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Flashcards in MCAT Chemistry Crash Course Deck (43):

What is the shortcut equation to calculate the approximate pH of an acid?

If [H+] = n x 10 -e
then pH = {e-1}.{10-n}

Ex:   if [H+] = 6.2 x 10-4, then n = 6.2, and e = 4, so the pH can be approximated as equal to

[4-1].[10-6.2] = 3.38

The real pH is 3.21, which is within the acceptable error for the multiple choice MCAT.


In the below titration of the acid H2A, what is the earliest point where the solution is entirely A2-?

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Point D

A2- production is not complete until 2 full equivalents of base have been added.  This must be at the second equivalence point, which is Point D.


What is the molecular formula of this molecule?

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The molecular formula is the total of all the atoms present in a single molecule of the substance.

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What is the empirical formula of this molecule?

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The empirical formula is the ratio of the number of atoms in a substance, expressed as the lowest common denominator. 

In this case, take the molecular formula, C4H8, and divide both subscripts by 4 to get to the final answer.

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Knowing that the oxidation state of the oxygens in NO3- is -2, how do you calculate the oxidation state of the nitrogen?

1) Add up the total oxidation for all the molecule's known atoms:

(-2) * 3 = -6

2) Subtract that amount from the molecule's net charge:

(-1) - (-6) = +5

The remaining amount is the unknown atom's oxidation state.


  • H2S
  • S8
  • SO2

  • In H2S, S = -2

H2=2(+1)=2, S must be -2 to compensate.

  • In S8, S = 0

This is Sulfur's standard state, all atoms are zero oxidation in their standard state.

  • In SO2, S = +4

The two oxygens (-2 each) have a total charge of -4, S must be +4 to compensate.


Briefly describe the Bohr Theory of the atom.

The Bohr Theory states:

  • Electrons can only exist in fixed orbits or energy levels.
  • These energy levels are at specific distances from the nucleus.
  • Any energy emitted/absorbed from/by an atom would be the result of an electron jumping from one energy level to another.


What is the quantum number s or ms called?

What does it represent?

s or ms is the spin quantum number.

It represents the spin direction of an electron.

s can have exactly one of two values, +1/2 and -1/2, corresponding to spin-up and spin-down. These two values are inherently equal in energy.


the Aufbau Principle

The Aufbau Principle describes the order in which subshells are filled with electrons as atomic number increases. Aufbau is German for 'Building Up'.

Shells/subshells of lower energy get filled with electrons before higher energy shells/subshells.

Ex: The 1s subshell fills first, then 2s, then 2p, and so on.


Hund's Rules

Hund's Rules (a.k.a. the stinky bus rules), describe the order of adding electrons to an unfilled subshell.

Hund's Rules explain that when electrons are added to a subshell that has more than 1 orbital (p, d, or f), each orbital first receives a single electron, each spin-up, until each orbital in the subshell has one electron contained within it.

Only once the orbital is half full will spin-down electrons be added, one per orbital, until the subshell is completely filled.


Where are the Alkaline Earth Metals located on the Periodic Table?

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Alkaline Earth Metals make up the second column of the Periodic Table (Group IIA).

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What is the valence shell configuration of all Alkaline Earth Metals?

What oxidation state do they ionize to?

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All Alkaline Metals have an s2 valence shell configuration.

They are relatively electropositive, so they will lose those 2 valence electron easily to take on a +2 oxidation state. 


What is the valence subshell for the elements in the last six columns of the periodic table?

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The elements of the last six columns have a valence p subshell.

Ex: Group IIIA has an s2pvalence configuration, while VIIIA is s2p6.

Note: Although Helium is typically listed on the farthest column with the Noble Gases in VIIIA, it actually has a valence s subshell.

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Which has a larger radius, Si or P?

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Remember: Atomic radius decreases from left-to-right across a column, and P is to the right of Si in the Period 3. 

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formal charge

Formal charge = (# of valence electrons) - (# of lone pair electrons) - (1/2 # of bonding electrons)

FC is essentially a fictious charge assigned to each atom in a Lewis structure just for the sake of helping distinguish the best Lewis structure for a molecule.


How would you actually calculate formal charge with an easier, shortcut formula?

In order to calculate formal charge in a given Lewis structure diagram use:

FC = # valence electrons - lines - dots. 

Ex: in CO2, Carbon has 4 valence, 4 lines and 0 dots, FC = 4-4-0 = 0.  Oxygen has 6 valence and 4 dots and 2 lines, FC = 6-4-2 = 0. 

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a bent molecule

A bent molecule:

  • has 3 atoms bonded.
  • has a 104.5 degree bond-bond angle.
  • contains two single bonds and two lone e- pairs.

Classic examples include: H2O and SCl2

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electronic geometry

Electronic geometry is the position of all bonding electrons and lone e- pairs around one central atom. 

Ex: in H2O, Oxygen has two Hydrogens bonded and two lone e- pairs skew to those bonds, for 4 total electron positions in a tetrahedral shape around the Oxygen. H2O has tetrahedral electronic geometry. 

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sp3 hybridization

  • One s orbital mixes with three p orbitals.
  • The resulting molecule will be tetrahedral in geometry.
  • There are 4 single bonds around a central carbon atom.

Common examples: CH4, CCl4

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What are standard conditions?

Standard conditions for a reaction require that:
  • Pressure = 1 atm
  • Temperature = 25 C = 298 K
  • Concentration = 1 M for all products and reactants


What is the enthalpy change when 2 moles of CH4 are formed, according to the following reactions?

rxn1: 2H2(g)⇒4H(g)
ΔHrxn= -870 kJ/mol

rxn2:  C(s) + 4H(g)⇒CH4(g)
ΔHrxn= +794 kJ/mol

-152 kJ

1) Adding the reactions together yields the formation reaction of CH4:

C(s) + 4H(g) + 2H2(g)  ⇒
CH4(g) + 4H(g)
Canceling common terms leaves:
C(s) + 2H2(g) ⇒CH4(g)

2) To complete the calculation, combine the reactions' enthalpies in the same way the reactions were combined.

ΔHrxn = ΔH1 + ΔH2 
-870 + 794 = -76kJ/mol

3) Finally, multiply by the number of moles (2) to get the final answer.


What are the enthalpy and entropy changes for this reaction?

H2O (l) ⇒ H2O (g)

ΔHrxn > 0
ΔSrxn > 0

The reaction is endothermic, since heat must be added to vaporize the water.

Since the reaction creates gas, it represents an increase in entropy.


What are the characteristics of an adiabatic thermodynamic process?

An adiabatic process is any process where heat cannot flow. Hence, Δq = 0 and ΔE = Δw; all energy change is due to work being done on or by the system.

Adiabatic processes are processes that happen in either heat-insulated systems, or that happen so quickly that heat cannot flow between system and environment.


If a chemical reaction is first order in [A], how does the reaction rate vary if [A] doubles?

The reaction rate doubles.

First order in [A] means the rate law is:

rate = kobs [A]1

Let [A]orig = x.
Then rateorig = kobs * x.
If [A] doubles, then

ratenew = kobs (2x) = 2 * rateorig


a second order reaction

A second order reaction is one whose overall reaction order is 2, and whose rate either depends on one reactant to the second order, or two separate reactants to the first order.

If the reactants for the reaction are A and B, the rate will be one of:

Rate = kobs [A]2
or Rate = kobs [B]2
or Rate = kobs [A]1[B]1


In the chemical reaction Energy Profile below, what is the quantity A?

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A is the Activation Energy Ea.

The Activation Energy determines how much energy the reactants require to convert to the activated complex, or transition state. The higher the Activation Energy, the slower the reaction proceeds.

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aA (g) + bB (g) ⇔ cC (g)

is in equilibrium, and is endothermic, what happens if the temperature is increased?

More C will be created.

For an endothermic reaction, heat must be added; hence it can be thought of as a reactant. Increasing the temperature is akin to adding more reactant, therefore, and shifts the reaction to the right.


What phase conversions are being shown with arrows A and B?

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  • A is Sublimation
  • B is Deposition

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What points are arrows A and B pointing to?

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  • A is the Triple Point.
  • B is the Critical Point.

Note: though the concept of Plasma exists, the MCAT does not explicitly test this as a phase.

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the van't Hoff factor

The van't Hoff factor, i, is the number of particles that an ionic solute yields upon dissociation. 

Ex: NaCl dissociates into Na+ and Cl- ions, a total of 2 per equivalent of NaCl. The van't Hoff factor for NaCl is i=2.


  1. KOH
  2. C6H12O6
  3. H2SO4
  4. CaCl2
  5. H3[CuNH3Cl5]

  • KOH: i = 2 (K+ and OH-)
  • C6H12O6:  i = 1 (glucose doesn't dissociate)
  • H2SO4:  i = 3 (2H+ and SO42-)
  • CaCl2: i = 3 (2Cl- and Ca2+)
  • H3[CuNH3Cl5]: i = 4 (3H+ and [CuNH3Cl5]3-)
    note: on the MCAT you are expected to know that a complex ion (in brackets) will not further dissociate.


osmotic pressure

Osmotic pressure is the pressure that results from osmosis.

As water molecules move across a semi-permeable membrane to create isotonicity on both sides, the pressure of both sides will change; this pressure is the osmotic pressure.

Ex: If red blood cells are placed in a hypo/hypertonic environment, they will expand/contract in response.

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Van Der Waals Equation

(P + a(n/V)2)(V - nb) = nRT


  • P = Pressure in atm or Pa
  • V = Volume in L
  • n = number of moles
  • R = Ideal gas constant  .082 L(atm)/mol(K) or 8.31 J/mol(K)
  • T = Temperature in K
  • a = attraction between molecules due to intermolecular forces
  • b = actual volume taken up by gas molecules


Explain whether a polar or nonpolar real gas will deviate more from ideal?

Polar gases will deviate more from ideal.

(P + a(n/V)2)(V - nb) = nRT

Remember: 'a' is the attractiveness between molecules. When 'a' is big (polar=attractive) the Pressure term will be larger. Effectively, the gas will experience more pressure holding it together than the ideal gas law predicts. 


What type of electrochemical cell is being depicted below?

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The cell is an electrolytic cell.

The two main types of electrochemical cells (electrolytic and galvanic) can be easily discerned by the presence or absence of a power source. In this case, the battery is the power source, indicating an electrolytic cell.

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In the galvanic cell depicted below, is the Cu cathode going to get lighter or heavier as current flows through the circuit?

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The Cu cathode will get heavier as current flows.

Reduction occurs at the cathode in any electrochemical cell. In this case, the reduction reaction will result in Cu2+(aq) going to Cu(s). As the Cu2+ ions move out of solution, they will plate on the Cu electrode, which will get heavier.

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Give the molecular form and charge for these common ions:

  1. ammonium
  2. chloride
  3. dichromate
  4. mercury(II) mercuric
  5. silver

  1. ammonium, NH4+, +1 charge
  2. chloride, Cl-, -1 charge
  3. dichromate, Cr2O7--, -2 charge
  4. mercury(II) mercuric, Hg++, +2 charge
  5. silver, Ag+, +1 charge


Give the molecular form and charge for these common ions:

  1. sulfate
  2. nitrate
  3. peroxide
  4. hydronium
  5. iron (II) ferrous

  1. sulfate, SO4--, -2 charge
  2. nitrate, NO3-, -1 charge
  3. peroxide, O2--, -2 charge
  4. hydronium, H3O+, +1 charge
  5. iron (II) ferrous, Fe++, +2 charge


the common ion effect

If a solution already contains ions that a salt would separate into when dissolved, they are referred to as common ions.  The salt will be less soluble than if dissolved in a pure solvent. 

Ex: NaCl dissolved into chlorinated water (Cl- ions present) will be less soluble than NaCl in pure water. The added Cl- ion (product) shifts equilibrium to the side containing the solid salt (reactant).


Within the general category of Gases, what are you expected to know?

  • Pressure, Absolute Temperature, and STP
  • Molar volume, partial pressure, and mole fraction
  • Kinetic theory of gases, and ideal gas definition
  • Ideal gas law, Boyle's law, Charle's law, Avogadro's law
  • Dalton's law
  • Van der Waals equation


Within the general category of Intermolecular Forces and Phases, what are you expected to know?

  • Hydrogen bonding, dipole-dipole, London dispersion forces
  • Phase changes naming, phase diagrams
  • triple point, critical point
  • colligative properties: Raoult's law, b.p. elevation, f.p. depression, osmotic pressure
  • Colloids
  • Henry's law
  • Properties of water


Within the general category of Thermodynamics and Thermochemistry, what are you expected to know?

  • State functions: H, G, S, Q, W, E
  • Endo vs Exo thermic reactions
  • Hess's law, bond energies, heats of formation
  • Specific heat, heat capacity, values for water
  • Entropy. Free energe (Gibbs), Spontaneity of reaction
  • Zeroth law, First law, Second law
  • Equivalence of energy units, Heat transfer types, PV diagram and work
  • Heat of fusion, heat of vaporization, difference between heat of phase change vs heat of temperature change.