Question

You need to prepare 1 L of the citric acid/citrate buffer. You have chosen to use Method 1 (see lab presentation). Calculate the pH of a solution prepared by mixing 300 mL of 0.45 M citric acid acid and 100 mL of 0.65 M NaOH plus water to a final volume of one liter. The Ka of citric acid is 7.24 x 10-4.

Answer 1

To calculate the pH of a citric acid/citrate buffer, mix citric acid and sodium hydroxide, then apply the Henderson-Hasselbalch equation using the equilibrium constant Ka for citric acid.

Step-by-step explanation:

The citric acid/citrate buffer system is a mix of citric acid and its conjugate base in solution. To calculate the pH, you need to account for the neutralization of the acid by the sodium hydroxide (NaOH) and then apply the Henderson-Hasselbalch equation using the equilibrium constant ka of citric acid.

First, calculate the moles of citric acid and NaOH:

• Moles of citric acid = 0.45 M * 0.3 L = 0.135 mol
• Moles of NaOH = 0.65 M * 0.1 L = 0.065 mol

Upon mixing, the NaOH will neutralize some of the citric acid:

• Moles of citric acid remaining = 0.135 mol - 0.065 mol = 0.070 mol

The moles of the citrate (conjugate base) formed will be equal to the moles of NaOH reacted since the reaction is a 1:1 molar ratio.

Using the Henderson-Hasselbalch equation:

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

Where [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid. Here, [A-] is the moles of citrate formed divided by the total volume in liters and [HA] is the moles of citric acid remaining divided by the total volume in liters.

This will give you the final pH of the buffer solution.

Answer 2

3.11 is the pH of the buffer

Step-by-step explanation:

The pH of a buffer is obtained using H-H equation:

pH = pKa + log [Conjugate base] / [Weak acid]

Where pH is the pH of the buffer, pKa = -log Ka = 3.14 for the citric buffer and [] could be taken as the moles of each species.

The citric acid,HX (Weak acid), reacts with NaOH to produce sodium citrate, NaX (weak base) and water:

HX + NaOH → H2O + NaX

That means the moles of NaOH added = Moles of sodium citrate produced

And the resulitng moles of HX = Initial moles - Moles NaOH added

Moles HX and NaX:

Moles NaOH = 0.100L * (0.65mol / L) = 0.065 moles NaOH = Moles NaX

Moles HX = 0.300L * (0.45mol / L) = 0.135 moles HX - 0.065 moles NaOH = 0.070 moles HX

Replacing in H-H equation:

pH = 3.14 + log [0.065mol] / [0.070mol]

pH = 3.11 is the pH of the buffer