Percent Composition What is the % mass composition (in grams) of the green markers compared to the all of the markers? % green markers = grams of green markers grams of all of the markers

Percent Composition % Comp. Is the mass of each element in a compound compared to the total mass of the compound, multiplied by 100% % mass = grams of element in 1 mole compound molar mass of compound

Calculate % Comp from a given chemical formula Find the mass % of H and the % of O in water 1 mole of water = g 1 mole of water contains 2 moles of hydrogen and 1 mole of oxygen

Calculating Percent Composition 2 moles of H= 2 g 2 g H x 100 = 11% 18 g H 2 O

Calculating Percent Composition 1 mole of O = 16 g O 16 g O x 100 = 89% O 18 g H 2 O

Try These! What is the % composition of Oxygen in lithium hydroxide ? An 8.2 g piece of magnesium combines completely with 5.4 g of oxygen to form a compound. What is the percent composition of magnesium?

What is the % composition of Oxygen in lithium hydroxide (LiOH)? % O 16 g x 100= 67% 24g An 8.2 g piece of magnesium combines completely with 5.4 g of oxygen to form a compound. What is the percent composition of magnesium? % Mg 8.2g x 100 = 60% 13.6g

A given compound is 80% C and 20% H. How many C and H grams are in this compound? Assume you have 100 g of the substance. 80 g C 20 g H

Find the number of moles of each atom in the sample. 80. g C x 1 mol C = 6.7 mol C atoms 12 g C 20 g H x 1 mol H = 20 mol H 1. 0 g

Find the mole ratio of C and H To find the smallest whole number ratio, divide the calculated mole values, by the smallest number of moles C: 6.7H: 20 = 2.98 ~ Ratio of C:H is 1:3 so formula is CH 3

Empirical Formula If given % composition, assume that there is a 100 gram sample. Convert grams to moles. Determine which mole value is smallest. Divide all mole values by the smallest mole value. Write the empirical formula using the ratio numbers as subscripts.

Empirical Formulas An empirical formula gives the simplest whole number ratio of atoms of elements It DOES NOT give the exact number of atoms in a molecule

Suppose that you are given a substance that contain hydrogen and oxygen. You are told that the ratio of moles of hydrogen to moles of oxygen is 1:1. What is the formula of the substance? HO, H 2 O 2, H 3 O 3, etc.

Try This! Calculate the empirical formula of 94.1% Oxygen and 5.9 % Hydrogen.

First convert to moles 94.1 g O x 1 mole = 5.9 moles 16 g 5.9 g H x 1 mole = 5.9 moles 1 g

Divide all mole values by the smallest whole value 5.9/5.9 = 1 Ratio is 1 oxygen to 1 hydrogen Empirical formula is OH

Try This! What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?

In 100 g of the compound, there are 25.9 g N and g O. These values are used to convert to moles g N x 1 mole N = 1.85 moles of N 14.0 g N 74.1 g O x 1 mole O = 4.63 moles of O 16 g O

To find the smallest whole number ratio, divide the calculated mole values, by the smallest number of moles 1.85 mol N = 1 mol N mol O = 2.50 mol O 1.85 Multiply by two! The empirical formula is N 2 O 5

Molecular Formulas Molecular formulas apply only to molecular compounds, NOT to ionic compounds Molecular formulas give the exact number of atoms of each element in a molecular compound

Molecular Formulas Are always a multiple of an empirical formula Determined by comparing the molar mass of an unknown compound with the molar mass of the empirical formula to find the multiple.

Finding Molecular Formulas Ex. The empirical formula of glucose is CH 2 O. From experiments, it was determined that the molar mass of glucose is 180 g/mol. What is the molecular formula of glucose?

Finding Molecular Formulas First, find the molar mass of the empirical formula CH 2 O 12 + (1 x 2) + 16=30 g/mol

Finding Molecular Formulas Find the ratio of molar mass (180 g/mol) to empirical formula mass (30 g/mol). 180 g/mol glucose = 6 mole glucose 30 g/mol CH2O 1 mole CH 2 0 This means the formula for glucose is 6 times the empirical formula

Finding Molecular Formulas If the molecular formula is 6 times the empirical formula, then the molecular formula is: 6 (CH 2 O) = C 6 H 12 O 6

Calculate the molecular formula of the compound whose molar mass is 60 g and empirical formula is CH 4 N. Molar mass of CH 4 N is 30 grams 60 grams/30 grams = 2 2 (CH 4 N) = C 2 H 8 N 2 Answer is C 2 H 8 N 2

Empirical vs Molecular Percent composition can be used to calculate empirical and molecular formulas. An empirical formula is a formula with the simplest ratio of atoms. Is C 6 H 18 an empirical or molecular formula? Molecular CH 3 is the empirical formula

More problems What’s the empirical formula of a molecule containing 65.5% carbon, 5.5% hydrogen, and 29.0% oxygen?

More Problems If the molar mass of the compound is 110 grams/mole, what’s the molecular formula?