# What is % of Al in Al 2O 3H2O?

bellw | (Level 1) Adjunct Educator

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% Composition of a hydrate - three step process

Step #1: Find the GFW of the compound

Step #2: Divide the elemental mass by the GFW

Step #3: Change quotient to % by multipling by 100

Al2O3 * H2O

Exact Weights                                   Approximate Weights

Al = 2 x 26.970 = 53.840                  Al = 2 x 27 = 54

O = 3 x 15.999 = 47.997                   O = 3 x 16 = 48

H = 2 x  1.008  =  2.016                    H = 2 x 1   =   2

O = 1 x 15.999 = 15.999                   O = 1 x 16  = 16

_______________________              _________________

GFW =  119.852 grams                      GFW =  120 grams

Al = 53.840 / 119. 852 = 0.44922       Al = 54 / 120 = 0.450

Al = 0. 44922 x 100 = 44.922 %         Al = 0.450 x 100 = 45.0%

llltkl | College Teacher | (Level 3) Valedictorian

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In order to find the % of Al in Al2O3, H2O, we need to find the molar mass of Al2O3, H2O and then calculate the mass of Al present therein as the percentage of the molar mass.

Atomic mass of Al is 26.98, or 27(approximately).

Molar mass of Al2O3,H2O = (2*27+3*16)+(16+2*1)=120

Therefore, % of Al in Al2O3, H2O = ( 2*27)*100/120 = 45

In other words, Al2O3,H2O contains 45 percent Al by mass.

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