Question

Aluminium sulfate hydrate al2(so4)3.xh2o contains 13.63% al by mass. calculate x, that is, the number of water molecules associated with each al2(so4)3unit.

Answer 1

mAl₂(SO₄)₃·xH₂O:
(mAl×2) + (mS×3) + (mO×12) + (mH₂O×x)
(27×2)+(32×3)+(16×12)+(x×18) = 342 + 18x [g]
mAl₂: 27×2 = 54 [g]

54g ---------- 13,63%
342+18x ---- 100%
0,1363(342+18x) = 54
46,6146 + 2,4534x = 54
2,4534x = 7,3854
x ≈ 3

>>> Al₂(SO₄)₃·3H₂O <<<<
:)

Answer 2

3 is the number of water molecules associated with each $Al_2(SO_4)_3$ unit.

What are molecules?

Molecules are made up of one or more atoms.

The molecular mass of Al₂(SO₄)₃·xH₂O

(27×2)+(32×3)+(16×12)+(x×18)

= 342 + 18x g

Molecular mass of Al₂:

27×2 = 54 g

54g contains 13.63% Al by mass.

(342+18x)g contains 100%

So,

0.1363 (342+18x) = 54

46.6146 + 2.4534x = 54

2.4534x = 7.3854

x ≈ 3

Hence, 3 is the number of water molecules associated with each $Al_2(SO_4)_3$ unit.

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