Kaplan MCAT General Chemistry Review

B. produces more product, which subsequently appears in the precipitate.

C. requires more of the reactant, so higher quantities of unreacted material are usually present in the precipitate.

D. requires less of the reactant, so higher quantities of unreacted material are usually present in the precipitate.

52. Which of the following BEST explains why the student’s initial results were correct for group A but incorrect for group B?

A. Group A contains very strong bases, while group B contains slightly weaker bases.

B. Group A contains compounds that dissociate into multiple ions, while group B contains compounds that dissociate into only two ions.

C. Group A contains compounds with insignificant solubility, while group B contains compounds with considerable solubility.

D. Group A contains cations with a + 2 or +3 oxidation state, while group B contains cations with a +1 oxidation state.

Practice Section 2

Time—70 minutes

QUESTIONS 1–52

Directions: Most of the questions in the following General Chemistry Practice Section are organized into groups, with a descriptive passage preceding each group of questions. Study the passage, then select the single best answer to the question in each group. Some of the questions are not based on a descriptive passage; you must also select the best answer to these questions. In you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain.

PASSAGE I (QUESTIONS 1–9)

Swimming pools are filled with water containing a number of dissolved ions for the purpose of purification and maintenance of pH. One chemical that is added to pools, chlorine, is used to kill bacteria and harmful contaminants and can be added to pools in a number of ways. Calcium hypochlorite, Ca(OCl)₂, is an inorganic chlorinating agent that contributes chlorine and calcium ions to the water.

Other chemicals and materials in swimming pools can also contribute calcium ions to the water. The concentration of calcium and other ions must be closely monitored so that the water does not become saturated with a particular compound. The solubility product constant, termed K_sp, describes the amount of salt in moles that can be dissolved in one liter of solution to reach saturation. No more salt can dissolve after reaching the point of saturation.

Plaster that lines swimming pools is a form of hydrated calcium sulfate, CaSO₄. The calcium from chlorinating agents along with the calcium from plaster that lines swimming pools makes it necessary to monitor the concentrations of Ca²⁺ and SO₄^2- to make sure that saturation is not reached. The following equation describes the dissociation of calcium sulfate in water:

CaSO₄Ca²⁺ + SO₄^2-

The K_sp value for the discussed dissociation reaction can be calculated by determining the values of [Ca²⁺] and [SO₄^2-] in a saturated solution. If the K_sp value is known, the ion concentrations in swimming pools can be used with the K_sp value to predict whether or not the levels are at or near saturation.

1. If the K_sp of CaSO₄ is calculated to be 4.93 × 10^-5 at 25°C, what is the minimum amount of CaSO₄ that can be added to 3.75 × 10⁵ L of water to create a saturated solution?

A. 2.63 × 10³ grams

B. 3.58 × 10⁵ grams

C. 7.16 × 10⁵ grams

D. 2.52 × 10³ grams

2. Which of the following compounds, when dissolved in water, has the highest concentration of calcium for one mole of the compound?

A. CaCO₃ (K_sp = 4.8 × 10^-9)

B. CaF₂ (K_sp = 3.9 × 10^-11)

C. Ca₃(PO₄)₂ (K_sp = 1 × 10^-25)

D. Ca(IO₃)₂ (K_sp = 6.47 × 10^-6)

3. The K_sp of CaSO₄ can be calculated by determining the concentration of a saturated solution of CaSO₄. The following graph shows the relationship between concentration and conductivity for CaSO₄, which was determined by finding the conductance for four CaSO₄ solutions of known concentration. Using the graph, estimate the concentration of a saturated solution that has a conductivity of 2.5 × 10³ µS/cm and then calculate the experimental K_sp for a saturated solution of CaSO₄:

A. 2.25 × 10^-4 M²

B. 1.5 × 10^-2 M²

C. 3 × 10^-2 M²

D. 1.22 × 10^-1 M²

4. In the experiment, a probe was used to measure the conductivities in each solution. The probe generates a potential difference between two electrodes and reads the current that is produced as a voltage. The computer then outputs the conductivity. If several different solutions were all heated from room temperature to 75° Celsius, how would the conductivity of the solutions change and how would the K_sp be affected?

A. The conductivity would increase and the K_sp would increase.

B. The conductivity would decrease and the change in K_sp would increase.

C. The conductivity would increase and the change in K_sp cannot be determined.

D. The conductivity would decrease and the change in K_sp cannot be determined.