Destroyer Gen Chem Question

[KappaSig]

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I think this problem might be written wrong...

How many ml of .120M KOH must be added to 60ml of .20M HF to produce a solution with a pH of 3.3? (pKa of HF is 3.3)

they say the answer is 5 ml

i believe it to be 50 ml


can someone please correct me if i'm wrong and tell me why?

 

[flower123]

you are right..it is 50ml and that's what the destroyer says too. In order to neutralize it MV=M2V2 you need 100ml. But to reach half equivalence pt where the pH=pKa you only need to add half of it which is 50ml.

 

[KappaSig]

i see i think they just messed up the numbers in mine to give me 5ml thank you


can you also explain another question for me please,


Consider the equation below: What is the coefficient needed for I^- to balance the equation:

Cr2O7 ^-2 + H ^+1 + I^-1 --> Cr ^+3 + H20 + I3 ^-1

so they do the reactions and do haf-reactions which i always have trouble with

they said they get:

6e^- + 14H^+1 + Cr2O7^-2 --> 2Cr+3 + 7H2O

please explain to me how they got 6e^-

then they get 3I^-1 --> I3^-1 + 2e^-

please explain to me how they got the 2e^-

 

[flower123]

all the questions are explained in the solutions.

But basically you have to split the equation into 2 parts.

First equation:
Cr2O7^-2 + H^(+1) --> Cr+3 now balance this...you need 7 oxygen on the right side so add 7H20 and then you need 14 Hydrogen molecules on the left side so balance that..also 2 cr on the right side. Then balance the electrons..you have +14 and -2 charge on the left side total or +12 and you have +6 on the right side. so now add 6 electrons on the left side to balance the charge.

second equation:
I- --> I3-
you need to balance the I's so add coefficient 3 on the left side. Now you have -3 charge on the left and -1 charge on the right so add 2electrons on the right side.

Now you have to add the equations together and make the electrons the same. So you multiply the 2nd equation by 3 so you have 6 electrons on the right side which will cancel out with the 6 electrons on the left side of the first equation. The answer to the question is that you need 6I- when the equation is balanced.

Not sure if this helps but try looking at the solution in the back so you can see it better.