Chem CatalystIf a class of 22 students has 15 girls and 7 boys,

what percentage of the class is male? What percentage is female?

Today’s Agenda:1. Catalyst2. Notes- percent composition/ empirical

formula3. HW: p. 306-7 #32-35

Percent Composition/Mass Percent- The percent by mass of each element in a

compound.

(mass of part) ÷ (mass of whole) x100

Percent CompositionWhen a 13.60g sample of magnesium oxide is

decomposed, 5.40g of oxygen are obtained. What is the percent composition of this compound?

1) Find the mass of each element.

2) Find the mass of the compound.

3) Calculate.

Percent CompositionWhen a 13.60g sample of magnesium oxide is

decomposed, 5.40g of oxygen are obtained. What is the percent composition of this compound?

Percent Composition: From Formula1) Find the total molar mass for each

element.2) Divide by molar mass of compound.3) Multiply by 100.

Percent Composition from FormulaFind the percent composition of propane

(C3H8).

Percent Composition from Formula: Your turn!Find the percent composition of iron (II)

oxide (FeO).

Find the percent composition of iron (III) oxide (Fe2O3).

Chemical Formulas:Empirical formula: the lowest whole-

number ratio of the atoms of elements in a compound

Molecular formula: the exact number of atoms of elements in a compound

For example, styrene (C2H2), and acetylene (C8H8) have different molecular formulas but the same empirical formula (CH)

Finding Empirical Formulas:Determining the MOLE RATIO of elements

in a compound.1. Find percent composition (if not given)2. Divide percent composition by molar

mass of the element to get mole ratios3. Simplify to whole numbers4. Write the formula with the subscripts

Empirical Formula: ExampleA compound contains 25.9% nitrogen, and

74.1% oxygen. What is the empirical formula of this compound?

Empirical Formula: ExampleCalculate the empirical of these compounds:1) 94.1% O, 5.9% H

2) 67.6% Hg, 10.8% S, 21.6% O