Question

A monoprotic weak acid (HA) has a Pka value of 4.338. Calculate the fraction of HA in each of its forms (HA,A-) at pH 5.797. Next whatis the quotient (A-)/(HA) at pH 5.797.

So there should be three answers total.

Answer 1

Let`s calculate the concentration of H+ in the solution:

pH= -log[H+]= 5.797 -----> [H+]= 1,596x10^-6M

pKa= -logKa = 4.338 -----> Ka= 4.592x10^-5M

HA <-----> H+ + A-

Ka=[H+][A-]/[HA]

The concentration of H+ is equal to the concentration of A-, because for each H+ formed, 1 A- is formed too. So we can use the expression of Ka to find the concentration of HA, let`s call it X.

Ka= (1,596x10^-6)²/X -----> X= 5.547x10^-8M= [HA]

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We can use the Henderson-Hasselbalch equation:

pH= pKa + log[A-]/[HA] = 4.338 + log[A-]/[HA]=5.797

1.459= log[A-]/[HA]

[A-]/[HA]= 28.77