titration problem

[pistolpete007]

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this problem came straight from achiever test 2...and there was already a thread on this and YES i did look at it the question is
How many milliliters of 0.05 M HCl are required to turn 20 ml of 0.05 M KOH into a solution of pH 2.00?

A.10B.15C.20D.25E.30

everyone on that thread said the answer was 28ml....but there is not option for that?
(40)(0.01)=0.05(x)
x=8
20 + 8=28ml
it makes sense what every1 did how come this isnt an option?

 

[TeamGuo]

I think answer is 30 ml.


You have to have this formula

(0.05 M) (x) = (0.01 M) (40 ml + x)

Since total volume is 40 ml + the amount of acid you are dumping in.

And you get 10 mL.

20 ml + 10 ml = 30 ml as the answer.

 

[osimsDDS]

I think answer is 30 ml.


You have to have this formula

(0.05 M) (x) = (0.01 M) (40 ml + x)

Since total volume is 40 ml + the amount of acid you are dumping in.

And you get 10 mL.

20 ml + 10 ml = 30 ml as the answer.

yes that is correct, because those ppl werent taking into account the .01M times x that you need to subtract from .05x when you bring it over to the other side...since the molarity is very small you do need to subtract .05x-.01x to get .04x and then divide .4/.04 and x=10 then you add 10 to the starting solution of 20 and you get 30ml

 

[Mstoothlady2012]

Obviously you didn't look at the thread properly🙄 Dr. Green corrected us and gave us a very good explanation, look at reply # 11

http://forums.studentdoctor.net/showthread.php?t=536957