1. Mass Relations in Formulas
In this section we will learn how to calculate the percent composition from a given formula. More importantly we will learn how to use percent composition to determine the formula. This skill is important in identifying unknown substances.

2. Percent Composition
The percent composition is given by using the mass percents of the elements present in an ionic or molecular formula.
From the correctly written formula, the amount of each element is available in terms of the number of atoms.
The subscripts in the formula not only represent the atom ratio, but also the molar ratio of atoms combined.
Water is a ratio of 2 Hydrogen to 1 Oxygen either in atoms or moles.
The molar mass is then used to calculate the mass percents of each element present in a formula.

3. Calculating Percent Composition
The percent of an element is found by taking the mass total of the element and dividing it by the total mass of the compound and then multiplying by one hundred.
When asked to find the percent composition, it is generally understood that you find the mass percent of each of the elements present.

4. Simplest Formula
The simplest formula, also called empirical formula, is the lowest whole number ratio of elements present in a formula.
For ionic compounds the simplest formula is the ionic formula.
For molecular compounds, the molecular formula is an integer multiple of the simplest formula.
Data leading to the simplest formula may be expressed 3 ways:
Masses of the elements present
Mass percents of the elements
The masses of product or reactant found in a chemical reaction
The strategy is to find the number of moles of each element, then the mole ratio, and finally the simplest formula.

5. Tips on Finding Empirical Formula
The mass of each element will be given to you in order of the formula.
Divide the mass of the element by its molecular mass. This is the moles present of each element.
Divide the moles by the lowest number of moles present in the compound (round off any number that are close to whole numbers). This gives you a ratio of one element to another.
If there are any elements with the number of moles that are not a whole number, multiply the whole compound by a multiple until there is a whole number on the subscripts.
This is the Empirical Formula.

6. Example 1 – given mass

7. Given Percent Composition
Often instead of mass of elements, the percentage of each element is given.
Change the percent into mass in grams. This assumes that you have a total mass sample of 100%.
Follow the same process as you would if you were given mass to find the empirical formula.
Homework P212/213 [35, 37, 38, 41, 42]

8. Example 2 – given percent