10.3. Chemical Formulas http://www.ck12.org
FIGURE 10.14
Ball-and-stick model of ethene, C 2 H 4.- In order to find a whole-number ratio, divide the moles of each element by the smallest value obtained in step
2. - If all the values at this point are whole numbers (or very close), each number is equal to the subscript of the
corresponding element in the empirical formula. - In some cases, one or more of the values calculated in step 3 will not be whole numbers. Multiply each of
them by the smallest number that will convert all values into whole numbers. Note that all values must be
multiplied by the same number so that the relative ratios are not changed. These values can then be used to
write the empirical formula.
Sample Problem 10.14: Determining the Empirical Formula of a Compound
A compound of iron and oxygen is analyzed and found to contain 69.94% iron and 30.06% oxygen. Find the
empirical formula of the compound.
Step 1: List the known quantities and plan the problem.
Known
- % of Fe = 69.94%
- % of O = 30.06%
Unknown
- Empirical formula = Fe?O?
Follow the steps outlined in the text.
Step 2: Calculate.
- Assume a 100 g sample.
→69.94 g Fe
→30.06 g O
- Convert to moles.
69 .94 g Fe×
1 mol Fe
55 .85 g Fe= 1 .252 mol Fe30 .06 g O×
1 mol O
16 .00 g O= 1 .879 mol O- Divide both values by the smallest of the results.