Empirical and Empirical and Molecular Formula Molecular Formula

and Percentage and Percentage CompositionComposition

Stoichiometry Math of

Chemistry

Percents

% = part

wholex 100

Percentage Composition

Percent Composition is finding the percent by mass of each element in a compound using the periodic table.

Percentage Composition

An 8.20 gram piece of magnesium combines with 5.40 grams of oxygen to form a compound.

What is the percent composition of this compound?

Percentage Composition

How many grams of Ca are in 64.3 grams CaCl2?

You Try

How many grams of Oxygen are in 19.2 grams Na3PO4?

Finding a ratio

If a school has 30 boys and 60 girls, what is the ratio of boys to girls?

Finding a whole number ratio in its lowest terms

If a school has 27 boys and 18 girls, what is the ratio of boys to girls?

Finding a whole number ratio in its lowest terms

If a school has 75 boys and 60 girls, what is the ratio of boys to girls?

Finding a whole number ratio in its lowest terms

A certain compound contains 4.14 moles of Mg and 2.76 moles of P.

Find the mole ratio between Mg and P.

Write the formula.

Empirical Formula

An Empirical formula gives the lowest whole number MOLE ratio between the atoms in a compound.

The Math

A compound contains 0.900 grams of calcium and 1.60 grams of chlorine.

Find its empirical formula.

The Math

A compound is analyzed and found to consist of 36.0 % aluminum and 64.2 % sulfur.

Find its empirical formula.

You Try

What is the empirical formula of a compound that is 66.0 % Ca and 34.0 % P?

Molecular Formula

A Molecular formula gives the actual ratio between the atoms in a compound.

For Ionic compounds, the empirical and molecular formulas are the same.

Molecular Formula

For molecular compounds, the molecular formula is a multiple of the empirical.

Example:C6H6 is benzene. What is

benzene’s empirical formula?

The Math

A compound contains 31.10 grams of carbon and 7.872 grams of hydrogen.

Find its empirical formula.

Molecular formula

Vitamin C is ascorbic acid. It has a percentage composition of 40.9 % carbon, 4.58 % hydrogen, and 54.5 % oxygen with a molecular mass of 176.1 u.

Find its empirical and molecular formulae.

Review Molecular formula

Aspirin, acetylsalicylic acid, contains 60.0 % carbon, 4.48 % hydrogen, and 35.5 % oxygen with a molecular mass of 180 u.

Find its empirical and molecular formulae.

Hydrates

Hydrates are compounds with water molecules trapped in their crystal structure.

The water is weakly bonded and can be removed by heating the substance.

Quiz Review

A particular hydrate of MgSO4 contains 51.14 % water.

Find the formula for the hydrate.

Quiz Review

Find the percentage of water in the hydrate, Na2CO3 * 10 H2O.

Quiz Review

Find the percentage of oxygen in P2O5.

How many grams of oxygen are in 25.5 grams of P2O5?