Kaplan MCAT General Chemistry Chapter 4: Compounds and Stoichiometry

Molecular weight

The mass (in amu) of the constituent atoms in a compound as indicated by the molecular formula.

mole

one mole of a compound has a mass in frame equal to the molecular or formula weight of the compound in amu.

ex. one molecule H2CO3 has a mass of 62 amu and one mole of the compound has a mass of 62 grams

Moles from mass equation:

Molar mass

The mass of one mole (Avogadro's number 6.023x10^23 particles) of a compound; usually measured in grams per mole.

units for molar mass vs molecular weight

g/mol vs amu/molecule

equivalents

how many moles of the thing we are interested in will one mole of a given compound produce

ex. sodium will donate one mole of electrons (one equivalent), magnesium will donate two moles of electrons (two equivalents)

Gram equivalent weight

A measure of the mass of a substance that can donate one equivalent of the species of interest.

Gram equivalent weight equation:

Normality

The ratio of equivalents per liter; it is related to molarity by multiplying the molarity by the number of equivalents present in one mole of a compound.

ex. 1N solution of acid contains a concentration of hydrogen ions equal to 1 mole/L

normality compared to molarity

1N HCl solution, molarity is 1M because monoprotic, in a 1N H2CO3 solution, the molarity of H2CO3 is 0.5M because diuretic

Equivalents

Moles of the species of interest, equivalents are most often seen in acid-base chemistry ( H+ ions or -OH ions) and oxidation-reduction reactions.

Equivalents from mass equation:

Molarity from normality equation:

In a titration, a diprotic acid requires ____

two equivalent so base to neutralize bth protons of this acid

The law of constant composition

any pure sample of a compound will contain the same elements in the same mass ratio.

formular for ionic compounds

only empirical

Percent composition equation:

1 mole of any ideal gas at STP

22.4 L

one mole of any substance=

its molar mass in grams (from the periodic table)

Limiting reagent

The reactant that will be consumed first in a chemical reaction. Determines the amount of product that can be formed.

all comparisons must be in moles

Excessive reagents

The reagents left over after limiting reagent is consumed.

Theoretical yield

The amount of product generated if all of the limiting reactant is consumed with no side reactions.

principles involving the determination of the limiting reagent

1. all comparisons of reactants must be done in units of moles. gram-to-gram comparisons will be useless and may even be misleading

2. it is not absolute mole quantities of the reactants that determine which reactant is the limiting reagent. rather, the rate at which the reactants are consumed (the stoichiometric ratios of the reactions), combined with the absolute mole quantities determine which reactant is the limiting reagent

how to find limiting reagent

first, determine the number of moles for each reactant

look at balanced equation

ex. Fe+S ->FeS

based on what's given you calculate 0.5 mole Fe and 0.75 mol S

because 1 mole of Fe is needed to react with 1 mole of S and there are 0.5 moles of Fe for the given 0.75 moles S, the limiting reagent is Fe. Thus, 0.5 moles of Fe will react with 0.5 moles of S, leaving an excess of 0.25 moles of S in the vessel. The mass of the excess reagent will be:

0.25mol x 32.1gS/1molS = 8g S

theoretical yield

the maximum amount of product that can be produced from a given amount of reactant

Percent yield

Calculated by dividing actual yield by theoretical yield and multiplying by 100.

how to calculate percent yield given mass of reactants and products

balance equation

calculate theoretical yield by using reactant, stoichiometry to convert to product

then determine percent yield

use grams

If 46g Na and 32g O2 are provided, find the maximum number of moles of sodium oxide produced. Identify the LR and find the mass of excess reagent left over once the reaction has run to completion.

4Na+O2-->2Na2O

covert 46g Na to moles to get 2 mol Na

convert 32 g O2 to moles to get 1 mol O2 (you have two oxygens so its 32 not 16)

You need 4 sodium atoms for every oxygen molecule, sodium will run out first then. TO determine how much Na2O is formed, use your stoic to get 1 mole Na2O

LR is Na, so 2molNa x 1molO2/4molNa = 0.5 mol O2 gets used so you have one mole so 1.0-0.5= 0.5mol )2 in excess. Convert to grams to get 16 g excess O2

ferrous vs ferric

ferrous Fe2+

ferric Fe3+

Monoatomic ions named?

Monoatomic ions are named by dropping the ending of the name of the element and then adding -ide

Polyatomic anions named?

Many polyatomic anions contain oxygen and are called oxyanions.

When an element forms two oxyanions, the smaller - ite and the larger = ate.

Nitrite vs Nitrate

NO2-

NO3-

Multiple oxyanions? aka extended series of oxyanions:

If an extended series of oxyanions, Hypo goes first and per goes last.

ClO-

ClO2-

ClO3-

ClO4-

Hypochlorite

Chlorite

Chlorate

Perchlorate

Polyatomic anions with H+?

Polyatomic anions often gain one or more H+. The resulting ions are named by adding hydrogen or dihydrogen in front of the anion's name.

older convention is bi- prefix (bisulfate HSO4-)

Mnemonic for -ite vs -ate?

The l-IGHT-est anions have the fewest oxygens, the heaviest anions ATE the most oxygens.

Common polyatomic ions to know:

ammonium

NH4+

Acetate

C2H3O2-

Cyanide

CN-

permanganate

MnO4-

thiocyanate

SCN-

Chromate

CrO4 2-

Dichromate

Cr2O7 2-

borate

BO3 3-

Ionic charge

Predictable by group number and the type of element (metal vs nonmetal) for respective elements.

What charges will metals form?

Metals for positively charged cations based on group number.

What change will nonmetals form?

Nonmetals form negatively charged anions based on the number of the electrons needed to achieve the octet.

Electrolytes

solutes that enable solutions to carry current

Contain equivalents of ions from molecules that dissociate in solution. The strength of an electrolyte depends on the degree of solvation

ionic compounds are good electrolytes because they dissolve readily, nonpolar covalent compounds are the weakest because they do not form current-carrying ions

Solvation

the process by which the positive and negative ions of an ionic solid become surrounded by solvent molecules

a solute is considered a strong electrolyte if

it dissociates completely into its constituent ions

NaCl and KI, highly polar covalent bonds that dissociate into ions when dissolved like HCl in water

nonelectrolytes

substances that form no ions in water and cannot conduct electricity

O2 CO2 and glucose, many nonpolar gases and organic compounds

ionic compounds are measured with

formula weights, not molecular weights like covalent compounds

the best electrolytes

dissociate readily and are ionic compounds with large amounts of cations and anions