# MCAT Physical : Solubility and Ions

## Example Questions

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### Example Question #1 : Ions In Solution

A U-shaped tube is filled with water and then split into two sections by a membrane at the lowest part of the tube. The membrane is permeable to water, but is impermeable to ions. 50g of salt are added to the left side of the tube and allowed to enter solution.

Which of the following would not result following the addition of salt?

The concentrations of both sides will become equal as water flows through the membrane.

The forces of entropy and pressure will be equal when the solution is at equilibrium.

The osmotic pressure of the left side will be greater than the right side.

The water will be higher on the side that has salt.

The concentrations of both sides will become equal as water flows through the membrane.

Explanation:

Osmotic pressure is defined as the tendency for water to diffuse into a solution via osmosis. Since the membrane is not permeable to ions, the salt ions are unable to cross the membrane and make the concentrations equal on both sides of the tube. Instead, water will flow to the side that has salt until the pressure in the tube with salt equals the forces of entropy. This will result in a higher level of water on the side of the tube with salt in it.

### Example Question #2 : Ions In Solution

The Haber-Bosch process, or simply the Haber process, is a common industrial reaction that generates ammonia from nitrogen and hydrogen gas. A worker in a company generates ammonia from the Haber process. He then dissociates the gaseous ammonia in water to produce an aqueous solution. Since ammonia is a base, it will accept a proton from water, generating  and ammonium ion products. The two reactions involved are:

The ammonium ion generated from the dissociation of ammonia is __________.

an anion because it gains an electron

a cation because it loses an electron

an anion because it loses an electron

a cation because it gains an electron

a cation because it loses an electron

Explanation:

Remember that there are two types of ions: cations and anions. Cations are produced when a atom of a molecule loses an electron. A neutral atom will contain no charge; however, when the atom loses an electron the atom becomes positively charged because it will contain more protons than electrons. In reaction 2, the nitrogen in ammonia loses an electron (because it shares the electron with the new hydrogen atom), which produces a positively charged ammonium ion. The ammonium ion is a cation because of this positive charge.

Anions are formed when an atom gains an electron, which makes the overall charge of an atom negative. The hydroxide ions from reaction 2 are anions.

### Example Question #3 : Ions In Solution

Which of the following solutions is NOT a good electrolyte?

CO2

CH3COOH

NaCl

NaOH

CO2

Explanation:

Electrolyte solutions are formed when a compound creates ions once in solution. Carbon dioxide will not create ions in solution, so it is not a good electrolyte.

(This is the acid dissociation for acetic acid.)

### Example Question #4 : Ions In Solution

What is the Vant Hoff factor of the molecule Li3PO4?

Explanation:

The Vant Hoff factor indicates how many particles a solid produces when dissolved in solution. When Li3PO4dissolves in solution, there are three Li+ molecules and one molecule of PO4-3. The Vant Hoff factor is equal to the sum of molecules: .

### Example Question #5 : Ions In Solution

Hydrochloric acid and sodium sulfite react in aqueous solution via the process:

Which of the following correctly expresses the net ionic equation for this reaction?

Explanation:

Start by writing the total ionic equation, using the following steps.

1. Break all soluble strong electrolytes, denoted by "(aq)," into their ions, indicating the number and charge of each ion.

2. Bring along unchanged any compounds denoted as "(s)," "(l)," or "(g)."

Remove any ions appearing on both sides of the reaction (any "spectator" ions) to get the net ionic equation.

Spectator ions:

### Example Question #6 : Ions In Solution

All of the following are true regarding a solution except __________.

A saturated solution is in a state of equilibrium

A solution can contain any of the three phases of matter

Solvation is the process by which a solute dissolves into a solvent and becomes a solution

An under-saturated solution is a solution that contains less solute than the solvent is capable of dissolving

A solution is composed of a solute in one phase dissolved in a solvent in another phase

A solution is composed of a solute in one phase dissolved in a solvent in another phase

Explanation:

Solutions can occur in all three phases of matter. They can also occur between two compounds in a single phase. For example, brass is a solution of two metals: zinc and copper.

### Example Question #7 : Ions In Solution

Electrolytes play a big role in maintaining blood pressure. Loss of electrolytes often leads to a drop in blood pressure. What can a doctor prescribe to a patient who has very low blood pressure?

Decrease the consumption of salt

Spend an hour a day in a hot sauna

Increase the consumption of sodium chloride

Increase the consumption of lithium chloride

Increase the consumption of sodium chloride

Explanation:

The question states that the patient has a low blood pressure, suggesting that the patient has a low concentration of electrolytes in his blood. To counter this, the doctor must prescribe a solution that will increase the concentration of electrolytes in blood. The best solution would be to increase the consumption of salt, or sodium chloride. Sodium chloride is a strong electrolyte and will dissociate into sodium ions and chlorine ions. This will increase the blood pressure of the patient and will remedy the problem.

Spending an hour a day in a hot sauna will cause excessive sweating. Remember that salts are excreted through sweat; therefore, the electrolyte concentration, and the patient's blood pressure, will decrease further if the patient spends time in a sauna. Although lithium chloride is a strong electrolyte (like sodium chloride), the doctor will not prescribe lithium chloride because it is a toxic substance.

### Example Question #8 : Ions In Solution

Adding ammonia to a solution containing a copper hydroxide precipitate has the effect of dissolving the precipitate because __________.

the addition of ammonia allows more hydroxide and hydrogen ions to form water molecules, increasing the solubility of copper hydroxide

an increase in hydronium ions shifts the reaction of copper and hydroxide to the left

covalent bonds form between copper ions and ammonia molecules

hydroxide ions bond ionically with ammonia

covalent bonds form between copper ions and ammonia molecules

Explanation:

The copper ion is able to form four covalent bonds. This can be explained with electron configurations.

The copper atom configuration:

The copper ion configuration:

The copper ion can accept electrons that fill empty  and  levels. In this example, the chemical equation is as follows:

### Example Question #9 : Ions In Solution

and  are alike in that __________.

both compounds are able to form coordinate covalent bonds with a metal ion

both involve hybridization of the  energy level

both involve coordinate covalent bonds that utilize empty electron orbitals in the metal ion

both involve ionic bonds that form due to polarity

both involve coordinate covalent bonds that utilize empty electron orbitals in the metal ion

Explanation:

Complex ions typically form when coordinate covalent bonds are formed between ligands, such as water or chloride, and metal ions that have lost electrons and have available orbitals. While one answer choice, "Both compounds are able to form coordinate covalent bonds with a metal ion," may seem correct, this happens within the compound. The compounds themselves are ions that will form ionic compounds.

### Example Question #10 : Ions In Solution

Which is an inaccurate depiction of the central iron atom in a hemoglobin molecule?

It has a coordination number of 6.

It is covalently bonded to five nitrogen atoms, one of which is a component of the globin protein.

It is covalently bonded to five water molecules that can easily be replaced with oxygen molecules.

It is able to form coordinate covalent bonds with water, oxygen, and carbon monoxide.