G.chem

[tonykangus]

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

I think that N1V1= N2V2
so, V1=N2V2/N1
=(1)(20)/1
=20 mL
,but topscore answer said 20.4mL.
anybody can explain why?

 

[drdeeeena]

i got 20.2 ml

since the final pH= 2 then the [H+]= 10^-2= 0.01

HCL--> H+ + Cl-
1M 0 M 0M
-0.01M +0.01M 0.01 M
----------------------------
0.99M 0.01M(from pH) 0.01M

then u do c1v1=c2v2=> V2= 20/0.99= 20.2 mL

i am not sure if this is the right answer but sometimes the solution in some textbook are wrong (keep that in mind)

 

[doc toothache]

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

I think that N1V1= N2V2
so, V1=N2V2/N1
=(1)(20)/1
=20 mL
,but topscore answer said 20.4mL.
anybody can explain why?

We know we will need 20 ml of 1M HCl to neutralize the 1 M NaOH. Once we finished the titration the total volume of pH 7 solution is 40ml not 20. To make it a pH of 2 we will need a 0.01M solution.

Then

M1V1=M2V2
0.01 x 40= 1V2
V2= 0.4 (total needed to make the 40 ml pH= 2); total 1 M HCl added 20 ml + 0.4 =20.4

i am not sure if this is the right answer but sometimes the solution in some textbook are wrong (keep that in mind)

And sometimes they are right.