Since acetic acid is a weak acid, it has a Ka that is rather small, we have to do a RICE table to determine the equilibrium amount of hydronium, H3O+ to then determine the pH.

R

I 0.1 M - 0 0

C -x +x +x

E 0.1 -x x x

So first we need to change our pKa to a Ka

where therefore

= =

If we assume that x is very small compared to 0.1...

Where

(note: when solving using the quadratic we come up with the same answer)

So if

The pH at half the equivalence point is equal to the pKa of the acid.

To answer this question we need to look at the reaction below:

An increase in the pH will result in a decrease in the concentration of hydrogen ions (). Using Le Chatelier’s principle we can find out which answer choices will decrease .

Removing carbonic acid will decrease the concentration of . To maintain equilibrium, the reaction will shift to the left and make more reactants from products; therefore, there will be a decrease in the and an increase in pH.

Recall that salts like sodium bicarbonate, or , will dissociate in water and form ions. Sodium bicarbonate will form sodium () and bicarbonate () ions. This side reaction will result in an increase in the bicarbonate ion concentration. Le Chatelier’s principle will shift the equilibrium of the given reaction to the left and, therefore, decrease the . Adding sodium bicarbonate will increase the pH.

In this question, we're presented with a variety of acid/base pairs and we're asked to identify which one could act as a buffer.

Remember that a buffer is a pair of acid and its conjugate base that acts to resist substantial changes in pH. In order for a buffer to work, the acid base pair needs to exist in equilibrium. This way, when the pH of the solution changes, the equilibrium of the acid/base reaction will shift, such that the pH will not change drastically.

To have an acid/base pair in equilibrium, we'll need to look for a pair that contains a weak acid. Acids like and are so strong that they will dissociate completely. Of the answer choices shown, only the carbonic acid/bicarbonate system ( and ) exists in equilibrium. Thus, this is the correct answer.

Buffer solutions can be made via two methods. The first method involves adding equal amounts of a weak acid and a salt of its weak conjugate base (or vice versa). The second methods involves adding a weak acid and a half equivalent of a strong base (or vice versa).

is a weak acid and is a salt of its weak conjugate base; therefore, this can form a buffer.

is a weak base and is a salt of its weak conjugate acid; this can also form a buffer. Note that this is the converse of the first method (weak base with salt of weak acid), but it can still form a buffer solution.

is a strong acid and is a weak base; therefore, adding and a half equivalent of will create a buffer solution. This is the converse of the second method (adding a weak base to a half equivalent of strong acid).

and are both strong reagents (acid and base, respectively); therefore, they cannot form a buffer solution.

When performing acid/base calculations, the Henderson-Hasselbalch equation is useful:

To use this equation we need to convert the to the , and can use the following definition to do so:

Since an individual will begin experiencing acidosis when the blood falls below 7.35 we can use 7.35 as the pH in the Henderson-Hasselbalch equation. Adding in the of 6.10 calculated from the gives the equation below:

Plug in known values and solve:

Correct Answer:

The answer is unitless because all units cancel out.

A buffer solution is formed from the equilibrium of a weak acid and its conjugate base, or from a weak base and its conjugate acid. It's ability to "buffer" the pH or keep it from changing in large amounts in from the switching between these two forms weak and its conjugate.

At the half end point, the . This can be determined by the Henderson-Hasselbalch equation if it is not clear.

Since the endpoint of the titration is that there are 10 mL of 0.1 M NaOH added, that means that there are 0.001 moles of acetic acid.

When 5 mL of NaOH is added, there are 0.0005 moles of acetic acid and 0.0005 moles of acetate formed.

Therefore pH= pKa

First to go through why the other ones are wrong:

Strong base + strong acid neutralizes and does not form a buffer solution

Strong base + weak base does not form a buffer - would need an acid

Strong base + weak acid = all weak acid converted to conjugate base

The correct answer is:

Strong base + weak acid = half converted to conjugate base with half leftover as weak acid, with all the components for a buffer

Nitric Acid is a strong acid and can't buffer. Rubidium Hydroxide is a strong base and thus can't buffer. Of the remaining, both are weak acids, but the one with a greater concentration has a greater buffering capacity.