Solubility and Ions - MCAT Chemical and Physical Foundations of Biological Systems

An electrolyte is a substance that creates ions when in solution. These compounds conduct electricity very well by donating ions to an aqueous solution. The worst electrolyte will be the compound that creates the smallest amount of ions in solution. Silver chloride is relatively insoluble in solution, meaning that it will make the smallest amount of ions out of the given options.

Van't Hoff factor can also be useful in determining electrolyte effectiveness, but is irrelevant in this particular question.

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.

Use the equation for molarity, , first to figure out the number of moles of solute present.

Solving this gives 0.225 moles of solute. Using the same equation again with the final molarity will give the final volume.

Finally, the amount of solvent added is the difference between initial and final total volumes.

0.643L – 0.300L = 0.342L added

20 grams of ammonia is approximately 1.17 moles of ammonia, so both the molarity and molality of this solution would not approach 2.

Now, we need to determine whether molarity or molality is more concentrated. It helps to remember for the MCAT that 1 liter of water is equal to 1 kilogram of water. Molality uses the denomination of kilograms of water, and molarity mixes the solute until liters of solution are created. Since the mixed solution will incorporate the solute, it will require less than 1 kilogram of water. As a result, 2M is more concentrated than 2m.

To solve this question we must first determine the number of moles of HCl in the initial solution. Converting 100mL into 0.1L, there is initially 0.6mol of HCl in the solution.

When we dilute the solution with 500mL of water, the final volume of solution is 600mL.

To find molarity we can divide number of moles of solute by the total volume of solution.

The final concentration is 1M.

Molality is the number of moles of solute divided by kilograms of solvent. Because we are given the solute in mmol and the solvent in grams, we must convert them to moles and kilograms, respectively.

300 mmol is 0.3 moles. Additionally 60 grams is 6 x 10-2 kg.

Now we can solve for the molality.

First, find the mass of copper (II) sulfate in one liter of solution, using the molar mass of copper (II) sulfate.

We now have the density of the copper (II) sulfate in the solution. Using the total density of the solution, we can calculate the contribution from water to this density.

Now that we know the mass of water per liter of solution, we need to convert to moles.

We now have the moles of copper (II) sulfate per liter and the moles of water per liter, allowing us to find the molar ratio in the solution.

Concentration can be measured in several different ways. Molality is moles of solute per kilogram of solvent. Molarity is moles of solute per liter of solution. Normality is equivalents of protons per liter of solution.

In our question, we are given the molality of the solution.

When using molality, it is important to realize that the solute occupies volume. One kilogram of water is equal to one liter of water, but there is an additional volume of four moles of solid sodium chloride that is added to the solution. A one-liter sample of this solution will, thus, contain less than one liter of water.

Recall that molality is defined as:

In this problem there is one half mole of sucrose per one kilogram of water.

Our goal is to calculate the mole fraction:

We need to calculate the number of moles of water in one kilogram of solution.

We now know that, in one kilogram of solution, there will be of sucrose and of water. Use these values in the mole fraction equation to solve for the fraction of sucrose.

In this particular scenario, molarity can be used interchangeably with normality. The product of the initial normality and initial volume will be equal to the product of the final normality and final volume.

Our initial normality is 0.7N and our initial volume is 0.02L. We also know our final volume is 1L.

Solve for the final concentration.

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: .

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.)

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.

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:

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.

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:

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.

The central iron in hemoglobin is the middle of a complex compound and is involved in six covalent bonds (coordination number of 6). It is bonded to five nitrogen atoms, one of which is part of an amino acid in the globin protein. This leaves one remaining bond, which can be with water, oxygen, or carbon monoxide (which forms a very stable bond, making it toxic).

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.