[AdiDex]

I have a little confusion in kinetics.
Suppose a reaction is given
2A ----> B
 I have given concentration with respect to time.
I have observed it is a first order reaction.
So i simply applied the formula for calculating the rate constant

k = (ln(A°/A))/t

I got some value by sovlving this so should i divide my answer  by 2 or not ?
I think i should not ( correct me if i'm wrong) .
But if it was an 0 order reaction then i will use

A = A° - kt

So by getting value of k so should i divide it by 2 or not....look at my answer ...here its explanation..!

If i would consider rate of formation of B (which is equal to rate of reaction)
Then i can use
B = B° + kt
But i will get a different value of k from this equation which will be just half of the previous .
So i think i should divide it by 2.

I'm not able to explain what i am thinking now...aaahhh i'm just superconfused ...please someone tell me the correct method with a proper explaination.
I want to write everything which is i'm just thinking right now but i'm not able to ...
Sorry for that..

[mjc123]

If we say
Rate of reaction = -1/2 d[A]/dt = d[B ]/dt = k[A]ⁿ
Then if n = 0, [B ] = [B ]₀ + kt
but d[A]/dt = -2*rate so [A] = [A]₀ - 2kt
If you calculated a rate constant k' for the disappearance of A, such that [A] = [A]₀ - k't, then k' = 2k, so if you want k, you should divide k' by 2.
Similarly, for a first order reaction, (ln([A]₀/[A]))/t will give you k', so divide by 2 to get k.
Always define the process for which your rate constant value applies.

[AdiDex]

http://schools.aglasem.com/?p=2970

Plz check 4.15 .
According to u it should be half of the given answer...!!

[mjc123]

See my comment in bold. The answer is correct for the question it asks. (Assuming the maths is correct, I haven't checked.) It isn't interested in [B ] at all, only in [A], so everything is defined in terms of [A]. They have calculated what I called k' above.
I grant there is some ambiguity in the question, arising from the concept of "rate of reaction". I have said more than once that I hate this usage, and wish people would always talk in terms of the rate of change of a specified reagent, but it is in use and you have to get used to it. In this jargon, for a stoichiometric reaction such as
aA +  bB  cC + dD
the rate of reaction is defined as
Rate = -1/a d[A]/dt = -1/b d[B ]/dt = 1/c d[C]/dt = 1/d d[D]/dt
The given answer violates this usage, and (implicitly, not explicitly) defines rate as -d[A]/dt, ignoring the stoichiometric coefficient, perhaps because there is only one reactant. You are expected to realise this from the question, which is where the ambiguity arises.
Oh, and please write English, not txt spk, see forum rules!