The pK of acetic acid is 4.76. Which statement is true at pH 5.0?

[A^-] = [HA]

[A^-] < [HA]

[A^-] > [HA]

To determine which statement is true at pH 5.0, we need to compare the concentrations of the conjugate base (A^-) and the acid (HA) at this pH.

Given that the pK of acetic acid is 4.76, we can use the pK equation to determine the concentrations of A^- and HA at pH 5.0.

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

5.0 = 4.76 + log([A^-]/[HA])

Subtracting 4.76 from both sides:

0.24 = log([A^-]/[HA])

To convert this equation from logarithmic form to exponential form:

10^(0.24) = [A^-]/[HA]

Thus, [A^-]/[HA] = 1.58

Comparing the concentrations, we can conclude that [A^-] is greater than [HA] at pH 5.0. Therefore, the correct statement is:

[A^-] > [HA]

To determine which statement is true at pH 5.0, we need to compare the concentrations of the conjugate base (A^-) and the acid (HA) at that pH.

The pKa of a weak acid (in this case, acetic acid) is the negative logarithm of its acid dissociation constant (Ka). It indicates the tendency of the acid to donate a proton in a solution.

In this case, the pKa of acetic acid is 4.76. The equation to calculate the pH of a solution containing acetic acid and its conjugate base is:

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

At pH 5.0, substituting the values into the equation:

5.0 = 4.76 + log([A^-]/[HA])

Now we need to consider each statement:

1. [A^-] = [HA] - This means that the concentrations of the conjugate base and the acid are equal. However, for a weak acid like acetic acid, this is not true in most cases.

2. [A^-] < [HA] - This means that the concentration of the conjugate base is less than the concentration of the acid. If this is the case, then the pH of the solution is less than the pKa.

3. [A^-] > [HA] - This means that the concentration of the conjugate base is greater than the concentration of the acid. If this is the case, then the pH of the solution is greater than the pKa.

Considering the equation for pH, it becomes clear that at pH 5.0, [A^-] < [HA], which means the statement " [A^-] < [HA]" is true.

pH = pKa + log (base)/(acid)

Why not plug some number into the base and another number for acid and evaluate pH. If the number goes up you will have your answer and if it goes down you will have your answer.