Molecular FormulaMolecular Formula

Chapter 7.7Chapter 7.7Molecular FormulaMolecular Formula

Objective:Objective:

(1)(1)To calculate the molecular formula To calculate the molecular formula for a compound.for a compound.

Determining a Molecular Formula from an Determining a Molecular Formula from an Empirical FormulaEmpirical Formula

Step 1: Find the molar mass of the empirical Step 1: Find the molar mass of the empirical formula.formula.

Step 2: Divide the molar mass of the molecular Step 2: Divide the molar mass of the molecular formula by the molar mass of the empirical formula by the molar mass of the empirical formula.formula.

Step 3: Multiply the empirical formula by the Step 3: Multiply the empirical formula by the factor obtained in step 2.factor obtained in step 2.

Example #1Example #1

The empirical formula for a compound is PThe empirical formula for a compound is P22OO55. Its . Its experimental molar mass is 284 g/mol. experimental molar mass is 284 g/mol. Determine the molecular formula of the Determine the molecular formula of the compound.compound.

Example #1Example #1

The empirical formula for a compound is PThe empirical formula for a compound is P22OO55. Its . Its experimental molar mass is 284 g/mol. experimental molar mass is 284 g/mol. Determine the molecular formula of the Determine the molecular formula of the compound.compound.

Step 1: The molar mass of PStep 1: The molar mass of P22OO55 is 141.94 g/mol is 141.94 g/mol Step 2: (284 g/mol) / (141.94 g/mol) = 2.00Step 2: (284 g/mol) / (141.94 g/mol) = 2.00

Step 3: 2 x (PStep 3: 2 x (P22OO55) = ) = PP44OO1010

Example #2Example #2

Determine the molecular formula for a compound Determine the molecular formula for a compound that has a molar mass of 232.41 g/mol and an that has a molar mass of 232.41 g/mol and an empirical formula of OCNCl.empirical formula of OCNCl.

Example #2Example #2

Determine the molecular formula for a compound Determine the molecular formula for a compound that has a molar mass of 232.41 g/mol and an that has a molar mass of 232.41 g/mol and an empirical formula of OCNCl.empirical formula of OCNCl.

Step 1:The molar mass of OCNCl is 77.47 g/molStep 1:The molar mass of OCNCl is 77.47 g/mol Step 2: (232.41 g/mol) / (77.47 g/mol) = 3.00Step 2: (232.41 g/mol) / (77.47 g/mol) = 3.00

Step 3: 3 x (OCNCl) = Step 3: 3 x (OCNCl) = OO33CC33NN33ClCl33

Example #3Example #3

Determine the molecular formula for a compound Determine the molecular formula for a compound that has a molar mass of 32.06 g/mol and an that has a molar mass of 32.06 g/mol and an empirical formula of NH2.empirical formula of NH2.

Example #3Example #3

Determine the molecular formula for a compound Determine the molecular formula for a compound that has a molar mass of 32.06 g/mol and an that has a molar mass of 32.06 g/mol and an empirical formula of NHempirical formula of NH22..

Step 1:The molar mass of NHStep 1:The molar mass of NH22 is 16.01 g/mol is 16.01 g/mol Step 2: (32.06 g/mol) / (16.01 g/mol) = 2.00Step 2: (32.06 g/mol) / (16.01 g/mol) = 2.00

Step 3: 2 x (NHStep 3: 2 x (NH22) = ) = NN22HH44

Example #4Example #4

Determine the molecular formula for a compound Determine the molecular formula for a compound that has a molar mass of 120.12 g/mol and an that has a molar mass of 120.12 g/mol and an empirical formula of CHempirical formula of CH22O.O.

Example #4Example #4

Determine the molecular formula for a compound Determine the molecular formula for a compound that has a molar mass of 120.12 g/mol and an that has a molar mass of 120.12 g/mol and an empirical formula of CHempirical formula of CH22O.O.

Step 1:The molar mass of CHStep 1:The molar mass of CH22O is 30.01 g/molO is 30.01 g/mol Step 2: (120.12 g/mol) / (30.01 g/mol) = 4.00Step 2: (120.12 g/mol) / (30.01 g/mol) = 4.00

Step 3: 4 x (CHStep 3: 4 x (CH22O) = O) = CC44HH88OO44