What is formula of Aluminium Sulphate?

Question

Accepted Answer

The correct option is (1):

Aluminum sulphate is an ionic compound formed from aluminum ions ($Al^{3+}$) and sulphate ions ($SO_4^{2-}$). To determine its chemical formula, we need to balance the charges of these ions to achieve electrical neutrality. Aluminum is in Group 13 of the periodic table and typically forms a +3 ion ($Al^{3+}$). The sulphate polyatomic ion has a charge of -2 ($SO_4^{2-}$). To balance these charges, we need two aluminum ions to provide a total positive charge of +6 (2 x +3 = +6), and three sulphate ions to provide a total negative charge of -6 (3 x -2 = -6). Therefore, the correct formula represents this ratio, with the polyatomic ion enclosed in parentheses when more than one is present. This leads to the formula $Al_2(SO_4)_3$.

The incorrect option is (2) $AlSO_4$: This formula suggests a 1:1 ratio between aluminum and sulphate ions, which would result in a net charge of ( +3) + (-2) = +1, making the compound not electrically neutral. Thus, it is an incorrect representation.

The incorrect option is (3) $Al_2SO_4$: This formula implies two aluminum ions and one sulphate ion. The total charge would be (2 x +3) + (-2) = +4, again indicating an unbalanced charge and an incorrect chemical formula.

The incorrect option is (4) $Al(SO_4)_2$: This formula suggests one aluminum ion and two sulphate ions. The total charge would be (+3) + (2 x -2) = -1, which is also not electrically neutral, making this formula incorrect.