# Question #3a761

Sep 5, 2015

The molecular formula of Aluminium Hydroxide is $A l {\left(O H\right)}_{3}$, the subscript being 3.

#### Explanation:

To get this one needs to know the valencies for the two respective constituents.

The hydroxide ion ($O H$) has a valency of -1 and is typically written as $O {H}^{-}$ whereas Aluminium has a valency of +3, due to its position in group 3 of the periodic table.

So therefore, because $O {H}^{-}$ has valency of -1 and $A l$ +3 the combination of the two's molecular formula must be equal to zero in order to be stable.

So you find the lowest common multiple (LCM) of the two, which is 3, so thus you have to multiply the -1 of the hydroxide ion to get -3.

Thus the formula is $A l {\left(O H\right)}_{3}$

*+3 of Aluminium -3 of the Hydroxide due to multiplication of -1 with 3, gives you a total of 0, which indicates compound is stable.

Hope I helped :)