Henderson-Hasselbalch equation

[ahhppull]

Homework Statement

A 200.0 mL buffer solution is 0.280 M in acetic acid and 0.280 M in sodium acetate. What is the pH after addition of 0.0050 mol of HCl?

Homework Equations

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

The Attempt at a Solution

I have a problem with the significant figures. The website won't accept my answer. Or maybe I'm just wrong.

The initial ph is 4.74 and when HCl is added, the concentration of sodium acetate decreases by .0050 and the concentration of acetic acid increases by .0050

Therefore the equation becomes pH = 4.74 + log [.275]/[.285]

My answers I have inputted were 4.72 and 4.73

 

[ahhppull]

Nvm, no help needed. I figured out what the problem is.

 

[Borek]

Would be nice if you can tell what it was.

 

[ahhppull]

Oh, I did the problem wrong. The right was to do it was:

.28 M acetic acid x .200 L = .056 moles acetic acid
.056 moles acetic acid + .005 moles (from HCl) = .061 moles acetic acid

.28 M sodium acetate x .200 L = .056 moles sodium acetate
.056 moles sodium acetate - .005 moles = .051 moles sodium acetate

pH = 4.74 + log [.051]/[.061]
pH = 4.67

 

[DeeNos]

, but the website says they are incorrect.

The Henderson-Hasselbalch equation is a fundamental equation in acid-base chemistry that is used to calculate the pH of a buffer solution. It relates the concentrations of a weak acid and its conjugate base to the pH of the solution. In this case, the equation can be used to calculate the pH of the buffer solution before and after the addition of HCl.

Based on the given information, the initial pH of the buffer solution is 4.74. After the addition of 0.0050 mol of HCl, the concentration of acetic acid increases to 0.285 M and the concentration of sodium acetate decreases to 0.275 M. Plugging these values into the Henderson-Hasselbalch equation, we get:

pH = 4.74 + log[0.275]/[0.285] = 4.74 + log[0.9649] = 4.74 - 0.015 = 4.725

The pH after the addition of HCl is therefore 4.725. This answer is within the acceptable range of significant figures, so it should be accepted by the website. If not, it is possible that there is a rounding error or a mistake in the calculations. Overall, the Henderson-Hasselbalch equation is a useful tool for understanding and predicting the behavior of buffer solutions in acid-base chemistry.