Empirical Formulas
The compound disilicon hexafluoride has the formula Si2F6. The percent composition would be
calculated from the atomic masses (atomic weights) as follows:
Si 28.092 = 56.18
56.18 100%
170.2
= 33.01 % Si
F 19.006 = 114.00
114.0 100%
170.2
= 66.99 % F
170.18 molecular mass (molecular weight)
To obtain the formula from the percent composition one can convert masses to moles and find the
mole ratio. If masses are not given, it is convenient to (arbitrarily) start with 100. g of compound
to obtain elemental masses. So in this case 100.0 g would contain 33.01 g Si and 66.99 g F.
Convert these masses to moles and scale to simple whole numbers.
33.01 g Si
1 mol Si
28.09 g Si
= 1.175
1.175
1.175
= 1 Divide both numbers by the smallest number.
66.99 g F
1 mol F
19.00 g F
= 3.526
3.526
1.175
= 3.00 Si1F3 SiF3
SiF3 is the empirical formula. The empirical formula is the molecular formula reduced to the
simplest whole number ratio of atoms. It is not possible to get back to the molecular formula,
Si2F6, from percent composition without further information.
A silicon bromide compound is 11.65% Si and 88.35% Br. Calculate the empirical formula.
11.65 g Si
1 mol Si
28.09 g Si
= 0.415
0.415
0.415
= 1 3 3
88.35 g Br
1 mol Br
79.90 g Br
= 1.106
1.106
0.415
= 2.667 3 8.00 Si3Br8
Notice that the division by the smallest number (0.415) did not give whole numbers in this case, so
a further step is required. 2.667 cannot be rounded to 3.0 to give the compound SiBr3, because
SiBr3 will not agree with the percent composition . ( SiBr3 is 10.5% Si and 89.5%Br) Since 2.667
= 22/3 it is appropriate to multiply both numbers by 3. If you are not certain what to multiply by,
try multiplying by integers 2, 3, 4, 5… until both subscripts reach integer values or nearly integer
values—say within 0.05. If the final numbers seem unusually large, double-check all
calculations. If the numbers came directly from an experiment, it may be necessary to try the
experiment again to get better data.
