Percent Composition, Empirical and Molecular Formulas Courtesy www.lab-initio.com

Calculating Percentage Composition Calculate the percentage composition of magnesium carbonate, MgCO3. Formula mass of magnesium carbonate: 24.31 g + 12.01 g + 3(16.00 g) = 84.32 g 100.00

Formulas Empirical formula: the lowest whole number ratio of atoms in a compound. Molecular formula: the true number of atoms of each element in the formula of a compound. molecular formula = (empirical formula)n molecular formula = C6H6 = (CH)6 empirical formula = CH

Formulas (continued) Examples: Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio). Examples: NaCl Al2(SO4)3 MgCl2 K2CO3

Formulas (continued) Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio). Molecular: H2O C6H12O6 C12H22O11 Empirical: H2O CH2O C12H22O11

Empirical Formula Determination Base calculation on 100 grams of compound. Determine moles of each element in 100 grams of compound. Divide each value of moles by the smallest of all the values calculated. If number are not in whole numbers then multiply each number by an integer to obtain whole numbers for each element.

Empirical Formula Determination Sample Problem: Adipic acid contains: 49.32% C, 43.84% O , & 6.85% H by mass. What is the empirical formula of adipic acid? 1.Treat % as mass (g) ASSUME 100g sample C - 49.32% = 49.32 g O - 43.84% = 43.84 g H - 6.85% = 6.85 g ---------------------------------------------------------------------------- 2. Convert mass (g) to moles by dividing by molar mass: C - 49.32 g / 12.00 g/mol = _______ mol of C atoms O - 43.84 g / 16.00 g/mol = ________mol of O atoms H - 6.85 g / 1.01 g/mol = ________mol of H atoms

Empirical Formula Determination Adipic acid contains: 49.32% C, 43.84% O , & 6.85% H by mass. What is the empirical formula of adipic acid? 1.Treat % as mass (g) ASSUME 100g sample C - 49.32% = 49.32 g O - 43.84% = 43.84 g H - 6.85% = 6.85 g ---------------------------------------------------------------------------- 2. Convert mass (g) to moles C - 49.32 g / 12.00 g/mol = 4.107 mol of C atoms O - 43.84 g / 16.00 g/mol = 2.74 mol of O atoms H - 6.85 g / 1.01 g/mol = 6.78 mol of H atoms

Empirical Formula Determination 3. Divide each value of moles by the smallest of the values. Carbon: Hydrogen: Oxygen:

Empirical Formula Determination 3. Multiply each number by an integer to obtain all whole numbers. Carbon: 1.50 Hydrogen: 2.50 Oxygen: 1.00 x 2 x 2 x 2 3 5 2 Empirical formula: C3H5O2

Your Turn: Determine the Empiracle Formula for a compound whose percentage composition is 21.6% Na, 33.3% Cl, and 45.1% O ? Set up a chart that looks like this…

Your Turn: Determine the Empiracle Formula for a compound whose percentage composition is 21.6% Na, 33.3% Cl, and 45.1% O ? Set up a chart that looks like this…

Finding the Molecular Formula Once the empirical formula is found, it can be used to determine the molecular formula of a substance. The molecular formula is just a multiple of the empirical formula. So if given the molar mass of a substance, we can calculate the molar mass of the empirical formula and divide the two masses to give us the value to multiply our empirical formula by.

Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 1. Find the formula mass of C3H5O2 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g

Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g 2. Divide the molecular mass by the mass given by the emipirical formula.

Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? (C3H5O2) x 2 = C6H10O4 3. Multiply the empirical formula by this number to get the molecular formula.