Lecture 10.3 (Honors only)- Percent Composition & Empirical Formulas

Bellwork- oxygen tank

1)A small portable oxygen tank holds 164 liters of oxygen gas. How many moles does it contain?

2)How many O2

molecules doesthe tank contain?

3)How manyOxygen atoms?

The percent by mass or Mass percent of an element is the number of grams of an element in a compound, divided by the molar mass of the compound, and then multiplied by 100%.

Percent composition gives the mass percent of each element in a compound.

The empirical formula of a compound shows the smallest whole-number ratio of the atoms in the compound.

Ethyne (C2H2) is a gas used in welder’s torches. Styrene (C8H8) is used in making polystyrene.

These two compounds of carbon have the same empirical formula (CH) but different molecular formulas.

10.11

STEP 1- assume a 100g sample (percentages become grams)

STEP 2- convert grams to moles

25.9g N

74.1g O

1mole N 14g N

x ------------- = 1.85 moles N

1mole O 16g O

x ------------- = 4.63 moles O

STEP 3- divide all quantities by the lowest #

_________ = 1 1.85 moles

_________ = 2.5 1.85 moles

x2 =2

x2 =5

N2O5

The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula.

Methanal, ethanoic acid, and glucose all have the same empirical formula—CH2O.

1. Which of the following is NOT an empirical formula?

A. NO2

B. H2N

C. CH

D. C3H6