MCAT Physical : Solubility Rules

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #1 : Solubility Rules

Which of the following reactions will produce a precipitate?

Possible Answers:

Correct answer:

Explanation:

Knowledge of basic solubility rules is enough to answer this question. Nitrates and salts of alkali metals are always soluble, as are salts of ammonium. Acetates are also always soluble. Acid-base reactions, such as with hydrofluoric acid, produce a soluble salt and water. Sulfates are commonly soluble, with certain exceptions (mostly alkaline earth metals).

The only reaction that produces an insoluble product is that between silver nitrate and potassium chloride. Silver chloride is a common precipitate obtained in double replacement reactions.

Example Question #2 : Solubility Rules

Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils.

Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here.

In this law,  is the mole fraction of the solvent, is the vapor pressure of the pure solvent, and  is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.

A scientist is studying boiling point changes with the addition of solutes, and creates a colloid. A colloid is similar to a solution because both will possess which of the following characteristics?

I. They both show the Tyndall effect on light

II. They both involve the suspension of only molecular-sized particles

III. They both involve the suspension of particles that are too small to be individually distinguished with the naked eye

Possible Answers:

I, II, and III

III, only

I and III

I and II

II and III

Correct answer:

III, only

Explanation:

A colloid is a suspension of particles larger than molecules, but too small to be individually distinguished by the naked eye. Only colloids disperse light by the Tyndall effect. Milk is an example of a colloid.

Example Question #3 : Solubility Rules

Which of the following compounds would be generally insoluble in an aqueous solution?

Possible Answers:

NH4OH

MgCl2

BaSO4

NaOH

Correct answer:

BaSO4

Explanation:

Knowing some of the general solubility trends will greatly facilitate your understanding of solubility in chemistry. Here are a couple guidelines you can follow in order to predict which compounds are soluble.

1. Compounds containing alkali metals, ammonium cations, or nitrate anions are soluble.

2. Compounds containing halogen anions are soluble. Key exceptions are halogens attached to silver, mercury, and lead.

3. Sulfates are soluble, except when attached to heavier alkaline metals, like barium (thus the correct answer).

4. Carbonates, phosphates, and hydroxides are generally insoluble; however, if they are attached to one of the ions mentioned in one of the above points, they are soluble.

Example Question #4 : Solubility Rules

Which of the following molecules is insoluble?

Possible Answers:

Mg(NO3)2

NaCl

AgNO3

AgCl

KBr

Correct answer:

AgCl

Explanation:

These solubility rules should be known for the MCAT and are presented as a hierarchy.

1. All group 1 salts and ammonium salts are soluble

2. All nitrates, perchlorates, and acetates are soluble

3. All mercury, lead, and silver salts are NOT soluble

Understanding these solubility rules as a hierarchy, we can understand why Mg(NO3)2 and AgNO3 are soluble and why AgCl is not soluble. 

Example Question #5 : Solubility Rules

Which compound is most likely to be soluble in pentane?

Possible Answers:

Correct answer:

Explanation:

Molecules are most soluble in solvents that possess similar properties and intermolecular forces to the solute.

Pentane is a non-polar compound, which interacts predominately through london dispersion forces. Ammonia, silver chloride, and phosphorus trichloride are all polar molecules. Ammonia has a lone pair on the nitrogen; silver is an ionic compound; phosphorus trichloride has lone pairs on both the phosphorus and chlorine atoms.

In contrast, carbon dioxide has lone pairs only on the oxygen atoms and has a linear geometry. This creates a line of symmetry in the molecule such that the net polarities of the carbon-oxygen bonds cancel each other. Even though carbon dioxide has polar bonds, it is not a polar molecule and would likely dissolve in pentane.

Example Question #6 : Solubility Rules

All of the following compounds are soluble in water except __________.

Possible Answers:

Correct answer:

Explanation:

Compounds that contain sulfate groups (  )are soluble in water, unless they are bound to mercury, strontium, lead, calcium, or barium. Hence, barium sulfate, , is insoluble in water.

Compounds that contain nitrates ( ), ammonium (  ), and alkali metals are generally soluble in water. Compounds containing halogens are soluble in water unless they are bound to mercury, lead, or silver.

Example Question #7 : Solubility Rules

Which salt will produce a basic solution when dissolved in water?

Possible Answers:

Correct answer:

Explanation:

When  is dissolved in water, it forms  and . Both of these are basic compounds.

Each of the other answer options will result in at least one acidic compound when dissolved in solution.

Example Question #8 : Solubility Rules

Substances for which pure water would not be an efficient solvent are __________.

Possible Answers:

those that have large dipole moments

those that have high charge densities

those with evenly dispersed electrons

those that tend to form hydrogen bonds

Correct answer:

those with evenly dispersed electrons

Explanation:

Since water is polar, it dissolves substances that have electrostatic charge due to electron arrangement. All of the answer choices describe molecules that would have positive and negative components except those with evenly dispersed electrons. This describes a nonpolar molecule, which would not dissolve in water.

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