Molar Mass & Percent Composition

MOLAR MASS

• The mass of 1 mole of an element or compound– The mass in grams from the Periodic Table

Unit = grams per mole (g/mol)

MOLAR MASS

• Step One – Identify the molecular formulaEx: calcium fluoride

• Step Two – Determine the number of atoms of each element

Ca = F =

• Step Three – Multiply the atomic mass by the number of atoms

Ca = x =F = x =78.08 g/mol

CaF2

1 2

40.08 1 40.0819.00 2 38.00 +

MOLAR MASS - Examples

1) PbCl2

Pb = x = Cl = x =

2) Aluminum sulfate Al = S = O =

g/mol+

207.2 1 207.2

35.45 2 70.90278.10

Al2(SO4)3

26.98 x 2 = 53.96

32.07 x 3 = 96.21

16.00 x 12 = 192.00 +342.17 g/mol

MOLAR MASS - Examples

3) Copper (II) chloride Cu = 63.55 x 1 = 63.55

Cl = 35.45 x 2 = 70.90

4) NH4Cl

N = 14.01 x 1 = 14.01

H = 1.01 x 4 = 4.04

Cl = 35.45 x 1 = 35.45

g/mol+

134.45

CuCl2

+53.50 g/mol

PERCENT COMPOSITION• Step One – Identify the molecular formula

Ex: iron (III) oxide • Step Two – Calculate the molar mass of the compound

Fe = 55.85 x 2 = 111.70 O = 16.00 x 3 = 48.00

• Step Three – Divide the mass of the individual elements by the total, then multiply by 100

Fe =

O =

Fe2O3

159.70 g/mol+

111.70159.70

x 100 = 69.9%

48.00159.70

x 100 = 30.1%

PERCENT COMPOSITION1) H2O2

H = 1.01 x 2 = 2.02

O = 16.00 x 2 = 32.00

2) Sn(OH)4

Sn = 118.71 x 1 = 118.71

O = 16.00 x 4 = 64.00

H = 1.01 x 4 = 4.04

34.02 g/mol

/ 34.02 x 100 = 5.9 %

/ 34.02 x 100 = 94.1 %

186.75 g/mol

/ 186.75 x 100 = 63.6 %

/ 186.75 x 100 = 34.3 %

/ 186.75 x 100 = 2.2 %