MCAT Gen Chem

Question 0

What are standard conditions?

Answer 0

298K (25 degrees C), 1 atm, and concentration of 1M

Question 1

What is the difference between the empirical and molecular formula?

Answer 1

molecular takes into account the number of every atom.

the empirical formula deals with the ratios of atoms.

example: C4H8 vs. CH2

Question 2

What equation do we get from combining Planck and Bohr?

Answer 2

E = hc/(lambda) = -R/([1/ni^2] - [1/nf^2])

R = Rydeberg constant = 2.18 x 10^-18 J/electron

this equation states that the energy of the omitted photon corresponds precisely to the direct difference between the higher energy state of the electron to the lower energy state.

Question 3

how do you calculate the percent composition?

Answer 3

% composition = (mass of X in formula)/(formula weight of compound) x 100

Question 4

What are the units of rate of reaction expressed in?

Answer 4

Moles/(liters x secs)

Question 5

what is the rate equation?

Answer 5

Rate = k[A]^x[B]^y

Question 6

What is the units for Normality and what does it measure?

Answer 6

Normality is equivalents per liter, and measures concentration.

Question 7

What is C6H12O6 the formula for?

Answer 7

glucose.

Question 8

What does Gibbs Free Energy tell us about a reaction?

Answer 8

Whether or not it will be spontaneous.

Question 9

What is the equation for the Law of Mass Action?

Answer 9

Keq = [C]^c[D]^d/([A]^a[B]^b)

Question 10

What is the difference between K and Q?

Answer 10

K and Q are very similar. K is for the equilibrium position, and Q is for the position of the reaction at any given time. Remember these:

1. Q < K: rxn has not yet reach equilibrium
2. Q > K: rxn has exceeded equilibrium
3. Q = K: rxn is at equilibrium

Question 11

Is Keq temperature dependent?

Answer 11

yes.

Question 12

If Keq = x in one direction, what does it equal in the reverse direction?

Answer 12

1/x

Question 13

What are the properties of isobaric, adiabatic, and isothermal systems?

Answer 13

Adiabatic: no heat is exchanged between the system and the environment.

Isothermal: the system’s temperature is constant.

Isobaric: the pressure of the system is constant.

Question 14

What defines “standard conditions” and what are these conditions used to measure?

Answer 14

Standard conditions: 298K and 1atm. Standard conditions are used for measuring enthalpy, entropy and Gibbs free energy changes of a system.

Question 15

What is the equation for the first law of thermodynamics?

Answer 15

delta U = Q - W

the total internal energy of a system is equal to the heat put into the system minus the work gotten out of the system.

Question 16

what is the heat equation for “q”?

Answer 16

q = mc(delta T)

Question 17

What is the second law of thermodynamics? What does deltaS equal?

Answer 17

Energy spontaneously disperses from being localized to becoming spread out if it is not hindered.

deltaS = Qrev/T

(T is in Kelvin)

Question 18

What are three state functions? What does it mean to be a state function?

Answer 18

Entropy, Enthalpy, and Gibbs free energy are state functions and are path independent.

Question 19

What is the equation for Gibbs free energy?

Answer 19

deltaG = deltaH - T(deltaS)

*mnemonic: Get High Test Scores

Question 20

What is the relationship of a system to different values of G?

Answer 20

deltaG > 0, reaction is not spontaneous
deltaG < 0, reaction is spontaneous
deltaG = 0, system at a state of equilibrium (deltaH = TdeltaS)

Question 21

What is the equation for free energy from equilibrium?

Answer 21

deltaGknot = -RTlnKeq

Question 22

What are the two reactions for deltaG, and when is it used?

Answer 22

deltaG = deltaGknot + RTlnQ
deltaG = RTln(Q/Keq)

These equations can be used to see if a reaction is spontaneous at some point other than equilibrium.

We use Q instead of K in these reactions to show that it is not at equilibrium.

Question 23

what are the equivalents of 1atm?

Answer 23

760mm Hg = 760 torr = 101.325 kPa

Question 24

One mole of a gas at STP will occupy what volume?

Answer 24

22.4L

Question 25

What are two values of R?

Answer 25

``` R = 8.21x 10^-2 (L x atm)/(mol x K) R = 8.314 J/(K x mol) ```

Question 26

What is Dalton's Law of Partial Pressure?

Answer 26

Some total pressure equals the sum of the partial pressures. Ptotal = Pa + Pb + Pc...

Question 27

What is the difference between molarity and molality?

Answer 27

Molarity: moles of solute per liter of solution. Molality: moles of solute per kilogram of solvent.

Question 28

how do you calculate the concentration of a silution after diluting?

Answer 28

MiVi = MfVf M: molarity V: volume

Question 29

What causes a negative and a positive deviation from Raoult's Law?

Answer 29

solutions that prefer to have solvent-solvent and solute-solute interactions will not mix well, do not want to stay in solution, and will evaporate more quickly, creating a higher vapor pressure. For example, ethanol and hexane. two liquids that interact well together prefer to stay in solution and have a lower vapor pressure than predicted by Raoult's Law. For example, an acid in an aqueous solution.

Question 30

Solubility of gas in a liquid is directly proportional to what?

Answer 30

atmospheric pressure.

Question 31

The phase diagram for water has a positive or negative slope along the liquid/solid equilibrium?

Answer 31

negative

Question 32

In general, symmetry vs. branching does what to melting points?

Answer 32

Symmetry raises. Branching lowers.

Question 33

What are the two factors that affect boiling temp?

Answer 33

1. Molecular weight | 2. Intermolecular forces

Question 34

Which is more important when considering boiling point: hydrogen bonding or size?

Answer 34

H-bonds.

Question 35

Which direction will adding heat send this reaction? X + Y => XY, delta H < 0

Answer 35

To the left. Heat is a product.

Question 36

What is the Ka of water? Of an acid? Of a base?

Answer 36

Ka water = 10^-7 Ka acid > 10^-7 Ka base < 10^-7

Question 37

What is the Kb of water? Of acid? Of base?

Answer 37

Kb water = 10^-7 Kb acid < 10^-7 Kb base > 10^-7

Question 38

In an electrolytic cell is the cathode negative or positive?

Answer 38

The cathode is negative.

Question 39

What is the gram equivalent weight?

Answer 39

Weight of product needed to produce one mole of H+ ions. For a diprotic acid, the gram equivalent weight is half the molecular weight.