MCAT Gen. Chem

Some distinctions to make:

• molarity (M) vs. molality (m)
• M: moles of solute / liters of solvent AND solute
• m: moles of solute / kg of solvent ONLY
• enthalpy (ΔH) vs. heat (Q) vs. temperature (T)
• H is a state function, whereas Q is a process function.
• H can be given as J or cal (1J=1.4184cal), but in tables for standard enthalpy or for Hess's law, they are given in J/mol, Q is also expressed in J or cal
• Under constant pressure, Q is used interchangeably with ΔH
• Temperature is given in K, °C, or °F, and is described as average kinetic energy of molecules
• exergonic (-ΔG) vs. exothermic (-ΔQ/-ΔH)
• -ΔG: spontaneous, decrease in Gibbs free energy
• -ΔQ: releases heat, decrease in enthalpy (-ΔH) 
• The difference is highlighted by ammonium nitrate cold packs: they are endothermic (absorb heat, feel cool) BUT ALSO exergonic (spontaneous)!
• ΔG = ΔH - TΔS says that endothermic reactions can be made spontaneous if there is a high-enough temp. and an increase in entropy.
• STP (for gases) and standard conditions (for ΔG°, ΔH°, ΔS° of reactions)
• STP: 1 atm, 0°C
• 1 mol of gas is presumed to occupy 22.4 L at STP
• standard conditions: 1 atm, 25°C, 1 M

Tricks for estimating pH and [H] (-logs and antilogs) without a calculator:

Fastest trick:

[H] 6x10⁻³ ≈ 2.4 pH 

take the exponent (3) and subtract 1:                                              2._ 
and 10 minus the mantissa (6) gives you the fractional part:           _.4 

This is from KAPLAN and helps me a lot.

  and another quick trick:
log of 2,4,8 is .3, .6, .9

  and a trick to narrow down:
the halfway point of pHs is a mantissa of 3, so 
[H+] of 3x10⁻¹º = 9.5 pH 
[H+] of 3x10⁻⁹ = 8.5 pH 
[H+] of 3x10⁻⁸ = 7.5 pH