MCAT Kaplan General Chemistry Chapter 4: Compounds and stoichiometry

Compounds

pure substances composed of 2 or more elements in a fixed proportion. Smallest unit is a molecule for covalent compounds.

Molecules

two or more atoms held together by covalent bonds

formula unit

the lowest whole-number ratio of ions in an ionic compound

formula weight

the sum of the atomic weights of the atoms in the formula unit of an ionic compound. Unit is amu

molecular weight

the sum of the atomic weights of all the atoms in a molecule. Unit is amu

molar mass

the mass of one mole of a pure substance (g/mol)

Equivalents

how many moles of the thing we are interested in (like e- or H+) will one mole of a given compound produce. Ex: Sodium donates one equivalent of e- but magnesium donates 2 equivalents.

Equivalents=mass of compound/gram equivalent weight

gram equivalent weight

mass (in grams) that provides one equivalent of the particle of interest. equals molar mass/n, where n is the number of particles (like H+ or OH-) produced per molecule

normality (N)

equivalent of solute per liter of solution, often denoted by N.

1 N of HCl is 1 M HCl, but 1 N of H2CO3 is 0.5 M H2CO3 because it is diprotic. Molarity=Normality/n, where n is the number of equivalents of thing of interest a molecule produces.

Law of Constant Proportions

law that states that, in chemical compounds, the ratio of the elements is always the same

empirical formula

a formula with the lowest whole-number ratio of elements in a compound

molecular formula

A chemical formula that shows the exact number and kinds of atoms in a molecule, but not the arrangement of the atoms.

percent composition by mass (for an element in a molecule)

molar mass of element/molar mass of molecule x 100%

Finding Molecular Formula given molar mass and percent of each element

40.9% carbon, molar mass of compound 264 g/mol

moles C=(0.409)*(264 g/mol)/(12 g/mol)

Alternative method shown in image.

combination reaction

a chemical change in which two or more substances react to form a single new substance

decomposition reaction

a reaction in which a single compound breaks down to form two or more simpler substances

Combustion

reaction involves fuel and oxidant, usually hyrdocarbon and oxygen. Fuel can also be sugar or sulfur. Form carbon dioxide and water.

single-displacement reaction

atom or ion in a compound is replaced by another. Often redox.

Ex: Cu+AgNO3-> Ag +CuNO3

double replacement reaction (metathesis)

elements from two different compounds swap places

Ex: CaCl2 + 2 AgNO3-> Ca(NO3)2+2AgCl

neutralization reaction

special type of displacement where acid and base react to form a salt.

laws of conservation of mass and charge

mass/ charges of the reactants consumed must equal the mass/charge of the products produced

stoichiometric coefficients

indicate # of moles of a given species involved in rxns

whole numbers

limiting reactant

the substance that controls the quantity of product that can form in a chemical reaction

excess reagents

reactants present in quantities greater than necessary to react with the quantity of the limiting reagent

theoretical yield

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

actual yield

the amount of product that forms when a reaction is carried out in the laboratory

percent yield

actual yield/theoretical yield x 100

-ous

suffix used to represent ions with the lesser charge

Ex: ferrous (Fe2+) vs ferric (Fe 3+)

-ic

suffix used to represent ions with the greater charge

Ex: ferrous (Fe2+) vs ferric (Fe 3+)

-ide

The suffix added to a negative ion's name

Oxyanions

polyatomic ions that contain oxygen;

"-ite" for oxyanion with less oxygen, "-ate" for oxyanion with more oxygen

an anion that has one fewer oxygen atom than the "-ite" anion is given the prefix "hypo-".

An anion that has one more oxygen atom than the "-ate" anion is given the prefix "per-".

Bi-

used to indicate addition of a single hydrogen to anion

Ex: bicarbonate

HCO3-

hydrogen carbonate (bicarbonate)

HSO4-

hydrogen sulfate (bisulfate)

NH4+

Ammonium

C2H3O2-

Acetate

CN-

Cyanide

MnO4-

Permanganate

SCN-

Thiocyanate

CrO4 2-

Chromate

Cr2O7 2-

Dichromate

BO3 3-

Borate

Electrolytes

-solutes that enable solutions to carry currents

- electrical conductivity is governed by presence & concentrations of ions in solution

strong electrolytes dissociate into ions completely. Weak ones dissociate incompletely, like Hg2I2 and acetic acid. Nonpolar gases are nonelectrolytes

solvation

the act of dissolving