1. Percent Composition, Empirical and Molecular Formulas
Courtesy

2. Calculating Percentage Composition
Calculate the percentage composition of magnesium carbonate, MgCO3.
Formula mass of magnesium carbonate:
24.31 g g + 3(16.00 g) = g
100.00

3. Formulas
Empirical formula: the lowest whole number ratio of atoms in a compound.
Molecular formula: the true number of atoms of each element in the formula of a compound.
molecular formula = (empirical formula)n
molecular formula = C6H6 = (CH)6
empirical formula = CH

4. Formulas (continued) Examples:
Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio).
Examples:
NaCl
Al2(SO4)3
MgCl2
K2CO3

5. Formulas (continued)
Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio).
Molecular:
H2O
C6H12O6
C12H22O11
Empirical:
H2O
CH2O
C12H22O11

6. Empirical Formula Determination
Base calculation on 100 grams of compound. Determine moles of each element in 100 grams of compound.
Divide each value of moles by the smallest of all the values calculated.
If number are not in whole numbers then multiply each number by an integer to obtain whole numbers for each element.

7. Empirical Formula Determination
Sample Problem: Adipic acid contains:
49.32% C, % O , & 6.85% H by mass.
What is the empirical formula of adipic acid?
1.Treat % as mass (g) ASSUME 100g sample
C % = g
O % = g
H % = g
2. Convert mass (g) to moles by dividing by molar mass:
C g / g/mol = _______ mol of C atoms
O g / g/mol = ________mol of O atoms
H g / g/mol = ________mol of H atoms

8. Empirical Formula Determination
Adipic acid contains:
49.32% C, % O , & 6.85% H by mass.
What is the empirical formula of adipic acid?
1.Treat % as mass (g) ASSUME 100g sample
C % = g
O % = g
H % = g
2. Convert mass (g) to moles
C g / g/mol = mol of C atoms
O g / g/mol = mol of O atoms
H g / g/mol = mol of H atoms

9. Empirical Formula Determination
3. Divide each value of moles by the smallest of the values.
Carbon:
Hydrogen:
Oxygen:

10. Empirical Formula Determination
3. Multiply each number by an integer to obtain all whole numbers.
Carbon: 1.50
Hydrogen: 2.50
Oxygen: 1.00
x 2
x 2
x 2 3 5 2
Empirical formula:
C3H5O2

11. Your Turn:
Determine the Empiracle Formula for a compound whose percentage composition is 21.6% Na, 33.3% Cl, and 45.1% O ?
Set up a chart that looks like this…

12. Your Turn:
Determine the Empiracle Formula for a compound whose percentage composition is 21.6% Na, 33.3% Cl, and 45.1% O ?
Set up a chart that looks like this…

13. Finding the Molecular Formula
Once the empirical formula is found, it can be used to determine the molecular formula of a substance.
The molecular formula is just a multiple of the empirical formula.
So if given the molar mass of a substance, we can calculate the molar mass of the empirical formula and divide the two masses to give us the value to multiply our empirical formula by.

14. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
1. Find the formula mass of C3H5O2
3(12.01 g) + 5(1.01) + 2(16.00) = g

15. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
3(12.01 g) + 5(1.01) + 2(16.00) = g
2. Divide the molecular mass by the mass given by the emipirical formula.

16. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
(C3H5O2) x 2 =
C6H10O4
3. Multiply the empirical formula by this number to get the molecular formula.