G chem question

[horns1011]

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Can someone please explain this question to me? I'm having some trouble understanding Topscore's explanation of the answer. It is question #68 on Test #2.


What volume of HCl was added if 20mL of 1M NaOH is titrated with 1M HCL to produce a pH=2?

answer is 20.4mL

 

[rejuvenatedsoul]

first, add enough HCl to neutralize the sol'n: 20ml (according to equation VaNa = VbNb where a=acid, b=base, N=normality V=volume)...When it is neutralized, a 40 ml NaCl sol'n will be produced.

Next, how much more HCl do you need to add to the neutralized sol'n to produce a pH of 2? pH of 2 = [H+] = 1 x 10^-2 M = .01 M

Let x = amount of HCl to be added. so the final volume of the sol'n w/ph 2 is going to be 40 + x.

set up a dilution, since you are basically adding x amount of HCl to get it to become ph = 2...follow formula M1V1 = M2V2:

(1M)(x ml) = (.01 M)(40 + x ml)

solve for x and we get x = .4

so the FINAL volume of the solution that you titrated/neutralized/whatever is 40.4, but you added 20.4 mL to the original solution in order to get it to be ph = 2 🙂

there must be an easier way to explain this. sorry, i was basically teaching myself while i wrote this up.

 

[kanudo]

bump!!! any one else with the easier trick or batter explanation !!!
"rejuvenatedsoul" sorry no offense but i think i am little slow to get what u explained!!!

 

[rejuvenatedsoul]

^haha no worries, i would also like an easier explanation!! 🙂

 

[nyr201]

what were the answer choices? u can prob narrow it down which would make it easier