Kaplan MCAT General Chemistry Chapter 4: Compounds and Stoichiometry

Compounds

Substances composed of two or more elements in a fixed proportion.

Molecular weight

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

Moles from mass equation:

How to find moles?

# moles= mass in grams/molecular weight

Molar mass

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

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.

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:

The law of constant composition

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

Empirical formula

The smallest whole number ratio of the elements in a compound.

Molecular formula

Either the same as the empirical formula or a multiple, it gives the exact number of atoms of each element in a compound.

Percent composition equation:

Types of chemical reactions?

1) Combination reaction

2) Decomposition reaction

3) Displacement reaction - single or double

4) Neutralization reaction

5) Combustion

Combination reaction

When two or more reactants combine to form one product.

Decomposition reaction

When one reactant is chemically broken down into two or more products.

Single displacement reaction

Occur when an ion of one compound is replaced by another element.

Double-displacement reaction

When elements from two different compounds trade places with each other to form two new compounds.

Neutralization reaction

double-displacement reaction in which an acid reacts with a base to form a salt (and, usually, water)

Combustion reaction

Occurs when a fuel and an oxidant (typically oxygen) react, forming the products water and carbon dioxide ( If the fuel is a hydrocarbon).

Balancing chemical equations:

1) balance least common atoms

2) Balance more common atoms ( usually H or O)

3) Balance charge (if necessary)

1 mole of any ideal gas at STP

22.4 L

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.

Actual yield

What you actually get; typically lower than theoretical yield.

Percent yield

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

Ionic compound nomenclature:

What does the roman numeral in Fe (II) mean?

Roman numeral denotes the ionic charge on atoms that posses more than one ionic state.

I.e., Fe(II) = Fe2+ vs Fe(III) = 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.

Multiple oxyanions?

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

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.

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:

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

Solvation

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

ionic compounds are measured with

formula weights, not molecular weights like covalent compounds