Determining Molecular formula given
molecular
l l mass andd % composition
iti .
Of the three ways of determining a formula from composition
information, this is the easiest. Let’s try a typical problem.
I have a compound with a molecular mass of 60.05g, and it is
6.71% H, 40.00% C and 53.29% O, what is the molecular
formula of this compound?
Determining Molecular formula given
molecular
l l mass andd % composition
iti .
I have a compound with a molecular mass of 60.05g, and it is
6.71% H, 40.00% C and 53.29% O, what is the molecular formula
of this compound?
STEP 1. Since we know the total molecular mass of the compound,
and the % of each element in the compound, the first step is to take
the total molar mass and multiply it by the % composition for each
element to find out the mass of each element. The onlyy trick you
y
have to remember is to divide the % number by 100% to get rid of
the % unit. Let’s try this on the next page.
Determining Molecular formula given
molecular
l l mass andd % composition
iti .
I have a compound with a molecular mass of 60.05g, and it is
6.71% H, 40.00% C and 53.29% O, what is the molecular formula
of this compound?
Molecular mass x (% element/100%) = mass of element
H
60.05gg x ((6.71%/100%)) = 4.029gg
C
60.05g x (40.00%/100%) = 24.02g
O
60.05g x (53.29%/100%) = 32.00g
Determining Molecular formula given
molecular
l l mass andd % composition
iti .
STEP 2. Since step one gives us the mass of each element in one
mole of the compound. All we do now is to divide that mass by
the mass of a single mole of that atom to find out the total number
of moles of the element in the compound.
H
4.029g of H in compound/1.008g in 1 mole of H = 4 moles H
C
24.02g of C in compound /12.01g in 1 mole of C = 2 moles of C
O
32 00g of O in compound/16
32.00g
compound/16.00g
00g in 1 mole of O = 2 moles of O
Determining Molecular formula given
molecular
l l mass andd % composition
iti .
STEP 3. Combine the known number of moles into a formula.
H: 4 moles of H
C: 2 moles of C
O: 2 moles of O
molecular formula = H4C2O2
For proper form, there are rules for the order that the atoms should
occur in the formula. That is a picky point for a more advanced
y list yyour atoms in,, jjust as longg
class,, so I don’t care what order you
as you have the numbers right.
Determining Molecular formula given
molecular
l l mass andd % composition
iti .
Practice Problem:
A compound has a molecular mass of 96.094g, and it is
composed of 88.39%
39% H
H, 29
29.16%
16% N
N, 12
12.50%
50% C and 49.95%
49 95% O.
O What
is the molecular formula of the compound?
Determining Molecular formula given
molecular
l l mass andd % composition
iti .
A compound has a molecular mass of 96.094g, and it is composed of
8.39% H, 29.16% N, 12.50% C and 49.95% O. What is the molecular formula
of the compound?
Step 1:
96.094g x (8.39%/100%) = 8.06 g H
96.094g x (29.16%/100%) = 28.02 g N
96.094g x (12.50%/100%) = 12.01 g C
96.094g x (49.95%/100%) = 48.00 g O
Step 2:
8.06 g H / 1.008g in 1 mole = 8 mole H
28 02 g N/ 14.01g
28.02
14 01 iin 1 mole
l = 2 mole
l N
12.01 g C/ 12.01g in 1 mole = 1 mole C
48.00 g O/ 16.00g in 1 mole = 3 mole O
Final formula: H8N2CO3, or the more chemically correct formula (NH4)2CO3