Since every calcium hydroxide molecule will dissociate and form two hydroxide ions, the concentration of the hydroxide ions will be twice as much as the initial concentration of the base.

This means that the concentration of hydroxide ions in the solution will be 0.4M.We can use this concentration to solve for the pOH fo the solution.

Since we are looking for the pH of the solution, we simply subtract the pOH form 14.

This results in a pH of 13.6.

Basic solutions have a pH level greater than 7, which is the neutral pH. Acidic solutions have a pH level lower than 7.

Basic solutions have a pH level greater than 7 and acidic solutions have a pH level lower than 7. A pH of 7 corresponds to a neutral solution.

Acidic solutions have a pH level lower than 7, which is the neutral pH. Basic solutions have a pH level greater than 7.

Potassium sulfide is soluble in water yielding the specie  which undergoes a two steps basic hydrolysis:

The respective hydrolysis constants are:

Analyzing the rounded values of both hydrolysis constants, we can see that the value of  is very large then we can assume the first reaction occurs to completion and controls the concentration of  in the solution which will be . Hence, the pH will be:

This question is presenting us with a reversible chemical reaction of a weak acid. It provides us with the pKa of the acid, and asks us to determine where on the pH spectrum will the acidic form of acetic acid be equal to its basic form.

There is a very important concept that this question highlights, which is that a compound will always have equal amounts of its acidic and basic forms at a pH that is equal to its pKa. This can be shown mathematically by using the Henderson-Hasselbalch equation.

As the above equation shows, when the acidic and basic forms are equal, their ratio is equal to . And the logarithm of  is equal to . Thus, what we're left with is:

For a buffer solution, you need a weak acid and its conjugate base, or a weak base and its conjugate acid. HCO₃ from the NaHCO₃ and CO_3– from K₂CO₃ are this pair. 

A weak acid/base best buffers about 1 pH point above and below its pKa. The pKA closest to the middle of 4 and 6 (so want as close to 5) is acetic acid at 4.7.

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.

The definition of a buffer solution is that it contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Since we are starting with a weak acid in this case, we need its conjugate base.