Normality question

[joonkimdds]

How many moles of hydrochloric acid must be added to neutralize a 1 liter saturated solution of X(OH)2 if Ksp of this is 4 x 10 ^-18?

First of all, I can set the equation N1V1 = N2V2.

I can get [OH] using Ksp.
and volume of this OH is 1 L

so I have N1V2 = (1L)(2)(1x10^-6) = 2x10^-6.

the answer sa ys hydrochloric acid needs 2x10^-6 moles.

However, it never told me how much volume hydrochloric acid has so why do we assume that its volume is 1?

one of the answer choices was "cannot be determined from the info given" and I picked it but that was not the answer.




Another question I have is,
Ksp = [x][2x]^2 while [2x] is [OH]. If there is a question asking to find [OH] when X solubility is 1x10^-6, is it still 1x10^-6 or 2x10^-6?
and if [OH] is 2x10^-6, shouldn't i multiply by 2 again and make it into 4x10^-6 since normality is number of mol (which is 2) times [OH]?

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[g23armani]

X(OH)2 --> X + 2 OH-

start with 1 mole X(OH)2, let it dissociate Y amount. Resulting concentrations are:

1-Y --> Y + 2Y
X(OH)2 --> X + 2 OH-

Ksp: = [X][OH]^2

Plug in your Y variables

Ksp = [Y][2Y]^2

Ksp = 4Y^3

4 x 10 ^-18 = 4Y^3

Y = 1x10^-6

[OH] = 2Y = 2x10^-6

This is equal to the [HCl] you'd need since it's a strong acid.

 

[GiTsticker]

Here's one that I don't understand from destroyer:
If 20 mL of .012 M solution of Ca(OH)2 is added to 48 mL of HBr, what is the concentration of the HBr?

Is this question flawed? How should we know the concentration of HBr just by adding Ca(OH)2 to it? Orgoman says to use N1V1=N2V2. I understand that we have an N1 a V1, and a V2, but they are from two different chemicals.

When I do dimensional analysis, it doesn't cancel out. I get .01mol OH-/L HBr which IS the right answer, but I don't see how that info is useful.

 

[joonkimdds]

X(OH)2 --> X + 2 OH-

start with 1 mole X(OH)2, let it dissociate Y amount. Resulting concentrations are:

1-Y --> Y + 2Y
X(OH)2 --> X + 2 OH-

Ksp: = [X][OH]^2

Plug in your Y variables

Ksp = [Y][2Y]^2

Ksp = 4Y^3

4 x 10 ^-18 = 4Y^3

Y = 1x10^-6

[OH] = 2Y = 2x10^-6

This is equal to the [HCl] you'd need since it's a strong acid.

Thank you for your hard work but I think you didn't read what I wrote except the very first line.

1st of all, what you found is [OH]. Do you need to x2 to that value since normality is # of mol times [OH] ?

if so, is this answer flawed?

2nd of all, we don't have V1. How do you know the amount of V1?

Here's one that I don't understand from destroyer:
If 20 mL of .012 M solution of Ca(OH)2 is added to 48 mL of HBr, what is the concentration of the HBr?

Is this question flawed? How should we know the concentration of HBr just by adding Ca(OH)2 to it? Orgoman says to use N1V1=N2V2. I understand that we have an N1 a V1, and a V2, but they are from two different chemicals.

When I do dimensional analysis, it doesn't cancel out. I get .01mol OH-/L HBr which IS the right answer, but I don't see how that info is useful.

N1V1 = N2V2 in titration, we use this to make acid = base so that pH=7, in other word neutralize.
They are supposed to be from two different chemicals because acid is different from base. We never use the same chemicals.


Anyway, back to my original question, could someone answer it for me 🙂

 

[azar87]

guy with crazy picture lol git sumn i forget right so u use N1V1=N1V2

you have to react the two species and write the balanced equation after that you have to realize that since there are two Hydroxide groups that the normality isn't equal to the molarity that its actually double after doing that plug in the numbers you should get the correct answer im sorry i didnt solve it out right now but i did that question like yesterday and it stuck out to me so i know what your talken abt.

So again you react the species convert the molarities to the normalities plug in the volumes and you should get it.

 

[GiTsticker]

Joon, once you find [OH-] you no longer have to multiply by 2. The only time you multiply by 2 is if you have something like Ca(OH)2, then you multiply the molarity of CaOH2 by 2 to get the normality which in actuality is moles OH-/L or [OH-].

In fact, for your first question you don't even need to use N1V1=N2V2. Just set up your Ksp equation like g23armani did and set your [OH-] as 2x to reflect the fact that you get 2 moles OH per mole Ca(OH)2. Once again, when you find [OH-] you don't need to multiply by two because you already did when you set it as 2x.

Oh and thanks guys for commenting on my question. Joon, I think you got it when you said the question is implying that we titrate to ph=7 (although the question doesn't specify that I can see that implication.)

 

[g23armani]

Here's one that I don't understand from destroyer:
If 20 mL of .012 M solution of Ca(OH)2 is added to 48 mL of HBr, what is the concentration of the HBr? .

I don't think that question is possible unless they give you the Ksp of Ca(OH)2 and you make the assumption that the OH- in solution from Ca(OH)2 is enough to neutralize the acid...is that all the information?

 

[GiTsticker]

That is the question exactly as it is asked. It could a messed up question...it is the 2008 destroyer. But if we assume that the final pH is 7, then orgoman's method is correct.

Oh and Ca(OH)2 is a strong base, so no Ksp necessary.

 

[joonkimdds]

Joon, once you find [OH-] you no longer have to multiply by 2. The only time you multiply by 2 is if you have something like Ca(OH)2, then you multiply the molarity of CaOH2 by 2 to get the normality which in actuality is moles OH-/L or [OH-].

In fact, for your first question you don't even need to use N1V1=N2V2. Just set up your Ksp equation like g23armani did and set your [OH-] as 2x to reflect the fact that you get 2 moles OH per mole Ca(OH)2. Once again, when you find [OH-] you don't need to multiply by two because you already did when you set it as 2x.

Oh and thanks guys for commenting on my question. Joon, I think you got it when you said the question is implying that we titrate to ph=7 (although the question doesn't specify that I can see that implication.)

so does that mean both the molarity of (OH)2 is same as the normality of (OH)2?

And how about the volume question?

 

[g23armani]

You're getting too caught up in the normality volume equation. That only works well if you have two strong acids / bases. You need to understand the concepts. In order for a base to be neutralized, all the OH- has to be neutralized with H+. Since you have a weak base, you need to use the Ksp (or Kb) to find the concentration of OH- in solution. Then, make sure to use enough acid to produce that same amount of H+.

That's exactly what we did. We solved for the concentration of OH- in a saturated solution of Ca(OH)2. Since HCl is a strong acid and we assume it to dissociate completely, we have just solved for the # of moles of HCl we need to add to 1 liter (since this is equal to the H+ concentration).

Presto, done! No normality equation at all.