acid and formate are required to prepare a buffer solution with a pH of 4.78 .The of formic acid is A)0.17 B)0.083 C)3.3 × 10³ D)0.041 E)9.8

Question

Quizplus · Accepted Answer

To calculate the pH of a buffer solution, we can use the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

where pKa is the acid dissociation constant, [A-] is the concentration of the conjugate base (in this case, formate), and [HA] is the concentration of the acid (in this case, formic acid).

We are given the pH of the buffer solution (4.78), which allows us to find the ratio [A-]/[HA]:

4.78 = 3.75 + log([A-]/[HA])
log([A-]/[HA]) = 1.03
[A-]/[HA] = 10^1.03
[A-]/[HA] = 10.47

We are also given the pKa of formic acid (3.75), which allows us to find the ratio of the concentrations of acid and conjugate base:

pKa = -log(Ka)
Ka = 10^-pKa
Ka = 1.78 x 10^-4
Ka = ([H+][A-])/[HA]

Using the ratio [A-]/[HA] = 10.47 that we found earlier, we can write [A-] = 10.47[HA]. Substituting this into the equation for Ka, we get:

1.78 x 10^-4 = ([H+][10.47HA])/HA
[H+] = 1.86 x 10^-5

Now we can use the concentration of formic acid to find the amount required to prepare the solution:

pH = pKa + log([A-]/[HA])
4.78 = 3.75 + log([A-]/(0.083 M))
log([A-]/0.083) = 1.03
[A-] = 10^1.03 * 0.083 = 0.843 M

We need an equal amount of formic acid, so we need 0.843 M of formic acid and 0.843 M of formate, which gives us a total of 1.686 M of acid and conjugate base.

The amount of acid and formate needed are in the ratio of 1 : 1, so we need 11.2 moles of each to prepare the solution:

11.2 moles formic acid x (46.03 g/mol) = 515.5 g formic acid
11.2 moles formate x (68.01 g/mol) = 761.2 g formate

Acid and Formate Are Required to Prepare a Buffer