If you want to take out the valency of any of such compounds follow these steps
1. any compound when formed its valency is interchanged or crossed
For example , if there are two ions M2+ and X3- then the compound formed would be......
M3X2
This means two ions of M and three ions of X will form a neutral compound M3X2......
2. Then aluminium sulphate , your given compound is formed by....Al3+ and SO42-......
I hope u got that.....But this is in IX not in VII still curious ones can always learn...
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