AP Chemistry

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3. Stoichiometry

Counting by weighing

chemical stoichiometry: studying quantities of materials consumed and produced in chemical reactions

atoms are small; samples of matter contain huge numbers of atoms

determine number of atoms in a given sample by finding its mass

figure out average mass of atoms

Atomic masses

modern system of atomic masses: based on ¹²C (carbon twelve) as weighing exactly 12 atomic mass units (amu); all other atoms are relative to this standard

most accurate method for comparing masses of atoms: using mass spectrometer

determine mass values for individual atoms

atoms/molecules passed into beam of high-speed electrons

knock electrons off the atoms/molecules being analyzed; change into positive ions

applied electric field accelerates ions into magnetic field; interacts with applied magnetic field, changing path of ion

amount of path deflection depends on mass (more mass moves less), causing ions to separate

comparison of positions → relative masses

also used to determine isotopic composition of natural element

average atomic mass: relative average of atomic masses based on isotopic composition

finding

multiply percentage and mass value of each isotope

add values together

natural carbon does not contain a single atom with mass 12.01, but for stoichiometric purposes, we can consider carbon to be composed of only one type of atom with a mass of 12.01

The mole

mole (mol): number equal to number of carbon atoms in exactly 12 g of pure ¹²C

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also called Avogadro’s number

one mole of something consists of 6.022 ⨉ 10²³ units of that substance

sample of natural element with mass equal to the element’s atomic mass expressed in grams contains 1 mole of atomsd

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Molar mass

molar mass: mass in grams of 1 mole of the compound (also called molecular mass)

found by summing masses of component atoms

units

molecular compounds: molecules

ionic compounds: formula units

Solving problems

two ways to approach a problem

memorization/pigeonholing

label the problem (which pigeonhole fits)

requires new set of steps when problem changes

conceptual problem solving

understanding the big picture

looks within the problem for a solution

problem guides you as you solve it

ask questions and use knowledge to answer them

where are we going?

read the problem and decide on the final goal

sort through the facts given

focus on key words

draw diagram if appropriate

state problem as simply and visually as possible

how do we get there?

decide where to start (e.g. with chemical reaction in stoichiometry problem)

ask questions as you proceed

what are the reactants and products?

what is the balanced equation?

what are the amounts of reactants?

understanding of fundamental principles of chemistry will enable you to answer the simple questions and final solution

reality check: is the answer reasonable?

Percent composition of compounds

two common ways of describing the composition of a compound

numbers of constituent atoms

percentages (by mass) of elements

compare mass of each element present in one mole of compound compared to total mass of 1 mole of the compound

mass percent (or weight percent): proportion of a particular substance in a mixture as a percentage of the total mass

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Formula of a compound

often determined by taking weighted sample and decomposing it into component elements or reacting it with oxygen to produce substances to collect and weigh

for each element present:

determine molar mass of produced substances

determine fraction of element present by mass for the substance (mass of element divided by total mass of substance)

multiply fraction by given amount of substance

determine mass percent of element in the compound

convert masses of elements to numbers of atoms

base calculations on 100 g of compound

determine number of moles of each element present in 100 g of compound using atomic masses

divide each value of number of moles by smallest of the values

if numbers are not whole numbers, multiply by an integer so results are all whole

gives empirical formula: simplest whole-number ratio of elements in the compound

to specify molecular formula (exact formula of compound), must know molar mass