Question

an element X forms an oxide with formula X2O3
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state the valency of X,write the formula of chloride of X and sulphate of X.

Answer 1

X- Aluminum
Valency- 3
Formula for chloride- AlCl3(3is in sub script)
Sulphate- Al2(SO4)3[3 & 2 in sub script]

Answer 2

Explanation:

We know that oxide is $O^{-2}$ ion hence in the given formula of $\mathrm{X}_{2} \mathrm{O}_{3}$;

X must be +3 by cross method of valency.  

So, we can say that valency of X must be +3. In the chloride, the $Cl^{-1}$ ion is present hence with $X^{+3}$ and $Cl^{-1}$; the formula must be  \mathrm{XCl}_{3}.

Similarly, sulphate has $\mathrm{SO}_{4}^{-2}$ ion therefore the formula of sulphate will be $\mathrm{X}_{2}\left(\mathrm{SO}_{4}\right)_{3}$. Overall we can say that we can write the formula with the help of cross method of valency.