Percent Composition and Empirical Formulas

Ch 7.3- 7.4

Percent composition: percent, by mass, of each element in a compound

Mass of element in 1 mol of compoundMolar mass of compound

% composition =x 100%

Ex: Find percent composition of Cu2S

1. Find molar mass of Cu2S 159.2g

127.1g Cu x 100% = 79.85% Cu159.2g Cu2S

32.1g S x100% = 20.2 % S159.2g Cu2S

Ex: dinitrogen pentoxide is 25.9% nitrogen by mass. How many grams of nitrogen and oxygen are in a 125g sample of the compound?

Molecular Formula: total number of atoms of each lement in one molecule of a compoundEmpirical Formula: smallest whole number ratio of the elements in a compound

Molecular or empirical? 1. C2H4

2. CH3. H2O4. H2O2

5. C6H12O6

6. C8H8

7. Na2SO4If given the molecular formula, find the empirical formula.

Find Empirical Formal from %Can go from % composition to empirical formula1. change % to grams for each given (assume 100g sample)2. change grams to mol (using molar mass from periodic

table)3. divide all by the lowest # mols (to get ratio)4. write ratio as formula (if not whole #s, try multiplying all

by 2)

Ex: Quantitative analysis shows that a compound contains 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. Find the empirical formula.

32.38 g Na, 22.65g S, 44.99g O32.38 g Na x 1mol/23.0g Na = 1.408mol Ns 22.65g S x 1mol/32.1g S = 0.706mol S 44.99g O x 1mol/16.0g O = 2.81 mol O

Divide each by 0.706

1.99 mol Na1mol S3.98 mol O

Na2SO4

Ex:A 5.325-g sample of methyl benzoate, a compound used in the manufacture of perfumes, is found to contain 3.758 g of carbon, 0.316 g of hydrogen, and 1.251 g of oxygen. What is the empirical formula of this substance?

Answer: C4H4O

Ch 7.4

Pg 244 Practice: #1-3Pg 249 section review # 1-2

3-4 finish next class

Warm up