Inorganic Chemistry

Empirical Formula Molecular Formula

Empirical FormulasIn chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. You can determine the empirical formula of a compound based on: a. mass relations of elements obtained from experimental data b. percentage and composition of the compound.

Example of Empirical Formulas Sample Problem: 10.545 of aluminum burns completely in oxygen, producing 1.030g of aluminum oxide. Find the empirical formula of the oxide. Solution: You may solve the given problem by tabulating the given masses(in grams) of the elements involved, then dividing each by the corresponding atomic mass to convert to moles.

To find the simplest whole number ratio, divide by the smallest value. If you get unusual decimals like 0.5, 0.33, and 0.25 are rounded off to the nearest whole number.ELEMENT MASS (g) ATOMIC MASS (g) 27.0 16.0 MOLE

Example of Empirical FormulasMOLE RATIO 1 1.5

Al O

0.545 0.485

0.020 2 0.030 3

1 Al x 2 = 2 Al 1.5 O x 2 = 3 O Thus, the empirical formula is AlO.

Example of Empirical FormulasSample Problem: 2 A platinum (II) compound, which is used to treat tumors, contains 65.0% Pt, 23.6% Cl, 9.35% N, and 2.05% H by mass. Calculate its empirical formula. Solution: Dividing the relative mass gives the relative number of moles of atoms of the element.

Example of Empirical FormulasA tabular solution is shown below:Element Pt Cl N H Mass (g) 65.0 23.6 9.35 2.05 Atomic Mass (g) 195.1 35.5 14.0 1.0 Mole 0.333 0.665 0.668 2.05 Mole Ratio 1 2 2 6

The empirical formula is PtCl N H

Molecular FormulaThe molecular formula of the substance is a whole number multiple of the empirical formula. Below are some samples that illustrates the relationship.Multiple x Empirical Formula 2 x NH Molecular Formula/Name

3 x CH 6 x CH O

hydrazine C H , propene C H O , glucose

N H ,

To determine the molecular formula of a substance, the empirical formula and the molecular mass must be known.

Molecular FormulasSample Problem: 1 Caffeine found in tea is a white solid that contains 49.5% carbon, 5.20% hydrogen, 28.8% nitrogen, and 16.5% oxygen by mass. Its molecular mass is 194.1 g/mole. Find the molecular formula. Solution: Calculate first for the empirical formula.Element Mass (g) Atomic Mass (g) 12.0 1.0 14.0 16.0 Mole Mole Ratio

C H N O

49.5 5.20 28.8 16.5

4.125 5.20 2.06 1.03

4 5 2 1

Molecular FormulasThe empirical formula of caffeine is CHNO. To get the molecular formula: Molecular mass = 194.1 Formula mass of CHNO = 97 (CHNO)x = 94.1 97x = 194.1 x=2 The molecular formula is (CHNO)2 = CHO

Sample Problem: 2 Cholesterol, which has a molar mass of 368 g/mole, is composed of 84.0% C, 11.85% H, and 4.15% O by mass. Find its molecular formula. Solution: Determine the simplest formula.Element Mass (g) Atomic Mass (g) 12.0 1.0 16.0 Mole Mole Ratio

Molecular Formulas

C H O

84.0 11.85 4.15

7.0 11.85 0.259

27 46 1

The simplest formula of cholesterol is CHO. To get the molecular formula: Molecular mass = 386 Formula mass = 386 (CHO)x = 386 386x = 386 x=1 The molecular formula of cholesterol is CHO

Molecular Formula