10-3: Empirical and Molecular Formulas

Percentage Composition

• The mass of each element in a compound, compared to the mass of the entire compound (multiplied by 100)

• Tells you the percent of the mass made up by each element in the compound.

How to calculate % comp:

• (grams of X)/(total grams) x 100%

• Complete sample problem #7 on pg 334

• Complete practice problem #14 on pg 334

Empirical Formula

• The smallest whole number ratio of the atoms of the elements in a compound

• Remember glucose?

• What is its molecular formula?

• What is its empirical formula?

Calculating Empirical Formula• 1) If given as percents, read them as

grams (accurate if a 100 g sample)• 2) Convert each gram amount to moles• 3) Find the smallest whole number ratio

by dividing each mole amount by the smallest number. Round as appropriate.

• 4) Use that ratio to write the empirical formula

Sample Problem #8 (pg 36)

Molecular Formula

• Gives the actual number of atoms of each element in a molecular compound.

• Is a whole number multiple of the empirical formula.

Finding the Molecular Formula1. You will be given the molar mass of the

molecular formula in the question.

2. Find the empirical formula.

3. Find the molar mass of the empirical formula.

4. Divide molecular formula mass by empirical formula mass.

5. This is the whole number multiple that you should multiply the empirical formula by.

Practice problem #18 (pg 339)• Sample problem 9 (pg 338)