Question

MCAT Tutorial: Solution Chemistry

Answer 1

\({\bf{Basic~Terminology:}}\)
- solution: homogenous mixture of 2+ chemicals
- solvent: the compound that does the dissolving, most common is water
- solute: the dissolved substance
- solubility curve: graph that plots solubility of a substance vs temperature
> typically given per 100g solvent
> line shows the exact amount of solute that would produce an exactly saturated solution; above the line would be supersaturated, below would be unsaturated

Answer 2

Monoatomic ions are pretty straightforward, just take the stem of the element and add the suffix "-ide", ex. chlorine becomes chloride

Polyatomic Ions (you just have to memorize these unfortunately, flashcards are your best friends here)

Pattern for halogen polyatomics:

Answer 3

Solvation: interaction between solvent and solute particles in solution formation
Hydration: solvation with water as the solvent
Hydration shell: the name given to the structure of water molecules surrounding the solute
Hydration #: number of water molecules around the solvent

\({\bf{Solubility~Rules:}}\)
these are a total pain but you will have to memorize these
1. group 1 and ammonium ions (cations) = soluble in water
2. nitrate ion (anion) = soluble in water
3. chlorides, bromides, iodides (ions) are soluble except silver, lead(II) and mercury(II)
4. sulfates soluble except calcium, strontium, barium, lead(I)
5. oxides insoluble except for rule 1
6. hydroxides are insoluble except rule 1, plus calcium, strontium, and barium hydroxide (which are slightly soluble)
7. caarbonate, phosphate, sulfide ions are insoluble except rule 1

\({\bf{Solubility~of~Solids:}}\) increases with temperature, not affected by pressure
increasing surface area increases the rate of dissolving but does not affect solubility (which is the total amount that will dissolve)

\({\bf{Solubility~of~Gases:}}\) decreases as temperature increases, increases as pressure increases (Henry's Law)

\({\bf{Miscibility~of~Liquids:}}\) the extent to which liquids can mix, based on polarity (polar dissolves with polar, nonpolar dissolves with nonpolar, ex. oil vs water)

Answer 4

\({\bf{Measuring~Solubility:}}\)

- molarity = moles solute/L solution
- molality = moles solute/kg solvent (not solute. you must subtract the weight of the solute first)
- mole fraction = moles of a particular substance/total # of moles
- mass percent: mass of a particular substance/total mass

rule for dilutions: M1V2 = M2V2

\({\bf{Solubility~Product~Constant~Ksp:}}\)

Ksp = [cation]^(coefficient)[anion]^(coefficient)

be careful with cation/anion concentrations, ex. an ion like Ca(OH)2 produces 2 mol hydroxide per mol of compound

Common Ion Effect: if one of the ions being dissolved is already present, adding more of that ion shifts equilibrium away from that ion, thus decreasing solubility

ex. for a generic reaction

[AB2] <--> [A]2+ + 2[B]-
where A is the cation and B is the anion

suppose we already have 0.1M of ion B

therefore ksp becomes

Ksp = (x)(0.1+2x) where x is the molarrity of dissolved ion, notice we multiplied [B]- by 2 since there are 2 moles being dissolved

assuming that x is small, we can re-write this as

Ksp = x(0.1) which produces a smaller x value than the default case

Answer 5

\({\bf{Complex~Ions:}}\) metal cation bonded to multiple smaller ions (ligands)
- soluble in water (ion-dipole)
- # of ligands approx double the charge [this rule doesn't always work but can be a rough estimate]

examples (don't need to memorize these):

\({\bf{Colloids~and~Suspensions:}}\)
- colloid: homogeneous, larger paticles than a solution, cannot be filtered out ex. aerosols, foams, emulsions
- emulsion: two immiscible liquids, thermodynamically unstable (will separate over time)
- Tyndall effect: scattering of light by colloids/suspension particles
- suspension: heterogenerous, affected by gravity, can be filtered, ex. salad dressing, muddy water

Answer 6

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT