Answer: 12.18 u
Explanation: The average atomic mass of an element is calculated by taking the weighted average of the atomic masses of its stable isotopes.
In other words, each stable isotope will contribute to the average mass of the element proportionally to its abundance.
Possibly C or D i wanna say sorry if i’m wrong
Answer : The rate law for the overall reaction is, ![Rate=k[NO]^2[H_2]](https://tex.z-dn.net/?f=Rate%3Dk%5BNO%5D%5E2%5BH_2%5D)
Explanation :
Rate law : It is defined as the expression which expresses the rate of the reaction in terms of molar concentration of the reactants with each term raised to the power their stoichiometric coefficient of that reactant in the balanced chemical equation.
As we are given the mechanism for the reaction :
Step 1 :
(slow)
Step 2 :
(fast)
Overall reaction : 
The rate law expression for overall reaction should be in terms of
.
As we know that the slow step is the rate determining step. So,
The slow step reaction is,

The expression of rate law for this reaction will be,
![Rate=k[NO]^2[H_2]](https://tex.z-dn.net/?f=Rate%3Dk%5BNO%5D%5E2%5BH_2%5D)
Hence, the rate law for the overall reaction is, ![Rate=k[NO]^2[H_2]](https://tex.z-dn.net/?f=Rate%3Dk%5BNO%5D%5E2%5BH_2%5D)