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Aluminium has valency 3 and sulphate radical SO4 has valency 2. The formula of aluminium sulphate is
AAlSO4 directly
BAl(SO4)2
CAl2(SO4)3
DAl3(SO4)2
Answer & Solution
Correct answer: C. Al2(SO4)3
1. Use the criss-cross rule: write the formula so charges cancel.
2. Al carries +3 and SO4 carries -2.
3. Lowest counts that balance: 2 Al and 3 SO4.
4. Hence Al2(SO4)3.
_Source: Selina Concise Chemistry Class 9, Ch 4 The Language of Chemistry (Mistral OCR'd PDF), section on chemical formula_
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