Explanation:
The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbers in a polyatomic (many-atom) ion is equal to the charge on the ion. The oxidation number of oxygen in a compound is usually –2. The oxidation state of hydrogen in a compound is usually +1.
The oxidation state of Al in Al(OH)
4
−
x+4(+1−2)=−1
∴x=+3
The oxidation state of Mn in MnO
2
y+2(−2)=0
∴y=+4
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