[Chickenuggets]

How do you calculate the amount of HCl needed to change the pH of a NH3/NH4+ buffer by 1 unit?
Concentrations of NH3 and NH4+ are both 0.5M and HCl is 1M. Original pH was 10.15 and final after adding HCl was 9.15.
Thanks.

[magician4]

Concentrations of NH3 and NH4+ are both 0.5M and HCl is 1M. Original pH was 10.15

there is something utterly, utterly wrong here ...

regards

Ingo

[Chickenuggets]

Those are probably wrong. Anyway how can you calculate the amount of HCl needed to change a buffer by 1 unit. Don't worry about my concentrations.

OK There was 125mL of NH3 2M and 13.37g of NH4 0.5M and then this was mixed with 375mL of water to make 500mL solution

[Dan]

This should get you started.

You must show you have attempted the question, this is a Forum Rule.

[magician4]

the calculation is done by using the Henderson-Hasselbalch-equation, resolving it for a Δ pH = -1 (due to what you've told us)

problem is, to do so as a control for a real experiment, you'd need good experimental data to begin with.

you don't have those, as the data you're giving are contradictory :

first, we'd need an exact description of what you've done. Now:

13.37g of NH4 0.5M

isn't exactly what I would call such an description: there's no such thing as "NH₄⁺ stand alone"  (let alone "NH4") (I presume you did use 13.37g of NH₄Cl instead), and having no solution, but the pure substance instead, "0.5 M" becomes completely pointless.
... but this is the least of our problems, as this can be corrected easily.

The next problem, however, is a real whopper:

If, indeed , you did prepare a buffer of ammonia and ammoniumchloride, 0.5 M each, 500 mL ...
... your pH must (!) have been exactly at or very very very close to pH = 9.25
pH = 10.15 , like you reported, is definitively impossible for this system.

Anyway, pH = 9.25  is not what you reported, and now plain guesswork begins:

- your pH -meter wasn't worth a damn, defective or you didn't know how to calibrate / use it properly
- your substances weren't what they should have been (but then again: it would take a huuuuuuge degree of degradation/impurities to result in the for-real measured pH ), or your glassware wasn't clean, your "water" came from an exhausted ion-exchange column ...

... or somebody just played a dirty trick on you, and contaminated your experiment with some stiff NaOH solution when you looked the other way , who knows.

Either way: your data are useless.

They are esp. useless, as the amount of 1 M HCl needed for an pH-shift from 10.15 to 9.15  would greatly differ from the amount needed for a shift from let's say 9.75 to 8.75  (it would take approx. double the amount of HCl to achieve the latter).

  missing foundation, hence nothing can be calculated here


regards

Ingo