Transcript Chem A Week 12 Empirical Forumla and Molecular Formula

Determining the Empirical Formula for a Compound
•
The empirical formula for a compound is the simplest __________
whole
number __________
of the atoms in the compound.
ratio
Examples: H2O is the empirical formula for water. (can’t be reduced)
C1H2O1
Glucose, C H O , has the empirical formula ____________.
6
12
6
Step 1: Change % to grams.
Step 2: Convert grams to moles using the mole chart.
Step 3: Divide each of these answers by the smallest number.
Step 4: Your answers will be the subscripts for the empirical formula.
Helpful Rhyme: % to mass, mass to mole, divide by small, times ’til whole.
Practice Problems:
Calculate the empirical formula if a compound is found to be
composed of 7.8% Carbon and 92.2% Cl
1. Assume 100g since % , just change to % sign to g.
2. Find moles of each element
7.8g C
1 mole
(% to mass)
(mass to moles)
=
0.65 moles C
=
2.63 moles Cl
12.0 g C
92.2g Cl
1 mole
35.0 g Cl
2
3. Divide each mole found by lowest mole. (Round to whole #’s)
(÷ by small)
0.65 mole C = 1
0.65
2.63 mole Cl = 4
0.65
4. Final answer, putting numbers in as subscripts.
C1Cl4
CCl4
3
Practice Problems:
Calculate the empirical formula if a compound is found to be
composed of 42.9% C and 57.1% O
1. Assume 100g since % , just change to % sign to g.
2. Find moles of each element
42.9g C
1 mole
=
(% to mass)
(mass to moles)
3.57 moles C
12.0 g C
57.1g O 1 mole
=
3.57 moles O
16.0 g O
4
3. Divide each mole found by lowest mole. (Round to whole #’s)
(÷ by small)
3.575 mole C = 1
3.569
3.569 mole O =1
3.569
4. Final answer, putting numbers in as subscripts.
C1O1
CO
5
Determining the Molecular Formula for a Compound
•
The molecular formula for a compound is either the same as the
empirical formula ratio or it is a “_________
whole # _________
multiple of this
ratio. It represents the true # of atoms in the molecule.
Examples: 1) H2O is the empirical & molecular formula for water.
2) CH2O is the empirical formula for sugar, ethanoic
acid, and methanol. The molecular formula for
6
glucose is C6H12O6, (___times
the empirical ratio!)
Step 1: Determine the empirical formula for the compound. (See the
previous steps in the notes.)
Step 2: Determine the “whole # multiple” by dividing the molecular
formula mass (given in the problem) by the empirical formula
molar mass. Multiply each of the empirical ratios by this
whole number.
Practice Problems:
1) An unknown compound is composed of 5.9% hydrogen and 94.1%
oxygen. The molecular formula mass is 34 g. Determine the
molecular formula for the compound.
(mass to moles) 5.9 g H ÷ 1.0 = 5.9 moles H
H = 5.9% = 5.9 g
O = 94.1% = 94.1 g
94.1 g O ÷ 16.0 = 5.9 moles O
H5.9O5.9
5.9
(÷ by small)
H1O1
HO
5.9
Molar mass of H1O1 = 17 g
(34 ÷ 17 = 2)
[H1O1 ]x 2 =
Molecular formula mass divided by molar mass
H2O2
Practice Problems:
2) An unknown compound is composed of 40% carbon, 6.6%
hydrogen, and 53.4% oxygen. Determine the molecular formula for
the compound if the mass of one mole of the compound is 120 g.
(mass to moles) 40 g C ÷ 12.0 = 3.3 moles C
C = 40% = 40 g
H = 6.6% = 6.6 g
6.6 g H ÷ 1.0 = 6.6 moles H
O = 53.4% = 53.4 g
53.4 g O ÷ 16.0 = 3.3 moles O
C3.3H6.6O3.3
3.3 3.3
(÷ by small)
3.3
Our formula mass = 30 g
(Compare)
C1H2O1
CH2O
(120 ÷ 30 = 4) [C1H2O1 ]x 4 =
C4H8O4