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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: quddit32 on March 17, 2024, 10:37:35 PM
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Deeply appreciate your help in advance
I'm a bit confused about how to calculate the molarity of an ammonium phosphate buffer solution. Could you please provide some guidance?
1.If there is instruction that "prepare a 5.0 mM phosphate buffer solution by mixing 2.5 mM of ammonium phosphate monobasic and 2.5 mM of ammonium phosphate dibasic." However, since monobasic and dibasic contain different amounts of phosphate per molecule, I'm unsure if we should just add the moles of the two chemicals together to determine the final phosphate molarity. Could you clarify if statement 1 is correct?
2. If there is instruction that "prepare a 5.0 mM phosphate buffer solution by adding 5mM of ammonium phosphate monobasic and then adjust the pH to x.x with H3PO4 solution", can we still call this solution a 5.0 mM phosphate buffer? My concern is that we don't know how much H3PO4 solution is added. It doesn't matter for molarity calculation?
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since monobasic and dibasic contain different amounts of phosphate per molecule
They contain the same amount of phosphate per mole of substance, but different amounts of ammonia.
If there is instruction that "prepare a 5.0 mM phosphate buffer solution by adding 5mM of ammonium phosphate monobasic and then adjust the pH to x.x with H3PO4 solution", can we still call this solution a 5.0 mM phosphate buffer?
Depends on the concentration of the phosphoric acid, if it is 0.005 M final concentration will be still 5 mM. If it is different final concentration of the buffer can be (almost) anything.
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Depends on the concentration of the phosphoric acid, if it is 0.005 M final concentration will be still 5 mM. If it is different final concentration of the buffer can be (almost) anything.
However, the instruction says that initial prep already added 5 mM of phosphate salt, and we add more phosphoric acid to adjust the pH - can we still say it is 5 mM phosphate buffer
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If you mix 10 mL of 5 mM phosphate with 10 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix 10 mL of 5 mM phosphate with 20 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix x mL of 5 mM phosphate with y mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
edit: you do understand "phosphate" in a the context of a buffer means ALL forms of HnPO4(3-n)-?
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If you mix 10 mL of 5 mM phosphate with 10 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix 10 mL of 5 mM phosphate with 20 mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
If you mix x mL of 5 mM phosphate with y mL of 5 mM phosphoric acid, what is the "phosphate" concentration?
edit: you do understand "phosphate" in a the context of a buffer means ALL forms of HnPO4(3-n)-?
So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
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Yes,
generally if we say to prepare a 1 M Phosphate buffer, then all phosphates and phosphoric acid is in summery.
To calculate use Henderson Hasselbalch equation and the pKa values for the different phosphates according the pH to achieve.
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So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
Have you tried to calculate?
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Yes,
That's not a correct answer.
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So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
Have you tried to calculate?
Why does it need a calculation to confirm? If you we prepare the 5 mM ammonium phosphate buffer by adding salt to have 5 mM, then adjusting the pH with phosphoric acid, isn't it very obvious that if some of the phosphoric acid will be included as phosphate as it is clear that phosphoric acid is phosphate as well?
My original question in the post was:
"If there is instruction that 'prepare a 5.0 mM phosphate buffer solution by adding 5mM of ammonium phosphate monobasic and then adjust the pH to x.x with H3PO4 solution', can we still call this solution a 5.0 mM phosphate buffer? My concern is that we don't know how much H3PO4 solution is added. It doesn't matter for molarity calculation?"
Based on what you said, it seems obvious that the adjusting the pH with H3PO4 will increase the amount of phosphate in the buffer which has already 5mM of ammonium phosphate monobasic, so the molarity of phosphate will increase and it can't stay the same or decrease.
It is the question doesn't need the calculation. This is the yes or no question. Why do I need to do the calculation?
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It is the question doesn't need the calculation. This is the yes or no question. Why do I need to do the calculation?
Learning opportunity - once you do the calculations you will realize where is the mistake in your thinking.
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Yes,
That's not a correct answer.
Why, what do you mean
You cut out only the Word yes
Should it be NO. For what reason?
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Yes,
That's not a correct answer.
Why, what do you mean
You cut out only the Word yes
Should it be NO. For what reason?
I am assuming you answered
So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
Neither "yes" nor "no" are correct answer.
Sad that you both attempt to guess instead of doing the math.
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Yes,
That's not a correct answer.
Why, what do you mean
You cut out only the Word yes
Should it be NO. For what reason?
I am assuming you answered
So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
Neither "yes" nor "no" are correct answer.
Sad that you both attempt to guess instead of doing the math.
I think YOU are wrong, unless you can explain.
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Based on what you said, it seems obvious that the adjusting the pH with H3PO4 will increase the amount of phosphate in the buffer which has already 5mM of ammonium phosphate monobasic, so the molarity of phosphate will increase and it can't stay the same or decrease.
What is missing in this paragraph is a consideration of the volume of H3PO4. Does that help?
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That is what I meant. If adjusted the pH with phosphoric acid, then the molarity will change, of course the volume will also increase. pH can only adjusted with an different acid like HCl . To increase pH NaOH can be used, what has no effect of the phosphate beside also volume effect.
Example
Lets have a 0,1 M buffer half dihydrogenphosphate and half phosphoric acid. Means both chemicals are 0,05 M
According HH equation is pH = pKa = 2,1 ( first pka)
Volumen should be 100 ml
If add now again 100 ml 0,05 m H3PO4 again the volume increase to 200 ml, the concentration of salt is half because dilution. The concentration of acid is the same, because same concentration is added.
In HH equation pH = pka + log( c salt/c acid)
pH = 2,1 + log (0,5 * csalt/ c acid)
pH drops to 1,79
Molarity changed to 0,075 M
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If add now again 100 ml 0,05 m H3PO4 again the volume increase to 200 ml
Molarity changed to 0,075 M
Now do the math adding 0.1 M H3PO4.
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pH drops more
pH = 2,1 + log (0,025/0,075) = 1,62
Molarity = 0,1 M
And now, what should it proof, pH is changed more and molarity is in this case the same but ratio phosphate and phosphoric acid is different.
The only thing what we can say it is possible to have a constant molarity with different pH by choosing different ratios of phosphate and phosphoric acid.
But it doesn't say if I add some phosphoric acid to a phosphate solution the molarity will be the same, only if molarity of both compounds are the same.
At least M total = M(H3PO4) - M(H2PO4-) + (MH2PO4-) - M(H3PO4)
0,1 = 0,05 + 0,05
0,1 = 0,025 + 0,075
0,1 = 0,01 + 0,09
etc.
pH will result in the quote in HH equation.
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0,1 = 0,05 + 0,05
0,1 = 0,025 + 0,075
0,1 = 0,01 + 0,09
And that's the point - these are all 0.1M phosphate buffers.
Question from the very beginning was whether it is possible to keep the buffer concentration constant while adding acid. OP doubted, but now that you did the math you have proven that it is perfectly possible.
So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
The correct answer is: it can, but it doesn't have to. What happens to the buffer concentration when adding acid depends on the acid concentration. It is not a simple yes/no question without additional clarification.
Actually preparing buffers of a constant concentration by mixing volumes of acid and base of the same concentration as the required one is a standard lab procedure. Makes calculations easier and removes source of possible errors, as the buffer concentration can't differ from the expected one. You still have to check the pH to be sure, but that's the easy part.