Which of the following weak acid-conjugate base combinations would result in an ideal buffer solution with a pH of 9.4? a) formic acid (HCHO 2 ) and sodium formate (K a = 1.8 x 10 -4 ) b) benzoic acid (HC 7 H 5 O 2 ) and potassium benzoate (K a = 6.5 x 10 -5 ) c) hydrocyanic acid (HCN) and lithium cyanide (K a = 4.9 x 10 -10 ) d) iodic acid (HIO 3 ) and sodium iodate (K a = 1.7 x 10 -1 )

Which of the following weak acid conjugate based combinations will result in an ideal buffer solution with a pH of 9.4? So the key terms they used here was ideal buffer. Remember, a buffer is ideal when its weak acid conjugate base components are equal to one another. And when that happens, pH equals p k a. So basically, we need to find the p k a's of each one of these buffer combinations and sees and see which one has a pKa closest to this pH. That'll be the best buffer to use. So remember, pKa itself equals negative log of Ka. So all you have to do is take the negative log of each one of these Ka values, and it'll give us a new pKa. So taking negative log of the first one will give us a pKa of 3.74. For the next one, pKa equals 4.19. Here, pKa equals 9.31. And then here p k a equals 0.77. Based on these p k a values, the one that has a p k a closest to the pH of 9.4 would be option c. We'd say that the hydrocyanic acid and lithium cyanide buffer solution would be the most ideal buffer to use if you want a pH of 9.4. Okay? Because if pKa is 9.31, the closest to this pH of 9.4. So just remember, if we're talking about an ideal buffer, that means pH equals pKa, look to see which combination gives us a pKa closest to the given pH.