Question

Why do some formulae have brackets for the OH? For example- Al(OH)3?

Answer 1

Brackets

Some formulae have brackets in them. For example, sodium hydroxide is NaOH, but magnesium hydroxide is Mg(OH)2. The 2 outside the brackets tells you that you have two of each atom inside the bracket. So in Mg(OH)2 you have one magnesium atom, two oxygen atoms and two hydrogen atoms.

Answer 2

Hey,

Ur answer is here,

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Actually, in formation of a formula by criss cross method the valency is transferred.

For example- Al+3 OH-1
=Al(OH) 3
In this the valency of Al I. E. 3 is transferred to OH.

If we take any another example like

H2SO4(Sulphuric acid)
=the valency of SO4 is - 2 and H is +-1
So when they combined the product formed is H2SO4

If we take a third example I.e.CaCl2
The valency of Ca is +2 and Cl is - 1

When they combined we get CaCl2
In which the valency of calcium Is transferred to chlorine.

In many combination if both elements have same valency I. E. +2 and - 2,then it is canceled by each other.

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Hope it helps you ☺️