Molecular formulas
Molecular formula – shows the actual number of atoms if each element in a molecule, as well as
the ratio of atoms.
The empirical formula can be determined experimentally, but it doesn’t tell us the whole story. For
example, glucose (C6H12O6), acetic acid (HCH3COO = C2H4O2) and formaldehyde (CH2O) all have the
same empirical formula = CH2O.
We can determine the molecular formula from a compound if we know:
a.
its empirical formula (we calculate from this the empirical formula mass)
b.
its gram formula mass, gram molecular mass or experimental molecular mass
c.
the known gram formula mass – GFM – (or other term above) is divided by the empirical
formula mass – EFM.
Example:
1.
The compound methyl butanoate smells like apples. Its percent composition is 58.8% C,
9.8% H, and 31.4% O. If its gram molecular mass is 102 g/mol, what is its molecular formula?
a.
determine the empirical formula:
58.8 g C
1 mol C
= 4.9 mol C
12.0 g C
9.8 g H
1 mol H
= 9.8 mol H
1.0 g H
31.4 g O
1 mol O
= 1.96 mol O
16.0 g O
4.9
1.96
:
9.8
1.96
:
1.96
1.96
the ratio is 2.5 : 5 : 1
multiply the ratios by 2 to get 5 : 10 : 2 for a formula of C5H10O2
now find the mass of the empirical formula
C 5 x 12.0 = 60.0 g/mol
H 10 x 1.0 = 10.0 g/mol
O 2 x 16.0 = 32.0 g/mol
Total = 102 g/mol
b.
c.
gram formula mass is given to us in the problem of 102 g/mol
gram formula mass
= 102 g/mol = 1
gram empirical mass = 102 g/mol
what this means is that the empirical formula IS ALSO the molecular formula.
2.
Dichloroacetic acid has an empirical formula of CHOCl and the gram formula mass is
129g/mol. Determine the molecular formula for dichloroacetic acid.
a.
Since we are given the empirical formula, we just need to find the mass of the empirical
formula.
C = 12.0
H = 1.0
O = 16.0
Cl = 35.5
Total = 64.5 g/mol
b.
c.
we are given the gram formula mass of 129 g/mol
divide gfm by efm:
129 g/mol
=
2
64.5 g/mol
what this tells us is that our empirical formula needs to be
increased by 2 for each element in the formula.
C2H2O2Cl2 is dichloroacetic acid
3.
Aspirin is made up of 60.0% C; 4.48% H; and 35.5% O. It has a gram formula mass of 180
g/mol, what is its molecular formula?
a.
60.0 g C
1 mole C
= 5.00 mol C
12.0 g C
4.48 g H
1 mol H
= 4.48 mol H
1.0 g H
35.5 g O
1 mol O
= 2.22 mol O
16.0 g O
5.00
2.22
:
4.48
2.22
:
2.22
2.22
the ratio is 2.25 : 2 : 1
multiply the ratios by a value to make the 2.25 a whole number
(2.25 : 2 : 1) x 4 = 9 : 8 : 4; so the empirical formula would be C9H8O4
now determine the empirical formula mass:
C 9 x 12.0 = 108 g/mol
H 8 x 1.0 = 8.0 g/mol
O 4 x 16.0 = 64.0 g/mol
Total = 180 g/mol
b.
The gram formula mass was given to us of 180 g/mol
c.
Divide the gfm by the efm:
180 g/mol
180 g/mol
The molecular formula for aspirin is C9H8O4
=
1
what this tells us is the empirical formula is
also the molecular formula