Mixtures of acids and bases

1. Oct 25, 2009

w3390

1. The problem statement, all variables and given/known data

You prepared a reaction table for 10.0 mL of 0.1 M NH4Cl with 5.0 mL of 0.1 M NaOH. Please choose all of the following that describe the solution that was produced.

The choices are:

1. a solution containing only a strong base
2. a solution containing a weak acid and a strong base
3. a solution containing a weak acid and a weak base
4. a solution containing only a weak base
5. a solution containing a conjugate acid/base pair
6. a solution containing only a strong acid
7. a buffer
8. a solution containing a weak base and a strong acid
9. a solution containing only a weak acid
10. a solution containing a strong acid and a strong base

2. Relevant equations

N/A

3. The attempt at a solution

I have already completed the table correctly and have found that the produced solution consists of .5 mmol NH4^1+ and .5 mmol NH3.

Using this information I've tried the answers: 5 by itself, and 3 and 5 together. These are both incorrect.

Can anyone help me out on what I'm missing. Any help would be much appreciated.

2. Oct 26, 2009

Staff: Mentor

Solution containing conjugate pair has a specific property - it resists pH changes. Does it ring a bell?

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3. Oct 26, 2009

symbolipoint

You are converting half of the moles of NH4Cl to NH3. So you have about equal amounts of a weak base and a salt of the weak base. What does this mean? What was the pH of the ammonium chloride solution before any strong base was added? What may be the pH after half of the ammonium chloride was turned back into ammonia?

4. Oct 26, 2009

w3390

So it would be 4 and 7? I'm not sure if it would also contain an acid/base conjugate pair.

Last edited: Oct 26, 2009
5. Oct 26, 2009

symbolipoint

w3390, excuse this next guiding question - it is meant to push you toward an answer but you may be still unprepared for it: Starting with the weak base, ammonia in solution, where would you expect the first buffer region if you were in the process of neutralizing it with hydrochloric acid?

6. Oct 26, 2009

w3390

I have no idea what that means or how to answer it.

7. Oct 26, 2009

symbolipoint

I noticed that in your original message question you did not know Relevant Equations, but you found in Attempted Solution, good values for ammonium ion and ammonia. What you need to know about are these, at least to use one of them:

Kb = $$\frac{[NH4][OH]}{[NH3]}$$

Ka = $$\frac{[NH3][H]}{[NH4Cl]}$$

Kw = [H][OH] = KbKa

Kb for ammonia is about 1.85*10-5

Please excuse the lack of showing charges for the ions. The asterisk was used to show multiplication in Kb value.

8. Oct 26, 2009

w3390

I know that the equilibrium concentration of NH4^1+ and NH3 are equal, so that is saying that the OH^1- concentration is equal to Kb. I do not see where this is going.

9. Oct 26, 2009

symbolipoint

You essentially have a formula to choose and only one variable is unknown. The process will be simple basic algebra.

10. Oct 26, 2009

w3390

I'm really sorry, but I cannot figure out what you are saying. What process will be simple algebra. From what it seems like you're telling me, the answer should be that the solution contains only a weak base since the OH^1- concentration is small. But I am probably completely wrong.

11. Oct 27, 2009

Lancelot59

Try writing out the reaction, see what you get.

12. Oct 27, 2009

Staff: Mentor

w3390: what is it in 7?

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methods

13. Oct 27, 2009

w3390

When I write the reaction I get:

NH4^1+(aq) + OH^1-(aq) --> NH3(aq) + H2O(l)

Using this reaction, I figured out that the equilibrium concentrations are as follows:

[NH4^1+]=.033
[OH^1-]=1.8e-5
[NH3]=.033
[H3O^1+]=5.6e-10

After writing the reaction and finding concentrations, I see that there are equal concentrations of acid and conjugate base and very small concentrations of hydroxide and hydronium.

With regards to Borek, what I know about buffer solutions is that they are mixtures of a weak acid and its conjugate base. I believe the NH4^1+ and NH3 combination constitute a buffer solution. Am I correct.

14. Oct 27, 2009

Lancelot59

DING! DING! DING! DING! DING!

Now if you can explain how buffers work you're golden.

15. Oct 27, 2009

Staff: Mentor

Seems like you have just stated that you should include 7 in your final answer.

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16. Oct 27, 2009

w3390

Okay, so just to make sure, am I correct in thinking that the answer should be number 5 and number 7, since I have a buffer and an acid/base conjugate pair?

17. Oct 27, 2009

Staff: Mentor

I think 3 still can be slassified as correct - NH3 is a weak base, NH4+ is a weak acid, they are both present in the solution.

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18. Oct 27, 2009

w3390

Alright, thanks a lot guys.

19. Oct 27, 2009

symbolipoint

I finally attempted two solutions of the main question to find the pH, including other concentrations. Many of my values are the same or close to same, as yours. My main trouble is that I came up with two different answers for the pH. (This could all be cleared up if I would just refer to the old quantitative textbook to reproduce the main Ka formula that I used to rely on so well). In either case, both pH answers were alkaline.