Percent Composition

 We go to school for 180 days a year. What % of the year are we in school? Similar to finding % in any other situation

Percent Composition  Tells us what % of each element makes up a particular compound  Use Molar Mass  Relative measure of mass for each element in a compound.

Percent Composition _________________________ x 100 Total Mass of the compound Mass of the element

Example Problem #1- when given only the formula  Consider the molecule NaCl  What is the percent composition? (we need to find the % of each element)  Step 1: Find the Molar Mass of the compound  58.5 g/mol.

Percent Composition  Step 2: Divide each part by the total and multiply by x 100 = 39.3% Na x 100 = 60.7% Cl

You Try  What is the percent composition for LiNO 3 ?  Molar Mass= 68.9 g/mol  Li = 10.0%  N = 20.3%  O = 69.7%

EMPIRICAL AND MOLECULAR FORMULAS

Two types of chemical formulas  Empirical-  is the simplest whole number ratio by which the elements are combined  Molecular-  the actual number of atoms in the molecule

Ex Sugar  Molecular formula Empirical formula  C6H12O6 Reduce  CH2O  ACTUAL amt of atoms RATIO 1:2:1

Finding Empirical Formula From % Composition 1.Assume that you have 100 grams of your substance, so you can convert the percents directly to grams. Ex: 48.64% C= g of C 8.16% H= 8.16 g of H 43.20% O= g of O * If you are given mass data, skip this step

Finding Empirical Formula From % Composition 

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4.Find the smallest whole number ratio by multiplying each number by the smallest number that yields whole numbers. Ex: 2 x 1.5 mol C = 3 mol C 2 x 3 mol H = 6 mol H 2 x 1 mol O = 2 mol O

Finding Empirical Formula From % Composition 5.Write the formula with the simplest whole number ratio. Ex: C 3 H 6 O 2

Finding Molecular Formula from Empirical Formula  Once you have figured out the empirical formula, you can find the molecular formula if you know the molar mass. Ex: Empirical Formula= C 2 H 3 O 2

Finding Molecular Formula from Empirical Formula 

2.Add them together to find the molar mass Ex: g g g = g/mol 3.Compare to the actual molar mass Ex: Actual molar mass= g/mol

Finding Molecular Formula from Empirical Formula 