Empirical Formula vs. Molecular Formula

Chemical Formula

• Gives the combining whole number ratios of the elements in a compound

• C6H12O6

Structural Formula

• Gives the spatial arrangement of atoms in the compound

• Structural formula for H2O is H – O – H

Empirical Formula

• Only gives the types of elements in the compound and the ratio of the elements in the formula

Empirical Formula

• Does not tell exactly how many of the elements are in the compound

Molecular Formula

• Gives you the exact elemental composition of the compound

• Formula of the compound as it would actually exist.

EF vs. MF

Sucrose or table sugar:

Molecular Formula = C6H12O6

Empirical Formula = CH2O

Sample Problem

• The compound adrenaline contains % C = 56.79 % H = 6.56 % O = 28.37 % N = 8.28 by mass. Find the empirical formula.

Empirical Formula

• EF Determination when % Masses are given

Steps to Solve for EF

• Step 1: Sum up all given percentages. If total equals 100%, go to step 2. If total does not equal 100, the missing % is due to one of the component elements.

• Step 2: Convert Mass % to grams.• Step 3: Calculate moles using mole =

gram/molar mass•

Empirical Formula

• Step 4. To get simplest ratios, divide the moles calculated by the smallest calculated mole. You must have a ration of 1 for at least one of the element. (Follow rule for rounding).

• Step 5. You now have the ratios or subscripts for the EF.

Rule of Rounding Molar Ratios

• Mole ratios can only be rounded to the nearest whole number if they are < 0.2 away from the nearest whole number. For ex: 1.95 = 2; 3.18 = 3 and 4. 13 = 4.

• If the mole ratio is > 0.2 away from the nearest whole number, multiply the mole ratio by a certain integer to get it close to the nearest whole number. For ex: 3.5 x “2” = 7; 6.33 x “3” = 18.99 = 19; 4.25 x “4” = 11.

Please Remember• If you have to multiply a mole ratio by an

integer to get close to a whole number, you MUST multiply all the other mole ratios by the same integer.

• “In short, what you do to one mole ratio, you also do to the rest.”

• The ratios give you the subscripts in the EF.

Steps To Determine the Molecular Formula

• Step 1. Now that you have the empirical formula, get the ratio of the “given” molar mass to the empirical formula mass. Ratio = Given Molar Mass

Empirical Formula Mass

* Round ratio to the nearest whole number.

• Please note that the Empirical formula Mass is the sum of the atomic masses of all the elements in the Empirical Formula.

• Step 2. Once the ratio has been determined, multiply all the subscripts in the empirical formula by the ratio. This gives you the Molecular Formula.

Sample Problem

• Caffeine, a stimulant found in coffee, contains 49.5 % C, 5.15% H, 28.9 % N, and 16.5 % O by mass. The molar mass of the compound is 195 g/mol. Determine the empirical and molecular formula of caffeine.

Sample Problem

• Ibuprofen, a headache remedy, contains 75.69 % C, 8.80% H, and 15.51 % O by mass. The molar mass of the compound is 206 g/mol. Determine the empirical and molecular formula of ibuprofen.