IIIIII II. % Composition and Formula Calculations Ch. 3 – The Mole

A. Percent Composition n the percentage by mass of each element in a compound

 100 = A. Percent Composition %Cu = g Cu g Cu 2 S  100 = %S = g S g Cu 2 S 79.85% Cu 20.15% S n Find the % composition of Cu 2 S.

n How many grams of copper are in a 38.0-gram sample of Cu 2 S? (38.0 g Cu 2 S)(0.7985) = 30.3 g Cu Cu 2 S is 79.85% Cu A. Percent Composition

%Fe = 28 g 36 g  100 = 77.78% Fe %O = 8.0 g 36 g  100 = 22.22% O n Find the percent composition of a sample that is 28 g Fe and 8.0 g O. A. Percent Composition

 100 = %H 2 O = g g 24.51% H 2 O n Find the mass percentage of water in calcium chloride dihydrate, CaCl 2 2H 2 O? A. Percent Composition

FORMULA CALCULATIONS

A. 2 Types of formulas C2H6C2H6 CH 3 reduce subscripts n Smallest whole number ratio of atoms in a compound Empirical Formula:

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

B. 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.

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

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

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

C. Molecular Formula n The molecular mass of the compound is 216 g/mol. 1.Empirical Formula: N 2 O 5 2.Empirical Form. Mass : g/mol 3.216/ = 2 = n 4.(N 2 O 5 ) 2 = N 4 O 10