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.