Question #140: Keep in mind that pH is defined as the -log[H+], or the negative log of the concentration of H+ ions. If Kw = k[H+][OH-], and Kw increases, then both the concentration of OH- and H+ increase. However, as the definition of pH only depends on the concentration of H+, you only consider the H+ increase. In reality, you are correct and the OH- concentration also rises, but this is not relevant to the definition of pH.
In fact, the sum of the two in the equation pKw = pH + pH just changes such that pKw is not equal to 14, but a greater number. It is interesting to see how acidity and basicity depend on temperature!
Question #142: This question can be a little tricky! Here, the key is that HCl and HClO4 are both strong acids in water, and that means they both fully dissociate in water. As a result, we do not know which acid is stronger in water, because they both fully dissociate. To figure out which acid is stronger, we need to create conditions where one acid fully dissociates and the other does not. As a result, we want to have something that “wants protons less” than water does. Acetic acid is less easily protonated than water is, and therefore it can show the difference in acid strength between HCl and HClO4.
This is known as the leveling effect. A good analogy is that of a test. For a teacher to figure out which of two students is more advanced in math, it is not sufficient to give the two students an easier test on addition and subtraction, because both of them will get 100%. What you need to see which student knows more is a harder test where one or both will not get 100%.
Question #143: Because water has constant concentration, it is not included in Keq equations like the Kw equation. The Kw equation is Kw = k[H+][OH-]. This is a great way to eliminate answers on the MCAT. If you are given a solid or liquid phase in a Keq equation, this is not possible as their concentrations are constant.