The order of a reaction with respect to a reactant with respect to a reactant is the power of concentration term of that reactant to which the rate of reaction is directly proportional. The overall order of a reaction is the sum of powers or exponents to which the concentration terms are raised in the rate law expression.

Molecularity is defined as the number of reactant species which collide simultaneously to form the products. It is always a positive integer like 1,2,3.. Higher molecularity (>3) are rare because chances of collisions of more than three molecules at a time are very less. Molecularity is defined only for elementary reactions. There is no meaning of molecularity for complex reactions. 

For the given reaction, A + B → products. Following data were obtained:

What is the order of reaction with respect to B?

Zero order

r = k [A]^x[B]^y 

0.05 = k [0.1]^x[0.1]^y       ......(i)

0.05 = k [0.1]^x[0.2]^y       ......(ii)

Divide (ii) by (i), we get

1 = [2]^y 

2⁰ = [2]^y 

y = 0

The order of reaction with respect to B is 0.