Calculate pH of an acetate buffer containing 27.5 mM acetic acid (pKa is 4.74) and 50 mM
sodium acetate. When a 30 mM aq solution of sodium acetate was prepared in the lab it had a pH
of 8.8. Calculate the amount of acetic acid (in g) to be added to 0.5 L of this sodium acetate
solution in order to bring its pH to the target value of 5.00.
I worked this below. I can find it if you wish.
im just stuck on how to do the last part.
5.00 = 4.74 + lot (15/x)
Solve for x = millimoles acetic acid and convert that to grams in 500 mL solution ? thank you very much
Your problem isn't chemistry. You just need a little help with algebra.
5.00-4.74 = 0.26 = log 15/x
15/x = antilog 0.26 = 1.82
So 1.82 = 15/x. Solve for x and convert to g/500 mL.
That's brilliant help :) thank you so much
To calculate the amount of acetic acid to be added to the sodium acetate solution, we need to use the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the weak acid in the buffer and the ratio of the concentrations of the weak acid and its conjugate base.
The Henderson-Hasselbalch equation is given by:
pH = pKa + log([A-]/[HA])
Where:
pH = desired pH of the buffer solution (5.00 in this case)
pKa = pKa value of acetic acid (4.74 in this case)
[A-] = concentration of the acetate ion (sodium acetate) in the buffer solution (in mM)
[HA] = concentration of acetic acid in the buffer solution (in mM)
In order to apply the equation, we need to determine the initial concentrations of acetic acid ([HA]) and sodium acetate ([A-]) in the solution.
Given:
Initial pH of sodium acetate solution (before adding acetic acid) = 8.8
Initial concentration of sodium acetate ([A-]) = 30 mM
To find the initial concentration of acetic acid ([HA]), we can use the fact that, by definition, the total concentration of acetic acid and sodium acetate must be constant. Therefore, we can write:
[HA] + [A-] = Total concentration of sodium acetate = 50 mM
Solving for [HA]:
[HA] = Total concentration of sodium acetate - [A-]
= 50 mM - 30 mM
= 20 mM
Now, we can substitute the values in the Henderson-Hasselbalch equation:
5.00 = 4.74 + log(30/20)
To simplify calculations, we can rewrite the equation as:
0.26 = log(30/20)
To solve for [HA], we rearrange the equation:
[HA] = (30/20) * 10^0.26
Now, we can calculate the concentration of acetic acid ([HA]):
[HA] = (30/20) * 1.778
To find the mass of acetic acid needed, we need to convert the molar concentration to mass using the molar mass of acetic acid, which is approximately 60.05 g/mol:
Mass of acetic acid = [HA] * molar mass of acetic acid
= (30/20) * 1.778 * 60.05
Therefore, to calculate the amount of acetic acid (in grams) to be added to 0.5 L of the sodium acetate solution, use the result obtained above with the appropriate calculations.