Chemistry Reference
In-Depth Information
crucible
Unknown compound 1
In another experiment an unknown organic
compound was found to contain 0.12 g of carbon
and 0.02 g of hydrogen. Calculate the empirical
formula of the compound. (
A
r
: H = 1; C = 12)
magnesium
ribbon
pipe clay
triangle
C
H
Masses (g)
0.12
0.02
0.12
12
0.02
1
Number of moles
heat
= 0.01
= 0.02
tripod
Ratio of moles
1
2
Empirical formula
CH
2
From our knowledge of bonding (Chapter 3, p. 46)
we know that a molecule of this formula cannot exist.
However, molecules with the following formulae do
exist: C
2
H
4
, C
3
H
6
, C
4
H
8
and C
5
H
10
. All of these
formulae show the same ratio of carbon atoms to
hydrogen atoms, CH
2
, as our unknown. To find out
which of these formulae is the actual formula for the
unknown organic compound, we need to know the
mass of one mole of the compound.
Using a mass spectrometer, the relative molecular
mass (
M
r
) of this organic compound was found to
be 56. We need to find out the number of empirical
formulae units present:
M
r
of the empirical formula unit
= (1 × 12) + (2 × 1)
= 14
Number of empirical formula units present
=
M
r
of compound
M
r
of empirical formula unit
=
56
14
= 4
Therefore, the actual formula of the unknown
organic compound is 4 × CH
2
= C
4
H
8
.
This substance is called butene. C
4
H
8
is the
molecular formula
for this substance and shows the
actual
numbers of atoms of each element present in
one molecule of the substance.
Sometimes the composition of a compound
is given as a percentage by mass of the elements
present. In cases such as this the procedure shown in
the next example is followed.
Figure 4.5
Apparatus used to determine magnesium oxide's
formula.
Table 4.1
Data from the experiment shown in Figure 4.5.
Mass of crucible
14.63 g
Mass of crucible and magnesium
14.87 g
Mass of crucible and magnesium oxide
15.03 g
Mass of magnesium used
0.24 g
Mass of oxygen which has reacted with the
magnesium
0.16 g
The data shown in Table 4.1 were obtained
from an experiment using the apparatus shown
in Figure 4.5 to find the formula for this white
powder, magnesium oxide. From these data we can
calculate the number of moles of each of the reacting
elements. (
A
r
: O = 16; Mg = 24)
Mg
O
Masses reacting (g)
0.24
0.16
0.24
24
0.16
16
Number of moles
= 0.01
= 0.01
Ratio of moles
1
1
Formula
MgO
This formula is the
empirical formula
of the
compound. It shows the simplest ratio of the atoms
present.
