Question 9e1a4

Feb 1, 2015

The pH values of the 1:1 and 1:4 buffers should remain unchanged on the addition of small amounts of strong acid or base.

In your experiment, you prepared 1:1 and 1:4 NH₃/NH₄⁺ buffers. You measured the pH of each with phenolphthalein. Then you added 1 drop of HCl and 1 drop of NaOH to separate test tubes of the buffers.

We can calculate the pH of an NH₃/NH₄Cl buffer from the equation:

"pH" = 14.00 - "p"K_"b" + log(("[NH"_3"]")/("[NH"_4^+"]"))

The value of $\text{p"K_"b}$ for NH₃ is 4.75. So,

"pH" = 9.25 + log(("[NH"_3"]")/("[NH"_4^+"]"))#

1:1 Buffer

If $\left(\text{[NH"_3"]")/("[NH"_4^+"]}\right) = \frac{1}{1}$, then

$\text{pH} = 9.25 + \log \left(\frac{1}{1}\right) = 9.25 + 0.00 = 9.25$

1:4 Buffer

If $\left(\text{[NH"_3"]")/("[NH"_4^+"]}\right) = \frac{1}{4}$, then

$\text{pH} = 9.25 + \log \left(\frac{1}{4}\right) = 9.25 - 0.60 = 8.65$

Phenolphthalein is pink between pH 8.2 and pH 10. Its colour at pH 8.65 might be a little less pink at pH 9.25, but it would be hard to tell the difference.

Your 1:1 and 1:4 solutions are both buffers. Their pH values should not change significantly on the addition of one drop of strong acid or base.