11.4 – Empirical and Molecular Formulas

Percent Composition Every chemical compound has a definite composition. What law is this referring to? The composition of a compound is usually stated as the percent by mass of each element in the compound. The type of chemist whose job it is to identify the elements & their percent by mass in a compound is an analytical chemist.

The equation used to determine the percent composition of an element in a compound is:

Percent Composition Example Problems: 1) Determine the percent by mass of each element in calcium chloride (CaCl 2 ). K:UK: 36.11% Ca and 63.89% Cl

2) What is the percent of oxygen in H 3 PO 4 ? K:UK: 65.31% oxygen

3) Calculate the percent composition of a compound that is 29.0 g of Ag with 4.30 g of S. K:UK: 87.1% Ag and 12.9% S

4) Which has the larger percent by mass of sulfur, H 2 SO 3 or H 2 S 2 O 8 ? K:UK: H 2 SO 3  39.06% S

Empirical Formula The information from the percent composition can be used to determine the formula for a compound. The empirical formula is the simplest whole-number ratio of atoms of elements in the compound. In many cases, the empirical formula is the actual formula for the compound. EX: H 2 O or H 2 SO 4

Molecular Formula For many compounds, the empirical formula is not the true formula. Examples: CH 2 (empirical formula) vs. C 2 H 4 (molecular formula) The molecular formula identifies the actual number of elements in a molecule. Sometimes the empirical and molecular formulas are the same. Ex: Water H 2 O To determine the molecular formula the molar mass of the compound must be determined through experimentation and compared with the mass represented by the empirical formula.

Notice that the molecular formula for acetic acid (C 2 H 4 O 2 ) has exactly twice as many atoms of each element as the empirical formula (CH 2 O). The molecular formula for a compound is always a whole-number multiple of the empirical formula.