Question

The sum of the oxidation no of the carbon atom in ch3cho

Answer 1

O is -2, unless it is peroxide = -1 
H is +1 unless bonded to a metal as metalic hydride = -1 

C is more electronegative than H, so C is negative and H is positive. 
The 1st C has 3 H’s = +3, so C = -3 

The bond between the 2 H’s doesn’t affect the oxidation number. 

O is more electronegative than C, so O is negative and C is positive. 
The 2nd C has 1H = +1 and 1 O = -2, so the 2nd C = +1 

The total must equal 0 
(+3 + -3) + (+1 + -2 + +1) = 0 

Carbon shares electrons with other elements. The bonding pair of electrons is attracted toward the atom which is more electronegative. In the C – H bond, H is positive and C is negative. In the C = O bond, O is negative and C is positive. You must examine each bond to determine the “charge” of the C atom. 

For the reason’s explained above, different carbon atoms can have different oxidation states in the same molecule. 

Answer 2

Answer: -2 will be it's answer. As one carbon has -1 oxidation no than for two carbon it will be -2