Chapter 9: Solutions Flashcards
What is a solution? What are they composed of (hint: solvents and solutes is the answer, which one is which)?
Solutions are homogenous mixtures, two or more substances that combined to form a single phase, usually the liquid phase.
A solution consists of a solute dissolved in a solvent. The solvent is the component of the solution that remains in the same phase after mixing.
The MCAT will focus almost exclusively on solids dissolved into aqueous solutions, but it’s important to remember that gases can dissolve in liquids, liquids can be dissolved into other liquids, solids can even be dissolved in other solids.
Why are gases mixed with gases considered mixtures? Are all mixture solutions?
Gases “dissolved” into other gases can be thought of as solutions, but are more properly defined as mixtures because gas molecules do not interact all that much chemically, as described by the kinetic molecular theory of gases.
All solutions are considered mixtures, but not all mixtures are considered solutions.
What is solvation?
Solvation, also known as dissolution, is the electrostatic interaction between solute and solvent molecules. When water is the solvent, it can be called hydration.
Solvation involves breaking intermolecular interactions between solute molecules in between solvent molecules and forming new intermolecular interactions between solute and solvent molecules together.
When is solvation exothermic? When is solvation endothermic?
Solvation is exothermic when the new interactions are stronger than the original ones. The dissolution of gases and liquids, such as CO2 into water, it’s an exothermic process because the only significant interactions that must be broken are those between water molecules. Le Chatelier’s principal tells us this is the reason that lowering the temperature of a liquid increases solubility of a gas in the liquid.
Solvation is endothermic when the new interactions are weaker than the original ones, and the process is favored at high temperatures. An example would be sugar and water.
What is an ideal solution?
An ideal solution is a type of solution for which the enthalpy of dissolution is equal to zero. This happens when the overall strength of the new interaction is approximately equal to the overall strength of the original interactions.
What is solubility?
The solubility of a substance is the maximum amount of that substance that can be dissolved in a particular solvent at a given temperature.
A solution is saturated when the maximum amount of solute has been added.
What are sparingly soluble salts? When are solutes considered soluble?
Sparingly soluble salts dissolve minimally in solvent (molar solubility of under 0.1M in grams per Liter).
Solutes are considered soluble when the Gibbs free energy change is negative at a given T, thus spontaneous, and are considered such when they have a molar solubility above 0.1 M.
Which salts (cations and anions) are considered always water soluble? What are some common insoluble ions?
All salts containing ammonium (NH4+) and alkali metal (group 1) cations are water soluble.
All salts containing nitrate (NO3-) and acetate (CH3COO-) anions are water soluble.
Some common insoluble ions include Pb+ and Ag+.
What are complex ions, also known as coordination compounds? What is a good example of a biologically relevant coordination compound?
Complex ions refer to a molecule in which a cation has s bonded to at least one electron pair donor. The electron pair donor molecules are called ligands.
The electron pair donor is called a Lewis base, the electron pair acceptor is called a Lewis acid.
A complex ion is an ion made up of a central metal ion and one or more surrounding ligands.
A biologically relevant example would be the iron cation in hemoglobin bonding with oxygen, CO2, and CO as ligands.
What is chelation? What is chelation therapy?
In some complexes, the central cation can be bonded to the same ligand in multiple places. This is called chelation. Chelation generally requires large organic ligands that can double back to form a second or even third bond with the central cation.
Chelation therapy is often used to sequester toxic metals (lead, arsenic, mercury, etc) The chelating agent, like ethylenediaminetetraacetic acid (EDTA), is administered intravenously and binds to metals in the bloodstream.
Describe the process of solvation.
Solvation refers to the breaking of intermolecular forces between solute particles in between solvent particles, with formation of intermolecular forces between solute and solvent particles. An aqueous solution, water is the solvent.
Describe the differences between solubility and saturation.
Solubility is the amount of solute contained in a solvent. Saturation refers to the maximum solubility of a compound at a given temperature; one cannot dissolve any more of the solute just by adding more at this temperature.
What is one way in which solubility of a compound can be increased?
Solubility of solids can be increased by increasing temperature. Solubility of gases can be increased by decreasing temperature or increasing the partial pressure of the gas above the solvent (Henry’s law).
Name two ions that form salts that are always soluble.
What is concentration?
Concentration denotes the amount of solute dissolved in a solvent. Concentrations are commonly expressed as percent composition by mass, mole fraction, molarity (moles / L), molality (moles solute / kg solvent), and normality (equivalents / L).
Example percent composition by mass page 327:
How do you calculate mole fraction (X) of a compound? When are mole fractions used?
Mole fractions are used for vapor pressure depression of a solution, and partial pressure of a gases in a system.
Example mole fractions page 328:
What are the following fractions expressed as decimals:
What is Molarity? Units? THIS IS THE MOST COMMON UNIT FOR CONCENTRATION ON THE MCAT.
Molarity calculation example page 329:
What is molality (m)? When is molality used?
The special situations were molality is required are boiling point elevation, and freezing point depression. As aqueous solutions become more concentrated with solute, their densities become significantly different from that of pure water; most water soluble solutes have molar masses significantly greater than that of water, so the density of the solution increases as the concentration increases.
Molality equation example page 329
What is Normality?
Normality of a solution is equal to the number of equivalents of interest per L. For example, there are 2 equivalents of H+ per mole of H2SO4. So one mole of H2SO4 in 1L solution would have a normality of 2N.
What is dilution? What is the equation for dilution? THIS ONE IS SUPER IMPORTANT TO ‘MEMBER.
A solution is diluted when solvent is added to a solution of higher concentration to produce a solution of lower concentration.
Solution example page 330
Concept check 9.2 page 331 (1)
Concept check 9.2 page 331 (2)
Concept check 9.2 page 331
Most solubility problems on the MCAT deal with solutions of sparingly soluble salts, which are ionic compounds that have very low solubility in aqueous solutions.
Let’s start with a common sparingly salt reaction using silver chloride, which dissociates sparingly in water according to the equation in the image.
What is Ksp?
Do Ksp equations have denominators? Why or why not? Are solubility product constants temperature dependent? Are they pressure dependent? Is there a difference for temperature and pressure with different varieties of solutes (hint gas and not gas)?
What is the ion product (IP) and how does it relate to solubility product constants?
What concept are these equations a special version of?
What can you say about the reaction if:
IP<Ksp
IP=Ksp
IP>Ksp
Example question Ksp (solubility) page 334
Example solubility page 335:
What would be the Ksp of a sparingly soluble salt with general formula MX? MX2? MX3?
What does a high Ksp tell you about a complex or ion?
A high Ksp value means that a substance dissolves a lot in aqueous solution.
For example complex ions with multiple polar bonds between ligands and central metal ions have very high Ksp meaning they readily dissolve a lot in aqueous solutions.
What is Kf (if hint needed: the formation of stability constant)?
When forming a complex ion, one must often use a mixture of solutions. For this reason, a distinction must be made between the Ksp of the solution, the net of the complex ion itself. The dissolution of the original solution is termed Ksp, and the subsequent formation of the complex ion in solution is termed Kf, the FORMATION of Stability Constant of the complex in solution. Example as follows:
Interesting example using Kf and Ksp of a reaction. Read this. Understand this.
Example of common ion effect page 338. This is one of the more challenging questions regarding solubility that can be found on the MCAT. I’m going to show the whole question and answer in the question slide. The answer slide will be my own notes.
Concept check 9.3 1 page 338 solubility
Concept check 9.3 2 molar solubility page 339
This one I need help on the math.
Concept check 9.3 3 page 339 molar solubility. Get to the point where you super know this and these kinds of equations.
What is Raoult’s Law (hint is needed: colligative properties)? Describe it in a graph of vapor pressure v % water. What happens at a molecular level to evaporation and condensation?
Raoult’s law account for vapor pressure depression, a colligative property, caused by solutes in solution.
As solute is added to a solvent, the vapor pressure of the solvent decreases proportionately.
On a molecular level, the presence of solute molecules can block the evaporation of solvent molecules (but not their condensation)
Fun pictures and stuff:
Example Raoult’s law (vapor pressure depression) page 341
Example vapor pressure page 342
What is the equation for vapor pressure depression (Raoult’s Law)?
What is boiling point elevation? What is the equation for boiling point elevation?
When a non volatile solute is dissolved into a solvent to create a solution, the boiling point of the solution will be higher than that of the pure solvent. The extent to which a boiling point is elevated relative to that of the more solvent is given by the formula:
Boiling point elevation example page 343
What is freezing point depression? What is the equation for freezing point depression?
The presence of a solute particles in a solution interferes with the formation of the lattice arrangement of solvent molecules associated with the solid state. The formula for calculating freezing point depression is:
Example freezing point depression page 344
Take note that these calculations use molality (moles/kg solvent). This makes sense because it needs to be standardized into the known quantity of how much solvent (typically water).
What is osmotic pressure? What is the equation for osmotic pressure? Is there a fancy picture that you could draw?
Osmotic pressure is the “sucking” pressure generated by solutions in which water is drawn into solution.
Formally, the osmotic pressure is the amount of pressure that must be applied to counteract the attraction of water molecules for the solution. For the technically correct kind of person.
Notice the units for M are MOLARITY (moles/liter SOLUTION). Molarity is concentration in solution defined as the number of moles per liter of SOLUTION.
Concept check 9.4 2 solutions page 345
Concept check 9.4 3 solutions page 345
Concept check 9.4 4 solutions page 345
Concept check 9.4 5 solutions page 345
Chapter 9 equations:
For working together with someone, the person can ask “what is the equation for….?” And ask me to dictate and talk about it.
Mastery assessment chapter 9 solutions page 312 question 1
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Mastery assessment chapter 9 solutions page 313 question 9
Page 353. Read this. Understand this.
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Answer on page 353
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