IIIIII Suggested Reading: Pages Chapter 7 sec 3 & 4

Formula Mass/Molar Mass H 2 O = 2 moles of hydrogen + 1mole of oxygen = 1 mole of water H 2(1) = 2 O 1(16) = g/mol Al 2 (SO 4 ) 3 = 2 moles of aluminum + 3 moles of sulfur + 12 moles of oxygen = 1 mole of aluminum sulfate Al 2(27)= 54 S 3(32)= 96 O 12(16)= g/mol

Mass Conversions How many moles are there in 21.0 g of aluminum sulfate? 21.0g1 mole=.0614 moles 342gAl 2 (SO 4 ) 3

Percentage Composition n the percentage by mass of each element in a compound

 100 = Percentage Composition %Cu = 128g Cu 160 g Cu 2 S  100 = %S = 32 g S 160 g Cu 2 S 80% Cu 20% S n Find the % composition of Cu 2 S.

%Fe = 28 g 36 g  100 = 78% Fe %O = 8.0 g 36 g  100 = 22% O n Find the percentage composition of a sample that is 28 g Fe and 8.0 g O. Percentage Composition

n How many grams of copper are in a 38.0-gram sample of Cu 2 S? (38.0 g Cu 2 S)(0.80) = 30.4 g Cu Cu 2 S is 80% Cu Percentage Composition

 100 = %H 2 O = 36 g 146 g 25% H 2 O n Find the mass percentage of water in calcium chloride dihydrate, CaCl 2 2H 2 O? *Percentage Composition

Empirical Formula n Smallest whole number ratio of atoms in a compound C2H6C2H6 CH 3 reduce subscripts

Empirical Formula 1. Find mass (or %) of each element. 2. Find moles of each element. 3. Divide moles by the smallest # to find subscripts. 4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.

Empirical Formula n Find the empirical formula for a sample of 25.9% N and 74.1% O g 1 mol 14 g = 1.85 mol N 74.1 g 1 mol 16 g = 4.63 mol O 1.85 mol = 1 N = 2.5 O

*Empirical Formula N 1 O 2.5 Need to make the subscripts whole numbers  multiply by 2 N2O5N2O5

Molecular Formula n “True Formula” - the actual number of atoms in a compound CH 3 C2H6C2H6 empirical formula molecular formula

Molecular Formula 1. Find the empirical formula. 2. Calculate the empirical formula mass. 3. Divide the molecular mass (given) by the empirical Formula mass. 4. Multiply each subscript by the answer from step 3.

Molecular Formula n The empirical formula for ethylene is CH 2. Find the molecular formula if the molecular mass is 28.1 g/mol? 28.1 g/mol 14 g/mol = 2.00 empirical mass = 14 g/mol (CH 2 ) 2  C 2 H 4