Question

A) If you make up a solution of 50 mL of 0.1 M TRIS in the acid form, what will be the pH?
(pka of TRIS: 8.3, molecular wt: 121.1 g)

B) if you add 2mL of 1 M NaOH to the solution, what will be the pH?

Thank you.

Answer 1

A) To calculate the pH of the solution, we need to find the hydrogen ion concentration [H+]. Since we have the pKa of TRIS (8.3) and the concentration of TRIS (0.1 M), we can use the Henderson-Hasselbalch equation to determine the pH:

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

In this case, the TRIS is in the acidic form, so the ratio of [A-] to [HA] is 1:1; therefore, the log term will be 0.

pH = 8.3 + log (1) = 8.3

So the pH of the 50 mL solution of 0.1 M TRIS in the acid form will be 8.3.

B) To find the pH after adding 2 mL of 1 M NaOH, we first need to determine how many moles of NaOH are added and how many moles of TRIS are present in the solution:

moles of NaOH = (2 mL) * (1 M) = 0.002 moles
moles of TRIS = (50 mL) * (0.1 M) = 005 moles

When NaOH is added it reacts with acidic TRIS (HA) to form the conjugate base (A-) and water, so the reaction is:

HA + NaOH → A- + H2O

Subtracting the moles of NaOH from the moles of HA:

moles of HA remaining = 0.005 moles - 0.002 moles = 0.003 moles

Since 0.002 moles of NaOH react with 0.002 moles of HA, we will have:

moles of A- formed = 0.002 moles

Now we can use the Henderson-Hasselbalch equation again to find the pH:

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

pH = 8.3 + log (0.002 / 0.003)

pH = 8.3 + log (2/3)

pH ≈ 8.1

So, the pH after adding 2 mL of 1 M NaOH to the solution will be approximately 8.1.