EMPIRICAL FORMULA VS. MOLECULAR FORMULA 

EMPIRICAL FORMULA VS. MOLECULAR FORMULA Empirical Formula: the simplest whole number ratio of atoms in a compound. Molecular Formula: shows how many atoms of each element are present in a molecule or compound.

Empirical vs. Molecular Identify the following as molecular or empirical. CH6 C2H2 C12H6 Empirical Molecular Molecular C2H4O C8H8S4 Empirical Molecular

Calculating Empirical Formulas  Use the following poem to remember the steps: "Percent to mass, Mass to moles, Divide by small, Multiply ‘til whole"

Calculating Empirical Formulas  Example #1: Find the empirical formulas for a compound with 13.5% of calcium, 10.8% oxygen, 0.675% of hydrogen.  Step 1: If the element is given in a percent, assume 100 g. 13.5% Ca = 13.5 g Ca 10.8% O = 10.8 g O 0.675% H = 0.675 g H

Example #1 Continue element to moles using the molar mass.  Step 2: Convert the mass of each element to moles using the molar mass.

Example #1 Continue smallest mole value.  Step 3: Divide each mole value by the smallest mole value.  Smallest mole value = 0.337

Example #1 Continue  Step 4: If CLOSE, Round to the nearest whole number.  If answer is NOT close to a whole number, you will need to multiply by a factor of 2 or 3. If answer ends with .5, multiply by 2. If answer ends with .3 or .6, multiply by 3.  You MUST multiply EACH element by the factor!  Example #1 Continue  Ca = 1  O = 2.0029 = 2  H = 1.9881 = 2

Example #1 Continue using answers as the subscripts. Step 5: Write Empirical Formula using answers as the subscripts. CaO2H2 = Ca(OH)2

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

Calculating Empirical Formulas Example #3 57.4% Carbon 6.16% Hydrogen 9.52% Nitrogen 27.18% Oxygen

Calculating Molecular Formulas Step 1: Find the molar mass of the empirical formula. Step 2: Divide the molecular mass by the empirical mass (big number by small number) Step 3: Multiply answer by each subscript in the empirical formula to get molecular formula.

Calculating Molecular Formulas  Example #1: What is the molecular formula of a compound whose molar mass is 60.0 g/mol and empirical formula is CH4N?

Calculating Molecular Formulas  Example #2: What is the molecular formula of CH3O if its molar mass is 62 g/mol?

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

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