Your laboratory supervisor directs you to prepare 500.0 mL of a buffer with a pH of 9.40. You have available all the weak acids and bases and the salts of those weak acids and bases. Describe how you could make this buffer.
To prepare a buffer with a pH of 9.40, you will need to choose a weak acid and its conjugate base or a weak base and its conjugate acid. Here's a step-by-step guide on how to make this buffer:
1. Select a weak acid or a weak base: In this case, since you need a pH of 9.40, it's better to choose a weak base and its conjugate acid. Let's assume you choose ammonia (NH3) as the weak base.
2. Write the equilibrium reaction: The equilibrium reaction for ammonia is NH3 + H2O ⇌ NH4+ + OH-. In this reaction, ammonia acts as a base and accepts a proton (H+) from water to form the ammonium ion (NH4+) and hydroxide ion (OH-).
3. Calculate the required amounts: To prepare 500.0 mL of buffer, you need to determine the amounts of weak base (ammonia) and its conjugate acid (ammonium) required. The buffer should have an equal amount of weak base and its conjugate acid for best buffering capacity.
4. Calculate molarities: You will need to know the pKa of the weak base and the pKa-pKw value to determine the initial molar concentrations of the weak base and its conjugate acid. The pKa of ammonia is 9.25 and pKw (at 25°C) is 14.00.
5. Use the Henderson-Hasselbalch equation: The Henderson-Hasselbalch equation states that pH = pKa + log ([A-]/[HA]), where [A-] and [HA] are the initial molar concentrations of the conjugate base and the weak acid, respectively.
6. Determine the initial concentrations: Rearrange the Henderson-Hasselbalch equation to calculate the initial concentrations of the weak base and its conjugate acid. Let's assume you want to use 0.1 M solutions.
Using the equation pH = pKa + log ([A-]/[HA]), substitute the given pH value of 9.40 for pH, the pKa value of 9.25 for pKa, and the 0.1 M concentration for [HA] to solve for [A-].
7. Calculate volumes: Calculate the volumes needed to make 0.1 M solutions. Since you already know the initial concentration and desired final concentration, you can use the equation C1V1 = C2V2, where C1 is the initial concentration, V1 is the initial volume, C2 is the final concentration, and V2 is the final volume.
8. Prepare the buffer: Once you have determined the volumes needed, measure the appropriate amounts of the weak base (ammonia in this case) and its conjugate acid (ammonium ion) using a balance or volumetric glassware. Dissolve each substance separately in the appropriate volume of water and then combine them in a 500.0 mL volumetric flask.
9. Adjust the pH if necessary: After preparing the buffer, check its pH using a pH meter. If the pH is not exactly 9.40, adjust it by adding small amounts of a strong acid (to decrease pH) or a strong base (to increase pH) until the desired pH is achieved.
Remember to always follow proper laboratory safety procedures and wear appropriate personal protective equipment when handling chemicals.