Answer :
Answer
(a) KNO₃: The oxidation number of the bolded element N is +5
(b) AlH₃: The oxidation number of the bolded element Al is +3
(c) H₂PO₄⁻: The oxidation number of the bolded element P is +5
Explanation
Note: The sum of the oxidation numbers of elements in a neutral compound is zero
(a) KNO₃
Potassium belongs to group 1, and oxygen belongs to group 6 so their oxidation numbers are +1 and -2 respectively.
The oxidation number of the bolded element N is calculated as follows:
KNO₃ = 0
K + N + 3(O) = 0
+1 + N + 3 (-2) = 0
+1 + N - 6 = 0
N = +6 - 1
N = +5
(b) AlH₃
Note: The oxidation state of hydrogen in hydrides is -1.
AlH₃ = 0
Al + 3(H) = 0
Al + 3(-1) = 0
Al - 3 = 0
Al = +3
(c) H₂PO₄⁻
H₂PO₄ = -1
2(H) + P + 4(O) = -1
2(+1) + P + 4(-2) = -1
+2 + P - 8 = -1
P = +8 - 2 - 1
P = +8 - 3
P = +5