Mass Relationships in Chemistry;
Stoichiometry

Mass
Atomic masses are relative, representing the mass of an atom of one element compared to the mass of another.
Carbon-12 is the standard and its mass is assigned a value of 12 amu.

Atomic Mass and Isotopic Abundance
Silicon consists of three stable isotopes whose atomic masses and abundances are as follows:
Si-28 27.98 amu 92.2%
Si-29 28.98 amu 4.7%
Si-30 29.97 amu 3.1%
Answer: 28.1 amu

Percent composition = total mass of element/total mass of compound X 100

 Determine the mass of each element.
 Determine the number of moles of each element by dividing by the atomic mass of that element
 Find smallest number of moles.
 Divide the number of moles of each element by the smallest number of moles
 Number of moles are subscripts.

A sample of compound is made up of 78.20 g of potassium and 32.06 g sulfur. What is the empirical formula?
78.20 g K x 1 mol K/39.10 g K = 2.000 mol K
32.06 g S x 1 mol S/32.06 g S = 1.000 mol S
Divide each by 1.000
K2 S1
Formula: K
2
S

A compound has the following percent composition.
31.9 % K 29.0 % Cl 39.1% O
To find the mass of each element, assume 100.0 g of compound.
31.9 g K 29.0 g Cl 39.1 g O
31.9 g K x 1 mol K/39.10 g K = 0.816 mol K
29.0 g Cl x 1 mol Cl/35.35 g Cl = 0.818 mol Cl
39.1 g O x 1 mol O/16.00 g O = 2.44 mol O
Ratios: K 0.816/0.816 = 1
Cl 0.818/0.816 = 1
O 2.44/0.816 = 2.99
Simplest formula: KClO
3

Rounding to determine integer ratios
To get an integer, it is ok to round down by at most 0.1.
 2.09 can be rounded down to 2
To get an integer, it is ok to round up by at most 0.1.
 2.93 can be rounded up to 3
 If that is not possible, multiply each ratio by small whole numbers until multiplication by one small number yields ratios that are all integers.
 1.50 x 2 = 3 1.33 x 3 = 4

Multiple: molar mass/simplest formula mass