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EXPERIMENT 12: Empirical Formula of a Compound

INTRODUCTION

Chemical formulas indicate the composition of compounds. A formula that gives only the simplest ratio

of the relative number of atoms in a compound is the empirical formula or simplest formula. The ratio

usually consists of small whole numbers. We call a formula that gives the actual numbers of each type of

atom in a compound the molecular formula. The numbers in a molecular formula will be whole number

multiples of the numbers in an empirical formula. To determine the molecular formula of a compound,

we need to know both the empirical formula and the molar mass of the compound.

Benzene, for example, has an empirical formula of CH. In a molecule of benzene, the number of carbon

atoms (C) and hydrogen atoms (H) are the same. The molar mass of benzene is 78.11 g/mol.

We can determine the molecular formula of benzene by first calculating the molar mass of the empirical

formula, which is 13.02 g/mol. We then determine the number of empirical units in a molecule by

dividing the molar mass of benzene by the empirical molar mass:

Number of empirical units = (78.11 g/mol) / (13.02 g/mol) = 6 empirical units

Multiplying the empirical formula by 6 gives the molecular formula of benzene, (CH)

6 or C

Experimentally, we can determine the empirical formula of a compound by first finding the mass of each

element in a sample of the compound. We then convert the mass of each element to the equivalent

number of moles of that element.

To find the simplest formula of a compound, we will combine the elements in the compound under

conditions that allow us to determine the mass of each element. From these data, the moles of atoms of

each element may be calculated. By dividing the numbers to the smallest number of moles, you obtain

quotients that are in a simple ratio of integers or are readily converted to such a ratio. The ratio of moles

of atoms of the elements in a compound is the same as the ratio of individual atoms that is expressed in an

empirical formula.

Example: Suppose that we want to determine the empirical formula of the oxide that formed when we

ignited 0.175 g of aluminum (Al) in an open container to produce a compound of Al and O that weighed

0.331 g. The gain in mass is due to the presence of oxygen atoms that combined with the aluminum

atoms in the reaction.

First, we find the number of grams of each element in the sample of the compound. We know we started

with 0.175 grams of Al, so we can calculate the grams of O in the compound by subtracting the mass of

Al from the mass of compound:

Mass of O = 0.331 g – 0.175 g = 0.156 g

Next, we find the number of moles of each element in the compound:

Empirical Formula Of A Compound Worksheets