`(i) C "in " CH_(4)`
Let the oxidation number of c in `CH_(4)` be x
Write the oxidation number of each atom above its symbol
`CH_(4)^(+1)`
Calculate the sum of the oxidation numbers of all the atoms
`x+4(+1)`
Since `CH_(4)` is a neutral molecule the sum of oxiation numbers must be zero.
`therefore " " x+4=0or x=-4`
(ii) Mn in ` KMnO_(4)`
Let the oxidation number of Mn in `KMnO_(4)` be x.
Write the oxidation number of each atom above its symbol
`overset(+1)K overset(x)Mn overset(-2)underset(4)O`
Calculate the sum of the oxidation numbers of all the atoms
`+1+x+4(-2)=1+x-8=x-7`
since `KMno_(4)` is a neutral molecule, the sum of the oxidation numbers must be zero.
`x-7=0 or x=+7`
(iii) Cr in `K_(2)Cr_(2)O_(7)`
Let the oxiodation number of Cr in `K_(2)Cr_(2)O_(7) be x`
Write the oxidation number of each atom above its symbol
`K_(2)^(+1) Cr_(2) O_(7)^(-2)`
Calculate the sum of the oxidation memebers of the atoms
`2(+1)+2(x)+7(-2)=2+2x-14=2x-12`
Since `K_(2)Cr_(2)O_(7)` is a neutral molecule , the sum of the oxidation numbers must be zero.
`therefore " " 2x-12=0 " " or " " x=12//2=6.`
`(iv) P in H_(3)PO_(4)`
Let the oxidation number of `P in H_(3)PO_(4)` be x
Write the oxidation numbers of each atom above its symbol
`overset(+1)H_(3) overset(x)P overset(-2)O_(4)`
Calculate the sum of the oxisdation numbers of all the atoms
`3(+1)+x+4(-2)=3+x-8=x-5`
Since `H_(3)PO_(4)` is a neutral molecule, the sum of the oxidation numbers must be zero.
`therefore x-5=0 or x=+5`
(v) N in `HNO_(3)`
"Let the oxidation number of N in" `HNO_(3)` be X
"write the oxidation numbers of each atom above its symbol".
`overset(+1)H overset(x)N overset(-2)O_(3)`
"Calculate the oxidation numbers of all the atoms"
`+1+x+3(-2)=1+x-6=x-5`
Since `HNO_(3)` "is a neutral molecule , the sum of the oxidation numbers must be zero"
`x-5=-0 or x=x+5`
