topscore T2 #68 gchem

[joefosho315]

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Hey guys, I'm having trouble understanding how to solve the following problem:

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

Somehow, the solution conjures up this equation:

(1M)(x mL) = (0.01M)(40 + x mL)

The answer is that 20.4mL of HCl was added. Can someone help me out, it'd be greatly appreciated! 👍

 

[barrett15]

First find how much HCl is needed to neutralize the solution.
N1V1=N2V2
Since both are 1 normality 20 ml HCl is needed to neutralize 20 ml NaOH. So the total solution is 40 ml.
Now use the dilution equation to find out how much more HCl is needed to get to pH = 2.
M1V1=M2V2
(1M HCl)(how much HCl you need to add) = (1.0x10^-2 which is the pH you want)(40 total volume + amount HCl you need to add)
In other words (1M HCl)(x ml) = (.01M HCl)(40 + x ml)
Solve for x get .404.
Now add that to the original amount of HCl added to get 20.4 ml HCl.

Good luck on your test, mine is this week too!