Kaplan MCAT General Chemistry Review

5. Instead of using calcium hypochlorite to introduce chlorine into the water, the owner of a water park decides to bubble Cl₂ gas into the pool. Will his decision affect the solubility of the plaster that is lining the pools at his water park?

A. Yes, the chlorine gas will increase the solubility of CaSO₄ in the plaster as compared with Ca(OCl)₂ because it will react with molecules of SO₄^2- and shift the equilibrium to the right.

B. Yes, the chlorine gas will increase the solubility of CaSO₄ in the plaster as compared with Ca(OCl)₂ because Cl₂ will not cause the same common ion effect that occurred with Ca(OCl)₂.

C. Yes, the chlorine gas will decrease the solubility of CaSO₄ in the plaster as compared with Ca(OCl)₂ because the absence of Ca²⁺ from the Ca(OCl)₂ will eliminate the common ion effect.

D. No, the chlorine gas will not change the solubility of CaSO₄ in the plaster as compared with Ca(OCl)₂.

6. Ca(OCl)₂ contributes OCl^- to the water, which acts to kill bacteria by destroying enzymes and contents of the cells through oxidation. In its ionic form, OCl^- exists in the following equilibrium:

HOCl H⁺ + OCl^-

In order for cleaning to occur properly, the pH must be at the right level to allow enough of the oxidizing agent, HOCl, to be present. If the pH is raised by the addition of sodium carbonate to the water, what will happen to the oxidizing power of the HOCl?

A. The higher pH will break the HOCl compound into single atoms and will eliminate its oxidizing power.

B. The pH cannot be raised due to the buffering system in the pool, and thus the oxidizing power of the chlorine will remain the same.

C. Fewer H⁺ ions will be present and the reaction will shift right. This will decrease the number of HOCl molecules and thus decrease the oxidizing power of chlorine.

D. A high pH will lower the concentration of H⁺ by associating H⁺ with OCl^-. This will increase the number of HOCl molecules and increase the oxidizing power of chlorine.

7. Due to changes in climate and poor management of ion content in the water, the swimming pool has now become supersaturated with calcium sulfate. What combination of events could have caused this to occur?

A. Cooling of the pool followed by addition of calcium sulfate

B. Warming of the pool followed by addition of calcium sulfate

C. Addition of calcium sulfate followed by cooling of the pool and then subsequent warming of the pool

D. Warming of the pool followed by addition of calcium sulfate and then cooling of the pool

8. Water “hardness” refers to the content of calcium and magnesium in water. When referring to swimming pools, water hardness mainly refers to calcium. One way to measure the balance of ions is to use the Langelier saturation index. The Langelier saturation index is derived from a combination of the following two equilibrium equations.

Which of the following accurately expresses the combination of the two equilibrium equations in terms of [H⁺]?

9. Phenol red is the most widely used indicator to determine the pH of water in swimming pools. The pK_a2 of phenol red is equal to 7.96. The acidic form of phenol red appears yellow and the basic form of phenol red appears red. In addition, the absorptivity (how strongly a species absorbs light) of the basic form is around three times greater than the acidic form. The color-changing region is indicated by an orange color. Due to the difference in absorptivity between different forms of phenol red, at what pH would the color change (to orange) be MOST likely to occur?

A. pH of 4

B. pH of 7.5

C. pH of 8.5

D. pH of 11

PASSAGE II (QUESTIONS 10–17)

Hydrogen is the first element of the periodic table. It contains one proton and one electron. According to one early model of the hydrogen atom developed in the early 20th century by Niels Bohr, that electron is found in any one of an infinite number of energy levels. These energy levels are sometimes called quanta, in that they can be described by a principal quantum number, n, that always has an integer value. As the electron moves from one energy level to another (n = 1 to n 8, or vice versa) it absorbs or emits some discrete quantity of energy accordingly. This quantity is directly proportional to the frequency of the light radiation that results from the energy change.

It took some time for atomic physicists to arrive at Bohr’s conclusions. They struggled to reconcile empirical data about light radiation from hydrogen atoms with their understanding that light photons moved and behaved as particles according to Newtonian mechanics. One discovery that led to Bohr’s quantum mechanics was a new quantitative interpretation of light emissions from hydrogen atoms. Hydrogen atoms emit light in characteristic patterns known as line spectra. These patterns are noncontinuous but predictable. In the early 1880s, Theodore Balmer derived a mathematical relationship between the energy emissions of a hydrogen atom and the wavelengths of light they radiated during transitions: