Question

I am lost. I have to create a buffer using NH4CL and 0.10 M NH3. I have to chose how much of the reagent to use in order to get a concentraion between 0.01 and .5 M. Then after I come up with the recipe for that I have to calculate the pH baces on the concentrations using the Henderson-Hasselbalch equation. We have not gone over this in class so I'm on my own and lost. Can anyone help??

Answer 1

Usually you know the pH but you don't show that. You know pKa for NH3 to be about 9.26 but use the number in your text/notes. If you want the buffer to be between 0.01 and 0.5, why not choose say 0.3.
If base + acid = 0.3, then
base = 0.1 and acid must be 0.2 so
pH = pKa + log base/acid
pH = 9.26 + log 0.1/0.2 and
pH = approx 8.8 and that's in the ballpark of where it should be. You also don't say how much to make. Let's make a liter of solution.
So you have 0.10 M NH3. Start with that.
You want NH4Cl to be 0.2 M.
So you want to add 0.2 mol NH4Cl to the 1L of 0.1M NH3 and mix.
grams NH4Cl = mols NH4Cl x molar mass NH4Cl or g NH4Cl = 0.2 x molar mass NH4Cl = ?

This should do it assuming there are no limits as to pH you want the buffer or the amount of buffer you are to prepare.

Answer 2

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