Titration question :|

[princessjasmine99]

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This is from topscore, "
What volume of HCl was added if 20mL of 1 M NaOH is titrated with 1M Hcl to produce a pH=2?"
I don't understand these titration problems when they add a desired pH! 🙁

 

[Member 6236]

That problem confused me too :/

 

[Daneosaurus]

If you add equal volume NaOH to HCl, you get a ph=7. So you know it'll 2omL HCl + the difference. Total V=40mL + x. Desired concentration is 10^-ph = 10^-2 = 0.01mol/L. Differnce V of HCl =x. Concentration of HCl = 1mol/L.
(40 + x) * 0.01 = x * 1
0.4 + 0.01x = x
0.4 = 0.99x
x = 0.404 mL

So you add 0.404mL HCl to 20mL HCl to get a total of 20.404 mL HCl.

Disclaimer: You may want to check to be sure, I haven't taken gen chem in almost 2 years and start studying it tomorrow for the DAT.

 

[Pillowcases]

excess * M1 HCl = total volume * M2 HCl
(x-20) * 1 = (x+20) * 10^(-2)
x = 20.40 mL