Aim: How can we calculate percent composition of a compound?

Do Now: 1.Take out a calculator and reference tables.

2.What is the gram formula mass of C6H5(OH)3?

How can we find the molecular formula of a compound given the empirical formula and

molecular mass?• Example: If the empirical formula is CH, and the

molecular mass is 26, what is the molecular formula?

• Three steps to solve: 1.Find the mass of the empirical formula. The mass

of CH =2.Divide the mass of the molecular formula by the

mass of the empirical formula. Example: 3.Multiply the empirical formula by the answer found

in step 2. Example:

Some More Practice Problems• A compound has an empirical formula of CH2 and

molecular mass of 56. What is its molecular formula?

• The empirical formula is CH3 and the formula mass is 30. What is the molecular formula?

What if we need to know the percent composition of a substance?

• The percent composition of a substance represents the composition as a percentage of each element compared with the total mass of the compound.

• The formula used can be found on reference table T.

• We can use this formula to find both the percent of individual elements in a formula or the percent of water in hydrated salt compounds (salts crystals that are attached to water).

Some Practice ProblemsFind the percent compositions of all the elements in

the following compounds: 1.CuBr2

2.NaOH

What if we are talking about a hydrated salt?

HINT: Treat the water as a unit. Example: What is the percentage, by mass, of water

in sodium carbonate crystals, Na2CO3• 10H2O?

The Lab Problem• What is the percent, by mass, of water in the

hydrated salt CuSO4•5H2O?

Activity/Homework

• Review Book page 48-49. Copy and answer questions 14-26.

• Review Book page 51-52. Copy and answer questions 37-44.

• A copy will be posted on wagnerhigh.net