Calculating

Empirical Formula

Empirical Formula

Tells the simplest whole number molar ratio of elements in a compound

To Calculate Empirical Formula

Assume 100 g of compound Convert percent composition of each element to

grams Find the number of moles of each element Use moles as subscripts for formula Find the simplest whole number ratio by

dividing all subscripts by smallest subscript

Sample Problem 7-12

A compound contains 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. What is the empirical formula?

Assume 100 g of the compound and convert each percent to grams: 32.38g Na 22.65 g S 44.99 g O

Sample Problem 7-12 Continued Calculate moles:

32.38 g Na x 1 mole/22.99 g = 1.408 22.65 g S x 1 mole/32.07 g = 0.7063 44.99 g O x 1 mole/16.00 g = 2.812

Use moles as subscripts Na

1.408S

0.7063O

2.812

Divide by smallest subscript Na

1.408/0.7063S

0.7063/0.7063O

2.812/0.7063

Na1.993

S1O

3.981

Round to get final answer: Na2SO

4

Sample Problem 7-13 Analysis of a 10.150 g sample of a compound

that contains only phosphorous and oxygen indicates the phosphorous content is 4.433 g. What is the empirical formula?

Find the mass of oxygen: 10.150 g – 4.433 g = 5.717 g O

Find the moles: P: 4.433 g x 1 mole/30.97 g = 0.14314 mole P O: 5.717 g x 1 mole/16.00 g = 0.35731 mole O

Sample Problem 7-13 Continued

Use moles as subscripts: P

0.1431 O

0.3573

Divide by smallest mole P

0.1431/0.1431 O

0.3573/0.1431

P1O

2.497

Multiply to get whole number ratio P

2O

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