In this section, you will create a buffer using the NH 3 /NH 4 + conjugate pair

Question

In this section, you will create a buffer using the NH3/NH4+ conjugate pair system.

After rinsing a graduated cylinder and 100 mL beaker with distilled water, add ~15 mL of ammonium chloride solution to the 100 mL beaker. Rinse the graduated cylinder with distilled water and measure ~15 mL of ammonia solution. Add this to the same 100 mL beaker as the ammonium chloride solution. This creates the buffer solution. Measure the pH of the concentrated buffer solution. Add ~30 mL of distilled water to the buffer solution. This creates a diluted buffer solution. Measure the pH of the diluted buffer solution.

Split the diluted buffer solution into a second rinsed 100 mL beaker. To one beaker, add five drops of 0.1 M HCl solution. To the other beaker, add five drops of 0.1 M NaOH. Measure the pH of each of the resultant solutions using the same technique that was used in part B.

Calculate the [H+] from both the concentrated buffer and diluted buffer in part C. In this case, these concentrations are the Ka of the ammonium. Report the average Ka for ammonium.

Buffer with solution: 9.15pH

Buffer with diluted: 9.13pH

buffer + HCl: 9.02pH

buffer + NaOH: 9.15pH

Explanation / Answer

Using the Henderson Hasselbach equation:

pH = pKa + log([salt]/[acid])

Since you have added equal quantities of both the salt and the acid solution, so the ratio inside the log term evaluates to one, and hence the log term evaluates to zero.

So, you get pH = pKa, and hence [H+] = Ka

From the data given:

pH of conc buffer = 9.15, so [H+] = 7.07*10-10 M

pH of diluted buffer = 9.13, so [H+] = 7.41*10-10 M

So, average [H+] = 7.24*10-10 M

So, avg Ka = 7.24*10-10

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