Chemistry: Determining Molecular and Empirical Formulas

What is molecular and empirical formula?

Molecular formulas tell how many atoms of each element are present in a compound, whereas empirical formulas tell the simplest or reduced ratio of elements in a compound.

Given 93.7 g of Carbon and 6.3 g of H in 100 g of naphthalene, how do we determine the empirical and molecular formulas for naphthalene?

Answer:

The empirical formula of naphthalene is C5H4, and the molecular formula is calculated by multiplying the empirical formula by 2, resulting in C10H8.

To determine the empirical and molecular formulas for naphthalene when given its percent composition by mass and molar mass, a common approach involves assuming a certain mass of the compound—typically 100 grams—to make the math simpler. This allows us to directly convert the percentages into grams. We can then convert these masses to moles by using the molar masses of carbon (12.01 g/mol) and hydrogen (1.008 g/mol).

For naphthalene with 93.7% carbon: 93.7 g C × (1 mol C/12.01 g C) = 7.802 mol C For the 6.3% hydrogen: 6.3 g H × (1 mol H/1.008 g H) = 6.250 mol H

Now we find the simplest whole-number ratio of moles of carbon to moles of hydrogen by dividing both by the smallest number of moles: 7.802 mol C ÷ 6.250 mol = 1.248 (approximately 1.25) 6.250 mol H ÷ 6.250 mol = 1

The empirical formula of naphthalene is thus C1.25H, which can be simplified to C5H4 (by multiplying both subscripts by 4). Finally, we determine the molecular formula by using the formula weight: Empirical formula mass of C5H4 is (5 × 12.01 g) + (4 × 1.008 g) = 62.052 g/mol. Since the molar mass of naphthalene is 128 g/mol, we divide this by the empirical formula mass: 128 g/mol ÷ 62.052 g/mol = ~2.06 (approximately 2)