Chemistry - Equivalence Point for Weak Base and Strong Acid Titration

[katrina007]

Question 4
- What is the approximate pH at the equivalence point when a weak base, like NH4OH (aq), is titrated with a strong acid like HCL (aq) ?

a) 7
b) 5
c) 9
d) 2
e) None of the above. It is...

Question 5
- Metals have loosely held electrons. This explains why metals usually...

a) have high ionization energies
b) have low thermal conductivity
c) form anions
d) are oxidized, rather than reduced
e) form few compounds

If anyone can help me with these questions please let me know. Thanks in advance

- Katrina

 

[symbolipoint]

Q4: Neutrality is pH 7; If you were titrating a strong base, end point will be at about pH 7, but a weak base will be already at a somewhat/slightly lower pH, so end point will be somewhat lower in pH.

Q5: Could you eliminate some possible poor choices and then make an intelligent guess? Loosely held electrons may be easily removed so, what is the electrical effect of this?

 

[Oaksinstructor]

Symbolipoint, be careful, endpoint and equivalence point are not the same.
End point is the point at which the indicator changes color (which would vary depending on what indicator you are using) equivalence point is the point at which the unknown acid or base is completely neutralized, this is pH 7. it doesn't matter what kind of acid or base are involved, at the equivalence point, you are neutral.

Now let's look at that number 5: It tells you that the electrons are only loosely held, and then gives you some options:
a: high ionization energy - Ionization energy is the energy required to remove an electron. If they are only loosely held, then it would not take much energy to remove them, so this one doesn't make much sense.

b:low thermal conductivity - this one is just common sense, does metal conduct heat? heck yes it does, so this one doesn't make any sense either.

c:form anions - ANION means "A Negative ION" you get negative by GAINING electrons, and if you are loosely holding them you're probably going to lose them, not gain them, so THIS one doesn't make sense either.

d:Oxidized, not reduced - again, by definition, you reduce something by adding negative electrons, and oxidize it by taking electrons away... hey... wait a minute, loosely held electrons are easy to take away... this answer could be the one! let's check the last answer just to make sure:

e:form few compounds - compounds are formed when atoms share electrons or give them up (or take them) since these electrons are only loosely held, it will be easy for other atoms to take them (or share them) and form a bond. if it is easy, it will probably happen a lot, so this answer doesn't make sense either meaning we were right with our choice of D.

 

[katrina007]

Hi Oaks,

in my chem book it talks about equivalence point:
- it occurs within the nearly vertical portion of the curve, is the point at which the number of moles of added OH ion equals the same number of moles of H3O ion originally present. At the equivalence point of a strong acid and strong base titration, the solution consists of the anion of the strong acid and the cation of the strong base. (These two ions do not react with water) so it is neutral: pH = 7

But for my question it has a weak base and a strong acid, so why does it have a pH of 7? I'm still confused.

 

[katrina007]

I checked the graphs and it says that with a weak base and strong acid the pH of the reaction is lower than 7.00, so its possible that the answer is either 5 or 2. But I think its (5) because the Cl ion doesn't react with water and same with the OH ion.

 

[Oaksinstructor]

No, it doesn't matter. When the H3O ion concentration is equal to the OH ion concentration, you are at pH 7

 

[katrina007]

how do i know that the the OH and H3O are equal? Are NH4OH equal to HCL ?

 

[chemisttree]

Careful Oaks! There is a trick to the question here. The key is WEAK base and strong acid.

In this situation you are asked to determine the pH of a solution of NH4Cl. This is the species present at the equivalence point. Equivalence point here refers to equal amounts of acid and base.

HCl + NH4OH -----> HOH + NH4Cl (at equivalence point)

But this happens as well:

NH4+ + HOH <------> NH4OH + H+

The pH of a solution of NH4Cl is NOT 7!

The Kb of NH4OH is 1.8 X 10-5. The (original) concentration of NH4Cl is [NH4CL]. Since we know that Ka = Kw/Kb, we substitute in what we know to obtain the Ka of NH4Cl.

Ka = 1X10-14/1.8X10-5 = 5.56 X 10-10

The expression for Ka is:

Ka = [H+][NH4OH-]/([NH4Cl]-[H+])

Lets use a typical concentration of 0.1M for [NH4Cl] even though this is not given in your example. The equation becomes:

Ka = x^2/(0.1-x) = 5.56 X 10-10

Solving for 'x' gives us 7.46 X 10-6.

pH is 5.13

You have good instincts, Katrina007!

 

[katrina007]

Thank-you chemisttree. I thought about what Oaks had said but then I went back in the book and searched for the graphs and said something different about equal point with weak/strong titrations. Anyways, thanks for everyone's help.

But still how did you come to know what the ka of NH4CL is? How do I find that number or is this something that's constant and given?

 

[chemisttree]

My original post had an error. I've since corrected it to read "The Kb of NH4OH is 1.8 X 10-5." Sorry for the confusion.

You might find that value in the back of your book in an appendix or in the CRC. I found it in 2nd edition of Biochemical Calculations by Irwin H. Segel but you can find it in the CRC Handbook of Chemistry and Physics, 84th edition, page 8-46. In the index it is listed under "Dissociation Constants of Inorganic Acids and Bases".

The pKa value is given for ammonia (NH3) as 9.25. These values are given for the conjugate acid of ammonia, NH4+, actually. This gives us a Ka of 5.62 X 10^-10 for the reaction

NH3 + HOH <--------> NH4+ + OH-

Since Kb = Kw/Ka, the Kb is 1 X 10^-14/5.62 X 10^-10 = 1.8 X 10^-5

 

[katrina007]

oh ok. Thank-you for that. I didn't know these values were given. :)
Thanks again.

 

[Oaksinstructor]

Yeah, chemistree is right, I went back and re-read the definitions, The equivalence point definition from my student's textbook was (GASP) wrong! From my own (old and dusty) textbooks the equivalence point is better defined as the point where the acid or base being titrated is completely reacted. And since the salts of weak bases and strong acids are acidic, the equivalence point should come at a pH below 7.

Although technically speaking, I wasn't wrong in the fact that when [OH-] and [H3O+] are equal, you have pH7, that's just not the case at the equivalence point for a weak base and a strong acid because the weak base does not completely dissociate and releases less of its OH- ions into solution.