### Video Transcript

The hydroxide ion concentrations of some bases are listed in the table below. Which of the bases listed in the table is the most acidic?

When we want to know if a solution is acidic, basic, or neutral, we often consider
the solution’s pH. pH is a way to represent the concentration of hydronium ions in a
solution. A pH value of seven represents a neutral solution. As the pH decreases, the solution becomes increasingly more acidic. And as the pH increases, the solution becomes increasingly more basic. As we want to determine which of the bases listed in the table is the most acidic, we
would need to determine which of the bases has the lowest pH. But the pH of a solution is based on the concentration of hydronium ions, and we’ve
been given the concentration of hydroxide ions.

So instead of using the pH scale to determine which is most acidic, we can use a
related scale, the pOH scale. The pOH of a solution is a representation of the concentration of hydroxide ions. In this scale, a pOH of seven represents a neutral solution. The rest of the scale is opposite the pH scale. As the pOH increases, the solution becomes increasingly more acidic. And as the pOH decreases, the solution becomes increasingly more basic. So the most acidic of the four bases is the one that has the highest pOH.

We can calculate the pOH of each solution using the equation pOH equals the negative
logarithm of the hydroxide ion concentration. So the pOH of solution A is equal to the negative logarithm of 2.7 times 10 to the
negative fifth, or 4.569. The pOH of solution B equals the negative logarithm of 6.5 times 10 to the negative
second, which is 1.187. The pOH of solution C is calculated to be 0.387, and the pOH of solution D is
2.267.

We can see that the base with the highest pOH is base A. Therefore, the base that is the most acidic is A.