Topscore 2 GC Question #68

[Ibracadabra]

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

----- I read the solution and it's not clicking.. any one give me some help?Thanks!! 😛

 

[jirotrom]

20.4...

strong base strong acid so there is acid left over if the pH is 2.

so you do M1V1= M2V2

M1= 1M
M2= .01 ( because this = pH2)
V1= x, because we are trying to solve for it
V2= 40ml + x ( because this is what the final volume will be)

 

[luv8724]

I will try my best...
This is a reaction between Hardcore strong base and acid reaction which leads to a complete dissociation.

what you do is start out writing
M1V1=M2V2
M1 = Molar conc. of HCl = 1M
V1 = X
M2 = concentration after adding additional HCl into the solution (since pH=2, [H+]=1X10^-2)
V2 = 0.04L+x
Since this is a strong acid/base dissociation we assume that they dissociate completely (So since we have 20ml of NaOH it will require another 20ml HCl to neutralize it.. then add x amount of volume to make it pH 2)

so... (1M)(X) = (1X10^-2)(0.04+X)
X = 0.0004L = 0.4mL
This means adding 0.4mL of HCl into the neutral solution will cause the pH to be 2.
so 20mL (HCl) + 0.4mL = 20.4mL

Correct me if im wrong..;;;

 

[Ibracadabra]

THe answer is 20.4 mL. Thanks a lot guys. 🙂

 

[OU11BB]

THe answer is 20.4 mL. Thanks a lot guys. 🙂

what were the rest of the answers?

 

[mustudent24042]

Thanks! this was a much better explanation then the one from Top Score!