SME
General Chemistry I · Problem type
The molecular formula from the empirical formula
Use this for “A sugar’s empirical formula is CH₂O and its molar mass is 180 g/mol. Find its molecular formula.”
This is an example. Type your own values to start a new problem.
r
=
M (g/mol)
÷
Mₑₘₚ (g/mol)
n
Empirical units per molecule
calculated
Formulas
r
=
M
Mₑₘₚ
5.994
=
180
30.03
n
=
r
rounded
to
the
ones
6 = 5.994 rounded to the ones
Assumptions
•
The molecular formula is a whole number n of empirical units: CH₂O × 6 is C₆H₁₂O₆.
•
A measured molar mass is rarely exact, so the ratio M ÷ empirical mass is rounded to the nearest whole number when it is within 2% of one.
•
Empirical formula mass is the sum of its atoms’ molar masses: CH₂O is 12.01 + 2 × 1.008 + 16.00 = 30.03 g/mol.
Step-by-step
WE KNOW
Mₑₘₚ = 30.03 g/mol, M = 180 g/mol
FIND
mass ratio (r), empirical units per molecule (n)
Step 1 · Find mass ratio (r)
Use r = M ÷ Mₑₘₚ
Divide the molar mass by the empirical formula mass: how many empirical units fit in one mole of molecules.
r
=
180
30.03
r
=
5.994
Step 2 · Find empirical units per molecule (n)
Use n = r rounded to the ones
A
molecule
holds
a
whole
number
of
empirical
units;
multiply
every
subscript
of
the
empirical
formula
by
n
.
n
=
5.994
rounded
to
the
ones
n
=
6
Check
Put the numbers back in. Both sides should match.
5.994
=
180
30.03
✓
6 = 5.994 rounded to the ones ✓
Topic check
See what you know: 5 questions on this topic
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