Empirical and Molecular FormulasChapter 10: Section 4

Balance the Following Compounds

• Ca Br

• Mg S

• Li N

• K S

• B O

• B (O H)

• Al O

• B I

• Sr F

• H (P O4)

Khan Academy

• Molecular and Empirical Formulas

Salman Khan

Percent Composition

•Percent composition explains to you, how much of an element, by mass, makes up a compound.

Percent Composition

• If you have 100 g of a compound which contains 55 g of element “X” and 45 g of element “Y”, to find the percent by mass you use the following formula:

• Percent by mass = [mass of element / mass of compound] * 100

Percent Composition

• [55 g / 100 g] * 100 = 55 % for element X

• [45 g / 100 g] * 100 = 45 % for element Y

Percent Composition

• Water

• Hydrogen mass = 2.016 g

• Oxygen mass = 15.999 g

• Molar mass = 18.015 g

• % Hydrogen

• % H = [2.016 g / 18.015 g] * 100

• % H =

• % O = [15.999 g / 18.015 g] * 100

• % O =

Percent Composition

• Na (H C O3)

• Na = 22.99 g

• H = 1.008 g

• C = 12.01 g

• O = 48.00 g

• Na (H C O3) = 84.01 g

• % Na = [22.99 g / 84.01 g] * 100

• % H = [1.008 g / 84.01 g] * 100

• % C = [12.01 g / 84.01 g] * 100

• % O = [48.00 g / 84.01 g] * 100

Percent Composition

• % Na = 27.37 %

• % H = 1.200 %

• % C = 14.30 %

• % O = 57.14 %

• Total = 100%

• The total is slightly off due to round off error.

Balance the Following Compounds Answers

• Ca Br2

• Mg S

• Li3 N

• K2 S

• B2 O3

• B (O H)3

• Al2 O3

• B I3

• Sr F2

• H3 (P O4)

Percent Composition

• Find the percent by mass for each of the following:

1. H Cl

2. H (N O3)

3. S2 (S O4)

4. H3 (P O4)

5. Sodium Carbonate

6. Iron (III) Bromide

7. Aluminum Oxide

Evaluate each answer… Your total should be 100% (or very close)

H Cl

• H 1.008 g

• Cl 35.453 g

• H Cl 36.461 g

• H% = [1.008 g / 36.461 g] * 100

• H% = 2.84

• Cl% = [35.453 g / 36.461 g] * 100

• Cl% = 97.24

• 2.84 + 97.24 = 100.08%

H (N O3)

• H 1.008 g

• N 14.007 g

• O 47.997 g

• Total = 63.012 g

• H% = [1.008 / 63.012] * 100

• H% = 1.60

• N% = [14.007 / 63.012] * 100

• N% = 22.23

• O% = [47.997 / 63.012] * 100

• O% = 76.17

• Total = 100.00

S2 (S O4)

• S 96.198 g

• O 63.996 g

• S2 (S O4) 160.194

• S% = [96.198 / 160.194] * 100

• S% = 60.05

• O% = [63.996 / 160.194] * 100

• O% = 39.95

• Total = 100.00

H3 (P O4)

• H 3.024 g

• P 30.974 g

• O 63.996 g

• H3 (P O4) 97.994

• H% = [3.024 / 97.994] * 100

• H% = 3.09

• P% = [30.974 / 97.994] * 100

• P% = 31.61

• O% = [63.996 / 97.994] * 100

• O% = 65.31

• Total = 100.01

Sodium CarbonateNa2 (C O3)

• Na 45.980 g

• C 12.011 g

• O 47.997 g

• Na2 (C O3) 105.988

• Na% = [45.980 / 105.988] * 100

• Na% = 43.38

• C% = [12.011 / 105.988] * 100

• C% = 11.33

• O% = [47.997 / 105.988] * 100

• O% = 45.29

• Total = 100.00

Iron (III) BromideFe Br3

• Fe 55.87 g

• Br 239.712 g

• Fe Br3 295.582 g

• Fe% = [55.87 / 295.582] * 100

• Fe% = 18.90

• Br% = [239.712 / 295.582] * 100

• Br% = 81.10

• Total = 100.00

Aluminum OxideAl2 O3