Unit: Chemical Quantities

1
Unit Chemical Quantities
Day 6 - Notes

• Determining the Empirical Formula from Percent
  Composition

2
After today, you will be able to

• Calculate the empirical formulas for various
  compounds given their percent makeup

3
Empirical Formula Determination from Composition

• To determine empirical formulas from
  composition follow these steps
• Assume 100.0g of the compound (change into
  grams)
• Convert g? moles for each element using molar
  mass
• Divide each mole answer by the smallest mole
  answer
• If you do not have a whole number, multiply by
  the smallest whole to get a whole ratio.
• Skip to this step if you already have a whole
  number ratio from 3. These whole s are the
  subscripts for each element.

4
To sum up these steps here is a poem for my
favorite little chemists

• Percent to mass
• Mass to Moles
• Divide by small
• Multiply by whole!

5

• Example What is the empirical formula for a
  compound that is 21.2 N, 6.10 H, 24.3 S, and
  48.4O?

1 mol N 14.01gN
1.51
21.2gN 6.10gH 24.3gS 48.4gO
x


2
1 mol H 1.01gH
6.04
x

7.97

8
N2H8SO4
1 mol S 32.07gS
0.758

x

1
3.03
1 mol O 16.00gO
x


4
6

• Example What is the empirical formula for a
  compound that is 69.8 Fe and 30.2 O?

1 mol Fe 55.85gFe
1.25
69.8gFe 30.92gO
x


1
(2)

2
1.89
1 mol O 16.00gO
1.5
(2)
3
x



Fe2O3
7
Time to Practice!Complete WS 6
HOMEWORK