Calculating Percentage Composition

1
Calculating Percentage Composition
Calculate the percentage composition of magnesium
carbonate, MgCO3.
24.31 g 12.01 g 3(16.00 g) 84.32 g
100.00
2
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
  n integer
• molecular formula C6H6 (CH)6
• empirical formula CH

3
Formulas (continued)
Formulas for ionic compounds are ALWAYS empirical
(lowest whole number ratio).
Examples
NaCl
MgCl2
Al2(SO4)3
K2CO3
4
Formulas (continued)
Formulas for molecular compounds MIGHT be
empirical (lowest whole number ratio).
Molecular
C6H12O6
C12H22O11
H2O
Empirical
H2O
CH2O
C12H22O11
5
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 the
  values.
• Multiply each number by an integer to obtain all
  whole numbers if needed to obtain integers

6
Empirical Formula Determination
Adipic acid contains 49.32 C, 43.84 O, and
6.85 H by mass. What is the empirical formula of
adipic acid?
7
Empirical Formula Determination(part 2)
Divide each value of moles by the smallest of the
values.
Carbon
Hydrogen
Oxygen
8
Empirical Formula Determination(part 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
C3H5O2
Empirical formula
9
Finding the Molecular Formula

• Succinic acid is a substance produced by moss
  this same compound is causes black streaks in
  asaphalt rooftops over time. Chemical analysis
  indicates that the compound is composed of 40.68
  carbon, 5.08 hydrogen and 54.24 oxygen. It has
  a molar mass of 118.1 g/mole. Determine the
  empirical formula and the molecular formula for
  succinic acid.

10
First, use the by mass and convert to moles.

• 40.68 g C 3.387 mole C
• 12.01 g C
• 5.078 g H 5.04 mole H
• 1.008 g H
• 54.24 g O 3.390 mole O
• 16.00 g O

11
Next, calculate the simplest ratio by dividing by
the smallest mole value.

• 3.387 mole C 1 mole C
• 3.387
• 5.040 mole H 1.49 mole H 1.5 mole H
• 3.387
• 3.390 mole O 1.001 mole O 1 mole O
• 3.387

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What is the simplest ratio?

• 1 mole C 1.5 mole H 1 mole O
• You must have whole numbers as subscripts so.
• C 2 x 1 2 mole C
• H 2 x 1.5 3 mole H
• O 2 x 1 2 mole O
• The empirical formula is C2H3O2

13
Now, you will calculate the molecular formula
using the molar mass given.

• 2 moles C x 12.01 g 24.02 g C
• 3 moles H x 1.008 g 3.024 g H
• 2 moles O x 16.00 g 32.00 g O
• The empirical mass of
• C2H3O2 59.04 g

14
Molecular Formula

• (Empirical mass) n Molecular mass
• (59.4 ) n 118.1
• n 2
• C2 (2) H3 (2) O2 (2) C4H6O4

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?
1. Find the formula mass of C3H5O2
3(12.01 g) 5(1.01) 2(16.00) 73.08 g
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?
1. Find the formula mass of C3H5O2
3(12.01 g) 5(1.01) 2(16.00) 73.08 g
17
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. Multiply the empirical formula by this number
to get the molecular formula.
3(12.01 g) 5(1.01) 2(16.00) 73.08 g
(C3H5O2) x 2
C6H10O4