Question

MCAT Tutorial: Introduction to Acid/Base Chemistry

Answer 1

no pikabats allowed

Answer 2

\({\bf{Acid/Base~Defintions:}}\)

Arrhenius: acids produce hydrogen ions (H+), bases produce hydroxide ions (OH-)
Bronsted-Lowry: acids donate H+ ions, bases accept them
Lewis Acid-Base Theory: acids accept electron pairs, bases donate them [definitely remember this for orgo]

conjugate acids: the compound corresponding to a base after it has accepted a proton/donated an e- pair
conjugate bases: the compound corresponding to an acid after it has donated a proton/accepted an e-pair

this is probably easier to see with an example so:

Answer 3

\({\bf{pH~and~Dissociation~Constants:}}\)

pH is a logarithmic scale used to describe the [H+] concentration
pH = -log[H+]
pOH = -log[OH-]
pH + pOH = 14

on the MCAT you will not be given a calculator so you must estimate to the nearest 10th, calculate the pH, and make adjustments accordingly

ex. for a strong acid of concentration 0.03, consider a concentration of 0.01, -log(0.01) = 2, then for a concentration of 0.03 it's gotta be a little lower than 2 since the hydronium ion concentration is greater than 0.01 but not greater than 0.1

below 7: acidic
7: neutral
above 7: basic

Ka is the acid dissociation constant, for a general acid this is: [H3O+][A-]/[HA]
Kb is the base dissociation constant, for a general base this is: [BH+][OH-]/[B-]
Kw describes the self-ionization of water, which happens regardless of the solution and must be accounted for for very small acid/base concentrations: [H3O+][OH-] = 1 * 10^(-14)

note: Ka * Kb = Kw (be careful for polyprotic acids)

\({\bf{Strong~Acids~and~Bases~(Memorize~This):}}\)
acids: HI, HCl, HBr, H2SO4, HNO3, HClO4
bases: most group 1 and group 2 hydroxides, NH2-, O^{2-}, H-

the following chart is more useful for organic chem but still worth looking at:

Answer 4

\({\bf{Weak~Acids~and~Bases:}}\) have a low dissociation rate/low Ka/Kb

as we stated before, Ka = [H3O+][A-]/[HA]

if we let "x" represent the amount dissociated and M = the molar concentration, then

Ka = x*x/(M-x)

for some calculations you will be able to assume that the dissociation is close to 0, making the denominator approximately M

Ka ≈ x^2/M

note: after you solve for x using this method, you must double check if the dissociation rate is <5% by calculating (x/M)*100 to get the percentage

Answer 5

\({\bf{Salts~and~Hydrolysis:}}\)
a salt is made from an acid ion and base ion; acid ion is negative, base ion is positive
- relative strength of the corresponding acids/bases determines the characteristics of the salt
> neutral salt: strong acid + strong base
> acidic salt: strong acid + weak base, ex: NH4NO3 since the NH4+ comes from a weak base and NO3- comes from a strong acid
> basic salt: weak acid + strong base, ex: NaC2H3O2 since the Na+ comes from a strong base (either NaOH or NaH) and the C2H3O2- comes from a weak acid (acetic acid)
> indeterminate: weak acid + weak base, would have to do another chemical analysis, beyond the scope of this tutorial

hydrolysis: process by which the ions of a salt react w/ water
can calculate the pH of a salt solution by considering the reaction of its corresponding base/acid

ex. for an acidic salt:

BH+ + H2O <--> B + H3O+

then apply the Ka (if given Kb, must convert to Ka) to calculate the [H3O+] concentration, and thus, the pH

Answer 6

\({\bf{Buffers:}}\) are a solution of a weak acid and its salt, or a weak base and its salt (must share a common ion)

when acid is added, the basic component will react with the hydronium ions, thus reducing the pH change

when base is added, the acidic component will react with the hydroxide ions, thus reducing the pH change

blood pH works on a buffer system involving carbonic acid and its CB, the bicarbonate ion

Henderson Hasselbach equation:

pH = pKa + log(base/acid)
note: pKa is the -log of the Ka
pOH = pkB + log(acid/base)

Answer 7

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