MCAT Solutions

Question 1

Solution

Answer 1

A mixture of substances that creates a homogenous mixture at the molecular level.

Question 2

What is solubility? What is it determined by?

Answer 2

Solubility is maximum amount of solute that’s dissolved in a solvent.

Determined by the intermolecular forces between the solute and solvent.

Question 3

Colligative properties

Answer 3

Properties of solubility that only depends on the concentration of solutes and not the properties.

Includes: boiling point, freezing point, vapor pressure, and osmotic pressure.

Question 4

How are solutions are formed?

Answer 4

Solvent- solvent interactions are broken

solute- solute interactions are broken

solute- solvent interactions are formed.

Energy input is required to break bonds while energy is released when creating bonds.

Solvent forms interactions with solute by surrounding them, creating a solvation shell.

Question 5

What causes the temperature of the solution to rise? Decrease?

Answer 5

The temperature will rise with an exothermic reaction due to the solute- solvent bond formation releasing heat in the form of energy and increasing temperature of surroundings.

The temperature will decrease with an endothermic reaction due to the solute- solvent bond formation requiring energy to break bonds.

Question 6

What are the ways we can measure the amount of solute in concentration?

Answer 6

By calculating either molarity, mass percent, or parts per million.

Question 7

Molarity

Answer 7

Moles of solute/ liters of solution

Question 8

Mass Percent

Answer 8

(Mass of solute/ Mass of solution) x 100

Question 9

ppm

Answer 9

Used for very, very small concentrations. Can be calculated via:

(mass of solute/ mass of solution) x 10^6

1 ppm = 1 g/ mL

1 ppm= mg of solute / L of solution

Question 10

Osmosis

Answer 10

The process of WATER moving from an area of low solute concentration to an area of high concentration.

Particles that cannot cross the semipermeable membrane are referred to as being osmotically active because they cause osmosis.

The act of water moving across the semipermeable membrane creates a pressure called osmotic pressure which we can calculate.

iMRT

i - van’t Hoff factor - number of osmotically active species that results from dissolution.

M- Molarity

R- ideal gas constant ( 0.8214)

T - temperature in kelvin

Question 11

How to solve for osmolarity?

Answer 11

Osmolarity = iM - RT/ (osmotic pressure)

Question 12

How is equilibrium established in solutions?

Answer 12

An equilibrium between the dissolution of a solute and the precipitation of the solute out of solution.

Question 13

Define solubility. What’s the relationship between the solute/ solvent ratio and solubility?

Answer 13

The amount of solute dissolved in a solvent at a given temperature and pressure.

Can be expressed in a table as: g of solute/ 100g H20.

When the solute/ solvent ration is below the solubility dissolution occurs. When the solute/ solvent ratio is above the solubility limit precipitation occurs.

Question 14

Solubility product constant ( Ksp)

Answer 14

The equilibrium constant that represents a dissolution and precipitation reaction.

Ksp = [ A] ^a [B]^b, where [A ] & [B] are resulting ions of the solution
* Pure liquids and solids are not included, just like in Keq*

a & b are stoichiometric coefficients.

Question 15

How can we use the Ksp to compare solubilities?

Answer 15

Higher the Ksp the more soluble it is, we compare the Ksp’s of compounds to each other BUT CAN ONLY COMPARE BETWEEN COMPOUNDS THAT DISSOCIATE INTO THE THE SAME IONS.

Question 16

Effect of temperature on Ksp?

Answer 16

Raising the temperature of an exothermic reaction will cause the equilibrium to shift to the solute (precipitation).

Raising the temperature of an endothermic reaction will shift the equilibrium towards the products (dissolution).

Question 17

Describe the common- ion effect

Answer 17

When compounds with similar ions are dissolved in a solution together the solubility decreases because there are more ions than either one of the compounds.

Result is the concentration to exceed the Ksp and the equilibrium to shift towards the solute (precipitation).

Question 18

How does changes in pH change solubility?

Answer 18

For acids, increase the pH increases their solubility but decreases it for bases.

For bases, decreasing the pH increases their solubility but decreases it for acids.

Question 19

Describe complex ions

Answer 19

A way to increase the solubility of an ion by combining it with a ligand to form a complex ion.

The complex ion shifts the equilibrium towards to the products because it decreases the concentration of the ion (because it’s being incorporated into ligand) thus shifting equilibrium towards products.

Question 20

Describe the solubility of a solid in a liquid v. a gas in a liquid?

Answer 20

Dissolving a solid in a liquid is an endothermic process because you must input energy to break the bonds. Imputing heat causes the bonds to break thus increasing the temperature increases the solubility.

Dissolving gas into a liquid is an exothermic process and so raising the temperature of solution decreases solubility.

Question 21

What does Henry’s law states?

Answer 21

Defines the solubility of a gas in a solvent at a given temperature.

States that the concentration of a gas dissolved is directly proportional to the partial pressure of the gas of the gas- solvent pair.

Cx = Kh * Px

Cx - concentration of dissolved gas

Kh - equilibrium constant for the gas- solvent pair.

Px- partial pressure of gas x in gas- solvent pair.