Question

Assign oxidation numbers to the underlined elements in the following species:

KAl(SO₄)₂.12 H₂O

Answer 1

+1 3+   x 2-        +1 -2
K   Al   (SO₄)₂ · 12H₂ O

Then, we have

1(+1) + 1(+3) + 2(x) + 8(-2) + 24(+1) + 12(-2) = 0

⇒ 1 + 3 + 2x - 16  24 -  24 =0 

⇒ 2x = 12

⇒ x = +6

or

We can ignore the water molecule as it is a neutral molecule. Then, the sum of the oxidation numbers of all atoms of the water molecule may be taken as zero. Therefore, after ignoring the water molecule, we have

1(+1) + 1(+3) + 2(x) + 8(-2) = 0

⇒ 1 + 3 + 2x - 16 =  0

⇒ 2x = 12

⇒ x = + 6

Hence, the oxidation number of S is + 6.