Question

the ability of a solution to resist changes to the ph from added acid or base depends upon the amount of weak acid and weak base. the buffering capacity is the ratio of weak acid to weak base and the best buffering occurs when these are equal, making the buffering capacity equal to 1. in the above buffer the buffering capacity is the ratio of $\ce{h2po4^{-1}}$ to $\ce{hpo4^{-2}}$, or
$\ce{h3o^{+1}} = \frac{k_a \ce{h2po4^{-1}}}{\ce{hpo4^{-2}}} = \frac{k_a (4.44 \times 10^{-5})}{(5.55 \times 10^{-5})}$
buffering capacity $= \frac{(4.44 \times 10^{-5})}{(5.55 \times 10^{-5})} = 0.8$
here the buffering capacity is close to 1 (0.8) and so the buffering capacity is 1
when the buffering capacity is 1, the $\ce{h3o^{+1}} = k_a$ and the $\ce{ph} = \ce{p}k_a$ this helps chemists choose the right buffer for the needed ph.
to buffer at $\ce{ph} = 7$, the weak acid should a $\ce{p}k_a$ of close to 7, true or false ? 2
the weak acid should have $k_a$ of close to $1 \times 10^{-7}$, true or false ? 3
a. good b. bad c. true d. false

Explanation:

Sub - question 1: Determine buffering capacity description

The buffering capacity here is 0.8, which is close to 1. A buffering capacity close to 1 means the buffer is good at resisting pH changes. So we choose the option related to "good".

Sub - question 2: pH and \(pK_a\) relationship

We know that when buffering capacity is 1, \([H_3O^{+1}]=K_a\) and \(pH = pK_a\). For a buffer to be effective at a certain pH (here pH = 7), the \(pK_a\) of the weak acid should be close to the desired pH. So the statement "To buffer at pH = 7, the weak acid should have a \(pK_a\) of close to 7" is true.

Sub - question 3: \(K_a\) and \(pK_a\) relationship

We know that \(pK_a=-\log(K_a)\). If \(pH = pK_a\approx7\), then \(K_a = 10^{-pK_a}\approx10^{-7}=1\times10^{-7}\). So the statement "The weak acid should have \(K_a\) of close to \(1\times10^{-7}\)" is true.

Answer:

1. A. good
2. C. true
3. C. true