If I take 5mL of a TRIS/HCL buffer with a pH of 7.4 and add 1mL of 1M HCL, how do I determine the new pH?

Don't I need the concn of the buffer?

The TRIS-HCl buffer is 0.1 M.

This is done the same way the phosphate buffer is done. The only difference is the pKa value for TRIS. If you have a follow up question, please repost at the top of the page. I have trouble finding these when they get so far at the bottom of the list.

To determine the new pH after adding HCl to the TRIS/HCl buffer, you need to consider the Henderson-Hasselbalch equation. This equation relates the pH of a buffer solution to the pKa and the ratio of the concentration of the acid to its conjugate base.

The Henderson-Hasselbalch equation is given as:

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

Where:
pH is the pH of the buffer solution
pKa is the acid dissociation constant of the acid component of the buffer
[A-] is the concentration of the conjugate base
[HA] is the concentration of the acid

In your case, TRIS/HCl is a buffer system, and HCl is the acid component. Adding 1 mL of 1M HCl to the buffer will increase the concentration of the acid, while the concentration of the conjugate base (TRIS) remains the same.

Step-by-step solution:

1. Calculate the new concentration of the acid, [HA]:
- The initial concentration of the TRIS/HCl buffer is not provided, so we cannot calculate the exact molar concentration of HCl.
- However, given that you added 1 mL of 1M HCl, the final concentration of the acid, [HA], will be 1M.

2. Calculate the new concentration of the conjugate base, [A-]:
- Since you added 1 mL of HCl to 5 mL of the buffer, the total volume of the solution is now 6 mL.
- The concentration of the conjugate base, [A-], remains the same as it is determined by the concentration of TRIS in the buffer.

3. Calculate the pH using the Henderson-Hasselbalch equation:
- To use the Henderson-Hasselbalch equation, you need to know the pKa of the acid component (HCl). The pKa value for HCl is -7, which can be used as an approximate value.

pH = -7 + log([A-]/[HA])

Substitute the concentrations into the equation:
pH = -7 + log([TRIS]/1M)

Since the initial pH of the TRIS/HCl buffer is 7.4, the [TRIS] is 1M (as the concentration of TRIS and its conjugate base are equal).

pH = -7 + log(1/1M)
pH = -7 + log(1)
pH = -7 + 0
pH = -7

Therefore, the approximate new pH after adding 1mL of 1M HCl to the TRIS/HCl buffer is around -7. Please note that this result assumes an approximate pKa value for HCl (-7) and considers that the concentration of the TRIS buffer component and its conjugate base remain the same.