Empirical & Molecular Formulas Student Notes

I. Empirical vs. Molecular Formulas A. Empirical Formula: the simplest whole number ratio of atoms in a compound. B. Molecular Formula: shows how many atoms of each element are present in a molecule or compound. C. Identify the following as molecular or empirical. i. CH6 ______iv. C2H4O ______

ii. C2H2 ______v. C8H8S4 ______

iii. C12H6 ______

II. Calculating Empirical Formulas A. Steps: i. Step 1: If the element is given in a percent, assume 100 g. ii. Step 2: Convert the mass of each element to moles using the molar mass. iii. Step 3: Divide each mole value by the smallest mole value. iv. Step 4: If CLOSE, Round to the nearest whole number. 1. If answer is NOT close to a whole number, you will need to multiply by a factor of 2 or 3. 2. If answer ends with .5, multiply by 2. 3. If answer ends with .3 or .6, multiply by 3. v. You MUST multiply EACH element by the factor! vi. Step 5: Write Empirical Formula using answers as the subscripts.

B. Example #1: Find the empirical formulas for a compound with 13.5% of calcium, 10.8% oxygen, 0.675% of hydrogen.

C. Example #2: Determine the empirical formula for a compound composed of 40.00% C, 6.72% H, and 53.29% O.

D. Example #3: 57.4% Carbon, 6.16% Hydrogen, 9.52% Nitrogen, 27.18% Oxygen

III. Calculating Molecular Formulas A. Steps: i. Step 1: Find the molar mass of the empirical formula. ii. Step 2: Divide the molecular mass by the empirical mass (big number by small number). iii. Step 3: Multiply answer by each subscript in the empirical formula to get molecular formula. B. Example #1: What is the molecular formula of a compound whose molar mass is 60.0 g/mol and empirical formula is CH4N?

C. Example #2: What is the molecular formula of CH3O if its molar mass is 62 g/mol?

D. Example #3: Find the molecular formula for a compound with an empirical formula of C2H8N and a molecular mass of 46 grams per mole.

E. Which of the following is an empirical formula? A. C3H6 B. C2H8 C. CH4 D. C4H10

F. The molecular formula for vitamin C is C6H8O6. What is the empirical formula? A. CHO B. C3H4O3 C. CH2O D. C2H4O2