I am given the task of preparing three buffer solutions at pH 10, 9.5, and 9.0. I have available concentrated ammonia and 3M hydrochloric acid.
Attempt at Solving
By adding HCl to a solution containing ammonia, it will completely consume the strong acid. Resulting solution will contain ammonia and it's conjugate acid ammonium.
NH3 + H+ --> NH4+
Where [NH4+]final = [HCl]initial
[NH3]final = [NH3]initial - [HCl]initial
I think at this point I should make some assumptions:
500mL buffer to be prepared pKaammonia=9.25
Conc NH3 = 2M Ammonia in ethanol solution
From here I think...
pH = pKa + log ([A-]/[HA])
pH = pKa + log ([NH3]/[NH4+])
pH = pKa + log ([NH3]/([NH3] - [HCl]))
making the pH 10 solution...
1010 = 109.25 + ( log([NH3] - log(1010))
It is pretty visible at this point though that I don't exactly know how to proceed. I have reached one variable but through processes that would not really hold up under scrutiny. The [NH3] that I have as my last variable is either [NH3]initial or [NH3]final but I have no idea how to determine which it is. I also feel I may have applied the wrong method from the start.
Any help at all in general guidance would be great! Thank you everyone.
I am an undergraduate chemistry research student currently in my senior year. I was given this task to be accomplished by next week. I of course will have to provide the calculations and necessary related information though before I will be able to perform any such task.
I am not trying to use this forum to circumnavigate any of that work that I must do. It is however difficult to get time with my research adviser currently so I am just trying to be as prepared as I can be walking into this situation.
I have taken classes were I have been walked through this same process but it definitely seems to be a "if you don't use it, you lose it" sort of deal.
I realize immediately I am new to this forum and am therefore being verbose simply to try and cover all of my bases. I hope that lengthiness in itself isn't more distracting. I have read through the posting rules and the homework questions rules and hope that my post reflects that I have.
I have also searched a great deal looking for related questions but many seem to have fallen victim to going off topic by simply asking a question and not putting any effort into its solution. Once again I hope I have navigated away from this pitfall.
If there is anything I can do to add clarity or stop this post from being thrown out please let me know so I can remedy the situation asap.
EDIT FROM COMMENT MADE BY OP - The buffer capacity desired is 0.1M and this was just gained as additional information. – hotmaildotcom1 16 hours ago
Thank you all for your assistance! Working through it after your guidance this is what I came to. There are more assumptions made that I will explain after the calculations as they will definitely be needed.
So after talking with everyone here I reassessed my approach. I will try and address each of my issues in a bullet point.
- I don't know why it wasn't immediately obvious to me that the NH3 would need to be diluted to a reasonable concentration. This was a point repeatedly made by the community so thank you all again so much. After I made this assumption I accepted the arbitrary starting concentration of 1.M NH3. I redid those calculations as I already had a 5mL volumeteric pipette calibrated, so those are the numbers shown.
- I was struggling with the concept of equilibrium in a significant way. After reading my textbook from analytical, I started to again use ICE tables as can be seen in my work. This greatly simplified the issue for me and I should have done this much sooner.
- I used the definition of pKa to solve for pKa rather than the Henderson-Hasselbalch equation. This is what my textbook used and I was doing well with those examples so I kept with it. It is my understanding that this doesn't change anything.
- I did not calculate any buffer capacity while doing these equations. I assumed that my overall concentrations of compounds used to make the buffer solution would be significantly larger than the molarity of the reaction being buffered. This proved to be a decent assumption (at least so I believe) as in practice a very large amount of reactant can be added and no noticeable change in pH is observed.
All of these calculations were ran by my PI and he signed off on all of them. Having performed the necessary precautions in order to work with these compounds, I carried out making all of these solutions. They all work like a charm without the need to add any additional acid or base in order to reach the desired pH's. I cannot thank everyone enough for your help and I apologize for taking so long to reply.
Finally I realize that this post is not really organized in a reasonable manner anymore in order to help anyone that might dig it up later. I am hesitant to delete any information in order to make things more legible. I simply did not have enough space in the comments sections in order to show that with everyone's help I was able to achieve a reasonable answer. If everything here checks out after review I will delete the "solutions" section and post it as the answer.