Consult the table of Ka values in the back of your textbook and suggest acid/con

Question

Consult the table of Ka values in the back of your textbook and suggest acid/conjugate base system (mono-protic acid only) that would yield a strong buffer capacity at pH of 8.60 (the buffer must be strong compared to others of the same molarity). Explain your selection (simply and succinctly). A buffer solution is prepared by mixing 20.0 mL of 0.032 M benzoic acid C_6 H_5 COOH_(aq) solution with 25.0 mL of 0.022 M NaC_6 H_5 COO_(aq). What is the pH of the buffer produced? (K_a of benzoic acid = 6.2 Times 10^-5).

Explanation / Answer

2.

consider this table of Ka and pKa

for pH = 8.6

Pka should be 8.6 for good buffer

pKa = -log Ka

8.6 = -log Ka

Ka = 2.512*10^-9

Which is true for hypobromous acid (HBrO)

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3.

After mixing,

[acid] =number of moles of acid/ total volume

         = 0.032*20/ (20+25)

= 0.0142 M

[salt] = 0.022*25/(20+25)

= 0.0122 M

Ka = 6.2*10^-5

pKa = -log Ka

= - log (6.2*10^-5)

= 4.2076

pH = pKa + log {[salt]/[acid]}

= 4.2076 + log {0.0122/0.0142}

= 4.142

Answer: 4.142

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