Define oxidation number. find the oxidation number of a. carbon in co2. b. sulfur in h2so4. c. phosphorus in po4 3- d. manganese in mno4 2-

An oxidation number is a number that is assigned to an atom in a substance. The oxidation number could be positive, negative, or zero depending on if the electrons are lost or gained. 

Generally Oxygen has -2 oxidation number because it gains 2 electrons.
and a hydrogen atom has +1 oxd. no.

Based on the above info, we can calculate for others.

a) CO2 
x + 2 (-2) = 0     [We equate with 0 because net charge is 0)
so, x = +4
Carbon ox. no. is = +4

b) H2SO4
    2(+1) + x + 4 (-2) = 0
    2  +x - 8 = 0
so, x = 6  (sulphur)

c) (PO4)3-
   x +4(-2) = -3
 x = +5 (Phophorus)

d) (MnO4)2-
 x + 4(-2) = -2
x = +6 (Manganese)

A.(nh4)(2co3)is the correct answer 

False, as a substance changes from a solid to a liquid and from a liquid to a gas the molecules get farther apart from each other