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# The rate of a certain chemical reaction is directly

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Intern
Joined: 25 Dec 2008
Posts: 41
The rate of a certain chemical reaction is directly [#permalink]

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25 Dec 2008, 09:12
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increase by 100%, which of the following is closet to the percentage change in the concentration of chemical A required to keep the reaction rate unchanged?

A) 100% decrease
B) 50% decrease
C) 40% decrease
D) 40% increase
E) 50% increase
Manager
Joined: 10 Aug 2008
Posts: 74
Re: source GMAT CAT [#permalink]

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25 Dec 2008, 09:49
Rate of Reaction is proportional = A^2 / B

If B increased by 100% means B changes to 2B to make the same rate A^2 should balance the effect of B, Means concentration of A should be INCREASED.

So A, B & C option are discarded.

Now if A increased by 40% than A replaces to 1.4A

A^2 should replace with 1.96 x A^2.

This is quite near and SO the answer is D
Director
Joined: 01 Apr 2008
Posts: 872
Name: Ronak Amin
Schools: IIM Lucknow (IPMX) - Class of 2014
Re: source GMAT CAT [#permalink]

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27 Dec 2008, 03:53
Rate = k A^2/B. Now if B=2B, in order to have the same rate we need to multiple the numerator by 2. i.e. A^2 becomes 2A^2.

So we have to increase A by 50%.
What is OA?
Intern
Joined: 25 Dec 2008
Posts: 41
Re: source GMAT CAT [#permalink]

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27 Dec 2008, 04:24
OA : is D

amit is right but can u explain it further pls
Director
Joined: 01 Apr 2008
Posts: 872
Name: Ronak Amin
Schools: IIM Lucknow (IPMX) - Class of 2014
Re: source GMAT CAT [#permalink]

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27 Dec 2008, 04:51
Yes I realized my mistake. Answer should be D.

Rate = K A^2/B , K is some constant( you can ignore for now )

Now if B becomes 2B, then for the equation to remain same we have to multiply numerator by 2.

So, A^2 becomes 2A^2. Now we concentrate only on this, that is how to make A^2 as 2 A^2.

If A is increased by 40% then A becomes 1.4A and so A^2 becomes 1.96 A =(approx) 2 A.
If A is increased by 50% then A becomes 1.5A and so A^2 becomes 2.25 A ( which is not closer than 1.96A)
So answer is D.
Re: source GMAT CAT   [#permalink] 27 Dec 2008, 04:51
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# The rate of a certain chemical reaction is directly

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