Formal charge on an atom in a Lewis structure
= [total number of valence electrons in free atom]
- [total number of non-bonding (lone pairs) electrons]
- \(\frac { 1 }{ 2 } \)[total number of bonding or shared electron]
Formal charge on H = 1 - 0 x 2 = 0
Formal charge on N = 5 - 0 x 8 = 1
Formal charge on O (1) = 6 - 4 - x 4 = 0
Formal charge on O (2) = 6 - 4 - x 4 = 0
Formal charge on O (3) = 6 - 6 - x 2 = -1


On solving we get, formal charges on O(1), N and O(2) as 0, +1 and -1 respectively. 
Similarly, on solving we get formal charge on H(1), H(2), O(2), O(3), O(4) ans S, as 0, 0, 0, -1 , 0, -1 and +2 respectively.

Nitrogen dioxide, NO₂

All the similar bonds in a polyatomic molecule do not have the same bond enthalpies. e.g. in H₂O (H-O-H) molecule bond undergoes some change because of changed chemical environment. Therefore, in polyatomic molecules the term mean or average bond enthalpy is used. It is obtained by dividing total bond dissociation enthalpy by the number of bonds broken. 
e.g H₂O(g) -> H(g) + OH(g);
\({ \triangle }_{ a }{ H }_{ 1 }^{ o }=502 \ KJ{ mol }^{ -1 }\)
Oh (g) -> H =  O(g);   \({ \triangle }_{ a }{ H }_{ 2 }^{ o }=427 \ KJ{ mol }^{ -1 }\)
Average O-H bond enthalpy
= \(\frac { 502+427 }{ 2 } \)  = 464.5 Kj mol^-1
The bond enthalpies of O-H bond in C₂H₅OH and H₂O are different because of the different chemical (electronic) environment around oxygen atom.

In H₂O, oxygen atom is sp³ hybridised with two lone pairs. The four sp³ hybridised orbitals acquire a tetrahedral geometry with two corners occupied by hydrogen atoms while other two by the lone pairs.
The bond angle is reduced to 104.5^o due to greater repulsive forces between lp - lp and the molecule thus acquires a V-shape or bent structure (angular structure).

In CO₂ molecule, carbon atom is sp-hybridised. The two sp hybrid are oriented in opposite direction forming an angle of 180^o \(O\overset { \pi }{ = } C\overset { \pi }{ = } O\\ \quad \quad \sigma \quad \quad \sigma \)
That's why H₂O molecule has bent structure whereas CO₂ molecule is linear.