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??
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.
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Of course, I can help you with that! Creating a buffer involves selecting suitable concentrations of an acid and its conjugate base (or a base and its conjugate acid) to maintain a relatively constant pH. In this case, you need to prepare a buffer using NH4Cl (the acidic component) and NH3 (the basic component). Let's break down the steps to solve your problem:
1. Determine the desired concentration range: You mentioned that you need to have a concentration between 0.01 and 0.5 M for the buffer solution. This will help us determine the amount of each reagent to use.
2. Understand the Henderson-Hasselbalch equation: The Henderson-Hasselbalch equation relates the pH of a buffer to the concentrations of the acid and its conjugate base (or vice versa). For an acid-base pair like NH4Cl/NH3, the equation is:
pH = pKa + log([base]/[acid])
where pKa is the negative logarithm of the acid dissociation constant (Ka), [base] is the concentration of the base (NH3), and [acid] is the concentration of the acid (NH4Cl).
3. Calculate the pKa value: The pKa value for NH4Cl/NH3 can be determined from the Ka value, which you can find in a reference book or online.
4. Determine the ratio of base to acid: To achieve a desired pH, you need to choose an appropriate ratio of [base] to [acid] in your buffer solution. Rearrange the Henderson-Hasselbalch equation to solve for the ratio:
([base]/[acid]) = 10^(pH - pKa)
By substituting the desired pH and pKa values into this equation, you can calculate the required ratio.
5. Calculate the concentrations of the reagents: Now that you have the ratio of [base] to [acid], you can select a suitable total concentration for the buffer. For example, if you choose a total concentration of 1.0 M, you can calculate the actual concentrations of NH4Cl and NH3.
6. Prepare the buffer solution: Weigh or measure out the appropriate amounts of NH4Cl and NH3, ensuring that the concentrations match the values calculated in the previous step. Dissolve the chemicals in water to obtain the desired buffer solution.
7. Determine the pH of the buffer solution: Now that you have prepared the buffer solution, you can calculate the pH using the Henderson-Hasselbalch equation. Plug in the concentrations of NH4Cl and NH3 into the equation, along with the pKa value, to obtain the pH of the solution.
These steps should guide you in creating the buffer solution and calculating its pH. You may need to consult reference materials or online resources to find the pKa value for NH4Cl/NH3. Good luck!