Chapter 7 Chemical Quantities 7.5 Molecular Formulas Learning Goal Determine the molecular formula of a substance from the empirical formula and molar mass.

Molecular Formula An empirical formula represents the lowest whole-number ratio of atoms in a compound. A molecular formula represents the actual number of atoms in a compound.

Molecular – Empirical Formula

Molar Mass, Empirical Formula CH2O

Relating the Empirical, Molecular Formula Once we determine the empirical formula, we can calculate its mass in grams. Given the molar mass of the compound, we can calculate the value of the small integer.

Relating the Empirical, Molecular Formula For example, when the molar mass of ribose is divided by the empirical formula mass, the integer is 5.

Relating the Empirical, Molecular Formula Multiplying the subscripts in the empirical formula (CH2O) by 5 gives the formula of ribose, C5H10O5.

Calculating a Molecular Formula Given the empirical formula of ascorbic acid, Vitamin C, C3H4O3, and the molar mass of the molecular formula (176.12 g/mol), we can calculate the molecular formula.

Calculating a Molecular Formula Given the empirical formula of ascorbic acid, Vitamin C, C3H4O3, and the molar mass of the molecular formula (176.12 g/mol), we can calculate the molecular formula.

Calculating a Molecular Formula Given the empirical formula of ascorbic acid, Vitamin C, C3H4O3, and the molar mass of the molecular formula (176.12 g/mol), we can calculate the molecular formula. Multiplying the subscripts in the empirical formula by 2 gives the molecular formula

Guide to Calculating a Molecular Formula from an Empirical Formula

Learning Check A compound is 27.4% S, 12.0% N, and 60.6% Cl. If the compound has a molar mass of 351 g, what is the molecular formula?

Solution A compound is 27.4% S, 12.0% N, and 60.6% Cl. If the compound has a molar mass of 351 g, what is the molecular formula? Step 1 Obtain the empirical formula and calculate the empirical formula mass. Write each percentage as the number of grams in a 100. g sample. S = 27.4 g N = 12.0 g Cl = 60.6 g

Solution Step 1 Obtain the empirical formula and calculate the empirical formula mass.

Solution Step 1 Obtain the empirical formula and calculate the empirical formula mass. Obtain the subscripts of each element.

Learning Check Step 1 Obtain the empirical formula and calculate the empirical formula mass.

Learning Check Step 2 Divide the molar mass by the empirical formula mass to obtain a small integer.

Learning Check Step 3 Multiply the empirical formula by the small integer to obtain the molecular formula. Multiplying the subscripts in the empirical formula by 3 gives the molecular formula.

Concept Map