UNIT 6: CHEMICAL QUANTITIES

Chapter 10: Empirical and Molecular Formulas

Empirical Formulas

Empirical Formulas The lowest whole-number ratio of

atoms of the elements in a compound.

Empirical Formulas are much like creating a recipe of elements. Orange Juice 1 can of concentrate to 3

cans of water = (OJ)1(H2O)3 = 1:3 ratio Double Batch

(OJ)2(H2O)6 = 2:6 = 1:3 ratio

Empirical Formulas

Steps in determining Empirical Formulas :1. Assume that the percent given is equal to the

number of grams in a 100 gram sample. Meaning…remove the % and add g for the

units. 2. Convert the grams to moles using the molar

mass.3. Divide each mole value by the smallest mole

calculated to determine a ratio. 4. If all the values are not near a whole number

(_.8-_.2 range), multiply ALL the values by 2 or 3 until the values are all whole numbers.

5. Give the Empirical Formula

Empirical Formulas

Practice with Steps A compound is analyzed and found to

contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 1: Assume percents are grams

Nitrogen = 23.9% = 23.9g Oxygen = 74.1% = 74.1g

Empirical Formulas

Practice with Steps A compound is analyzed and found to

contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 2: Convert to moles using molar mass

Nitrogen = 23.9 g 1 mol =1.85 mol Nitrogen

14 g Oxygen = 74.1 g 1 mol = 4.63 mol Oxygen

16 g

Empirical Formulas

Practice with Steps A compound is analyzed and found to

contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 3: Divide each mole by the

smallest mole value Nitrogen = 1.85 mol ÷ 1.85 mol =

1mol of NOxygen = 4.63 mol ÷ 1.85 mol = 2.5

mol of O

Empirical Formulas

Practice with Steps A compound is analyzed and found to

contain 23.9 % nitrogen and 74.1 % oxygen. Find the empirical formula of the compound. Step 4: Multiply the moles by a

coefficient to have whole number ratios. Nitrogen = 1 mol x 2 = 2 mol of NOxygen = 2.5 mol x 2 = 5 mol of O

Step 5: Give Empirical FormulaN2O5

Molecular Formulas

Molecular Formulas Based on Empirical Formulas Most molecular formulas are empirical

formulas, but some molecular formulas are not the lowest ratios.Example: Hydrogen Peroxide

Empirical Formula HOMolecular Formula H2O2

Requires two things Mass of the Compound Empirical Formula of the Compound

Molecular Formulas

Practice Calculate the molecular formula of a

compound whose mass is 60.0 g and empirical formula is CH4N.

Find the molar mass of the empirical formula CH4 N = 30.0 g/mol

Molecular Formulas

Practice Calculate the molecular formula of a

compound whose mass is 60.0 g and empirical formula is CH4N.

Divide the molecular mass by the empirical mass Molecular mass/ empirical mass (60.0 / 30.0 ) = 2

Molecular Formulas

Practice Calculate the molecular formula of a

compound whose mass is 60.0 g and empirical formula is CH4N.

Multiply the empirical formula subscripts by the coefficient found in step 2. C (1x2) H (4x2) N (1x2) = C2H8N2