Kaplan MCAT General Chemistry Review

After crossing out equal terms from the numerator and denominator, you are left with this:

Dividing 18 and 6 into our numerators will yield 50 grams of glucose.

13. A

What you are shown is a net ionic equation. To answer this question, work backward from the amount of product to the reactant with its spectator ions (in this case, sulfate).

First, for every two moles of FeSCN we create, we must react two moles of Fe³⁺. Therefore, you’re looking for the mass of iron sulfate that can provide two moles of iron.

Next, determine the charge on sulfate, which is -2 and iron, which in this case is +3. Therefore, iron sulfate must be present as follows:

Fe₂(SO₄)₃

The molecular formula tells you that each mole of iron sulfate releases two moles of atomic iron. Therefore, you only need one mole of iron sulfate for this reaction, which means the molar mass of iron sulfate is the answer.

Chapter 5: Chemical Kinetics and Equilibrium

It’s Friday night, and you’re on the first call of your pediatrics rotation. You’re a third-year medical student now, and you’re anxious to see some action. Hanging out in the call room near the ED of the children’s hospital, you get a page from the resident: Come to the emergency room, now, she says. They just brought in a kid with DKA. DKA, you know, stands for diabetic ketoacidosis and is a fairly common way for undiagnosed Type I diabetes to present. Entering the child’s room, the examination is already under way; he’s young—about 10 years old—conscious but agitated, and the most obvious sign—what you notice immediately—is his rapid, shallow breathing. You see he’s already receiving IV fluids and an insulin drip.

Later that evening, after the boy has stabilized, you and the resident are talking about diabetes and DKA. You remembered from your second-year lessons about endocrine pathophysiology that ketoacidosis can arise as a result of the body’s metabolism of fatty acids when insulin production finally shuts down in Type I diabetes. Because most of the cells of the human body can’t import glucose without the aid of insulin, the glucose accumulates in the plasma of the blood, producing hyperglycemia even as the cells of the body are in a state of glucose starvation. Fatty acids are metabolized into ketone bodies as an alternative energy source. Some of the ketones produced are ketoacids, and as the diabetic crisis continues and worsens, the concentration of these ketoacids increases, resulting in a plasma pH below 7.35 (metabolic acidosis). The combination of the acidosis, progressively severe dehydration due to the osmotic effect of glucose “spilling into” the urine, and other negative effects of the severe insulin depletion result in the host of signs and symptoms of diabetic ketoacidosis. You ask the resident why the boy was hyperventilating, and she takes a piece of paper and writes out the following:

H⁺ (aq) + HCO₃^- (aq) H₂CO₃ (aq) CO₂ (g) + H₂O (l)

It’s Le Châtelier’s principle, she deadpans, disappointed that you didn’t remember that. The respiratory system is trying to compensate for the metabolic acidosis; the increased breathing rate allows the patient to blow off more CO₂, which causes the equilibrium to shift to the right. Hydrogen ions combine with bicarbonate ions to produce carbonic acid, which dissociates into CO₂gas to replace the gas that’s being expelled from the lungs. Of course, the desired result is a decrease in the hydrogen ion concentration, which stabilizes the pH and keeps it from getting crazy low. It’s not perfect, but if you catch them soon enough, the pH hasn’t gone so low that they’ve essentially become a scrambled egg. You should know all of this by now.

You recognize the equation. In fact, you even remember studying it for your MCAT. What was that all about? Oh yeah, chemical equilibrium. Wow, chemistry really is essential for medical school!