 24
= 48 g
C
H
+
→
O
2
(
g
)
1 mole
1 × (16 × 2)
= 32 g
2MgO(
s
)
2 moles
2 × (24 + 16)
= 80 g
% by mass
92.3
7.7
Masses in 100 g
92.3 g
7.7 g
92.3
12
7.7
1
Number of moles
You will notice that the total mass of reactants is
equal to the total mass of product. This is true for
any chemical reaction and it is known as the
Law
of conservation of mass
. This law was understood
by the Greeks but was fi rst clearly formulated by
Antoine Lavoisier in 1774. Chemists can use this
idea to calculate masses of products formed and
reactants used in chemical processes before they are
carried out.
= 7.7
= 7.7
Ratio of moles
1
1
Empirical formula CH
M
r
of the empirical formula unit CH
= 12 + 1
= 13
Number of empirical formula units present
=
M
r
of compound
M
r
of empirical formula unit
=
78
13
= 6
The molecular formula of the organic compound is
6 × CH = C
6
H
6
. This is a substance called benzene.
Example 1 - using a solid
Lime (calcium oxide, CaO) is used in the
manufacture of mortar. It is manufactured in large
quantities in Europe (see Figure 4.6) by heating
limestone (calcium carbonate, CaCO
3
).
Questions
Use the following values of
A
r
to answer the questions
below: H
=
1; C
=
12; O
=
16; Ca
=
40.
1
Determine the empirical formula of an oxide of calcium
formed when 0.4 g of calcium reacts with 0.16 g of oxygen.
2
Determine the empirical formula of an organic
hydrocarbon compound which contains 80% by mass of
carbon and 20% by mass of hydrogen. If the
M
r
of the
compound is 30, what is its molecular formula?
●
Moles and chemical
equations
When we write a balanced chemical equation we are
indicating the numbers of moles of reactants and
products involved in the chemical reaction. Consider
the reaction between magnesium and oxygen.
magnesium
2Mg(
s
)
+
+
oxygen
O
2
(
g
)
magnesium oxide
2MgO(
s
)
→
→
Figure 4.6
A rotary kiln for burning (calcining) limestone into lime,
located in Moha, Belgium.
