Titration explanation - TopScore

[FreeFallin]

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Here's the question on TopScore:

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



It gives an explanation that's not making much sense. Can anyone explain how to do this problem? Thanks!

 

[Danny289]

Here's the question on TopScore:

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



It gives an explanation that's not making much sense. Can anyone explain how to do this problem? Thanks!

first you have to get rid of your OH- in soultion that you need 20 ml of HCl:

M1V1=M2V2 ===. 20 x1 = V x 1 ==> V HCl = 20 ml
( honestly You don't need this part you can do that without any calculation. I wrote it for clearification 🙂 )

Now you have 40 ml total soultion that want to have pH= 2 that you must add extera HCl
set above formula one more time just for your original HCl and new Volume;
pH= 2=== > [H+] =0 .01
1 x V = .01 x 40 ====> V =0.4 ml

20+ 0.4= 20.4 ml HCl < ===== answer

I don't know about topscore solution 🙂

 

[herkulease]

You have a strong base and a strong acid. so you would get a neutral product right?

But here the pH is 2 so that tells us we have added a bit more of the HCl to make the solution acidic.

so with a pH of 2 that means our [H+] concentration in the final solution is 0.01. (10^-2)

our total volume will be 20ml of naoh, 20 ml of HCl plus more of HCl which we'll call X.

why 20 of each? again strong acid + strong base = neutral so you need to have the same volume in order to neutralize it. So the final volume will be 40mL + X

Now we'll use the equation M1V1 = M2V2

M1 is out concentration of HCl
V1 is the volume of extra HCl needed

1M(x) = 0.01(40+x)
x = 4 + 0.01x
0.99x = 0.4
x = to about 0.4

so now we know that the extra amount of HCl added was 0.4 ml

But it wants the Total volume of HCl added.

so its 20 + 0.4 = 20.4 ml of HCl.

Hope this helps.

I'm wondering was 0.4 of of the answer choices?

 

[FreeFallin]

Thank you guys!! And yes, 20.4 is the correct answer. I really appreciate the explanations!