Physical Chemistry : Solubility Rules

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #12 : Phases And Properties Of Matter

Which of the following salts is insoluble?

Possible Answers:

Sodium phosphate

Potassium nitrate

Magnesium carbonate

Barium hydroxide

Correct answer:

Magnesium carbonate

Explanation:

Generally speaking, carbonate salts are insoluble, unless the carbonate is paired with an alkali metal. Magnesium carbonate, as a result, is considered an insoluble salt.

Example Question #1 : Solubility Rules

Which of the following solubility rules is incorrect?

Possible Answers:

Ammonium salts are considered soluble

Ionic compounds with halogens are generally soluble

Salts containing a nitrate ion are considered soluble

Phosphate salts are generally considered soluble

Correct answer:

Phosphate salts are generally considered soluble

Explanation:

Salts containing alkali metals, ammonium, or nitrate ions are considered soluble. Phosphates, on the other hand, are generally considered insoluble, unless paired with an alkali metal or ammonium ion.

Example Question #2 : Solubility Rules

A researcher is analyzing two bases. He puts both bases in a solution of water and notices that base A precipitates at the bottom of the solution whereas base B doesn’t. What can you conclude from the given information?

Possible Answers:

Base B likely has the higher dissociation constant

Base A produces more hydroxide ions than base B in solution

Base A could be calcium hydroxide

Base B could be barium hydroxide

Correct answer:

Base A could be calcium hydroxide

Explanation:

The question states that base A precipitates in a solution of water. We can conclude from this information that base A must be water insoluble. Recall that compounds that are hydrophobic tend to aggregate together when placed water, forming solid precipitates in the solution; therefore, base A must be a hydrophobic base. Base B, on the other hand, is soluble in water and doesn’t precipitate; therefore, base B must be a hydrophilic base.

Solubility rules state that all hydroxide compounds are insoluble in water, except sodium and potassium hydroxide. Calcium hydroxide is water insoluble and could possibly be the identity of base A.

Example Question #3 : Solubility Rules

Which of the following is a property of an atom that always makes a hydrophilic molecule?

Possible Answers:

It is an alkali earth metal

It has one valence electron

None of these are true

Both of these are true

Correct answer:

It has one valence electron

Explanation:

Hydrophilic molecules, by definition, are soluble in water. A compound’s solubility in water can be determined qualitatively using the solubility rules. If we look at the solubility rules, we will notice that there are three main cations that are always soluble in water. This means that a compound containing one of these three cations will always be soluble in water. The three cations are sodium, potassium and ammonium ions. Recall that sodium and potassium are alkali metals (column I of periodic table); therefore, these elements have one electron in its outermost shell (valence electron).

Alkali earth metals are on the second column of periodic table and have two valence electrons. Not all alkali earth metal containing compounds are water soluble (for example, calcium hydroxide).

Example Question #4 : Solubility Rules

Which of the following compounds will readily pass through the interior of a lipophilic membrane?

I. Potassium nitrate

II. Sodium chloride

III. Calcium carbonate

Possible Answers:

None of these can pass through the membrane

III only

I and II

I only

Correct answer:

None of these can pass through the membrane

Explanation:

Recall that interior of a cell membrane is highly nonpolar (due to the nonpolar tails of the cell membrane). Compounds that traverse the interior of a lipophilic membrane must be nonpolar. To solve this question, we need to figure out which compounds are nonpolar. Remember that all nitrates are soluble in water and, therefore, are polar. All alkali metal containing compounds (column I of periodic table) are soluble in water. This means that sodium chloride is water soluble because it contains sodium. Most carbonate salts are insoluble in water, including calcium carbonate; however, note that the calcium carbonate has polar groups with different electronegativities. This means that even though calcium carbonate is insoluble in water it contains polarity and therefore is not lipophilic. None of the compounds listed can traverse through a lipophilic membrane.

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