1. Chapter 7 Chemical Quantities
7.4 Mass Percent Composition and Empirical Formulas
Learning Goal Given the formula of a compound, calculate the mass percent composition; from the mass percent composition, determine the empirical formula of a compound.
The odor of pears is due to propyl acetate, C5H10O2.

2. Mass Percent Composition
Given the mass of an element in a compound, we can calculate the mass percent composition of that element.
The mass percent of an element in a compound is the mass of an element divided by the total mass of the compound multiplied by 100%.

3. Mass Percent Using Molar Mass
Mass percent composition of a compound can be
calculated using molar mass.
The total mass of each element is divided by the
molar mass of the compound, multiplied by 100%.

4. Guide to Calculating Mass Percent

5. The odor of pears is due to the organic
Learning Check
The odor of pears is due to the organic
compound propyl acetate, C5H10O2. What is its
mass percent composition?
The odor of pears is due to propyl acetate, C5H10O2.

6. Solution
Propyl acetate has the formula C5H10O2; what is
its mass percent composition?
Step 1 Determine the total mass of each element in the formula.
Given: C5H10O2
Need: mass percent composition

7. Solution
Propyl acetate has the formula C5H10O2; what is
its mass percent composition?
Step 1 Determine the total mass of each element in the formula.

8. Solution
Propyl acetate has the formula C5H10O2; what is
its mass percent composition?
Step 2 Divide the total mass of each element by the molar mass and multiply by 100%.

9. Solution
Propyl acetate has the formula C5H10O2; what is
its mass percent composition?
Step 2 Divide the total mass of each element by the molar mass and multiply by 100%.
The total mass percent for all the elements should
be 100%.
58.80% C % H % O = %

10. Empirical Formulas
The empirical formula
is the simplest whole-number ratio of the atoms
is calculated by dividing the subscripts in the actual (molecular) formula by a whole number to give the lowest ratio
C5H10O5  = C1H2O1 = CH2O
actual (molecular) empirical formula
formula

11. Empirical Formulas
The molecular formula is the same or a multiple of
the empirical formula.

12. Learning Check
A. What is the empirical formula for C4H8? (1) C2H4 (2) CH2 (3) CH
B. What is the empirical formula for C8H14? (1) C4H7 (2) C6H12 (3) C8H14
C. Which is a possible molecular formula for CH2O? (1) C4H4O4 (2) C2H4O2 (3) C3H6O3

13. Solution
A. What is the empirical formula for C4H8? (2) CH2
B. What is the empirical formula for C8H14? (1) C4H7
C. Which is a possible molecular formula for CH2O? (2) C2H4O (3) C3H6O3

14. Guide to Calculating Empirical Formula

15. Learning Check
A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.

16. Solution
A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.
Step 1 Calculate the moles of each element.

17. Solution
A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.
Step 2 Divide by the smallest number of moles.

18. Solution
A compound contains 7.31 g Ni and 20.0 g Br.
Calculate its empirical (simplest) formula.
Step 3 Use the lowest whole-number ratio of moles as subscripts.
Ni1.00Br2.00 = Ni1Br2, written as NiBr2

19. Mass Percent
The mass percent of CH4 is 74.9% carbon and
25.1% hydrogen.

20. Converting Decimal Numbers to Whole Numbers
When converting decimals to whole numbers,
decimals greater than 0.1 or less than 0.9 should
not be rounded off.

21. Learning Check
Aspirin is 60.0% C, 4.5 % H, and 35.5 % O.
Calculate its empirical (simplest) formula.

22. Solution
Aspirin is 60.0% C, 4.5 % H, and 35.5 % O.
Calculate its empirical (simplest) formula.
Step 1 Calculate the moles of each element.
A g sample of aspirin contains
60.0 g C, 4.5 g H, and 35.5 g O.

23. Solution
Aspirin is 60.0% C, 4.5 % H, and 35.5 % O.
Calculate its empirical (simplest) formula.
Step 1 Calculate the moles of each element.

24. Solution
Aspirin is 60.0% C, 4.5 % H, and 35.5 % O.
Calculate its empirical (simplest) formula.
Step 2 Divide by the smallest number of moles.

25. Solution
Aspirin is 60.0% C, 4.5 % H, and 35.5 % O.
Calculate its empirical (simplest) formula.
Step 3 Use the lowest whole-number ratio of moles as subscripts.
Multiply each of the subscripts by 4 to obtain a whole number.
C(4×2.25)H(4×2.0)O(4 × 1.00)= C5H8O4