The molecular formula of Aluminium Hydroxide is
To get this one needs to know the valencies for the two respective constituents.
The hydroxide ion (
So therefore, because
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
*+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 :)