
The percent composition of fuel is 81.7%
carbon and 18.3% hydrogen. What is the
empirical formula of the fuel?

Start by assuming 100 g of the compound
(81.7 g) / (12.01 g/mol) = 6.80 mol C
(18.3 g) / (1.008 g/mol) = 18.2 mol H

Recall that many compounds could potentially
have the same empirical formula, but the
molecular formula is specific to a given
compound


Molecular formula subscripts are a whole
number multiple of the empirical formula
subscripts
Similarly, the molar mass of a compound is a
whole number multiple of the molar mass of
the empirical formula

So, molar mass of the compound = n x molar mass
of the empirical formula, where n = 1, 2, 3…

A chemist determines that a substance has a
empirical formula of CH and a molar mass of
78 g/mol. What is the molecular formula of the
substance?


The empirical formula of CH is 13.0 g/mol
The ratio of the empirical formula to the molecular
formula is 78 = 6
13

Therefore, we multiply the subscripts of the
empirical formula by the ratio (6) to get the
molecular formula: C6H6

The empirical formula for ribose (a sugar) is
CH2O. A mass spectrometer determined the
molar mass of ribose to be 150 g/mol. What is
the molecular formula of ribose?


Many ionic compounds crystallize from water
solutions and end up with water incorporated into
their structures, forming a hydrate (hydrated =
contains water, anhydrous = without water)
For example, the chemical name for Epsom salts is
magnesium sulfate heptahydrate (use prefixes to
tell the number of water molecules in the
compound)
MgSO4 · 7H2O
 The dot represents a weak bond
 The molar mass of a hydrated compound must
incorporate the mass of the water molecules in the
compound


A 50.0 g sample of a hydrate of barium
hydroxide contains 27.2 g of Ba(OH)2. What is
the percent, by mass, of water in the
compound?

What is the formula for the hydrated barium
oxide compound ?

1. Moles of Ba(OH)2?

2. Moles of H2O?

3. Molar ratio?


The mass of an atom is expressed in atomic mass
units (u or amu)
Relative measure
One atom of carbon 12 has a mass of 12 u (ie. One u has a
mass of 1/12 that of a C-12 atom)
 Average atomic mass (what we see on the periodic table)
takes into account all isotopes (and their relative
abundances)


One mole of an element has a mass (in grams)
numerically equivalent to the element’s average
atomic mass (in amu)

Take home: grams are numerically equivalent to amu.