Stoichiometry & Quantative Chemistry

mole

counting unit → can count atoms, molecules and ions

the mole connects

1. Mass → substances in grams

2. number of particles → 1 mole always contains 6.022×10²³ particles

3. gas volume → 1 mole occupies a certain volume (at RTP: 24dm³)

quantitative chemistry

calculating amounts in chemistry (allows to predict how much product forms, how much reactant is needed, how efficient a reaction is)

Avogadro`s Constant

NA=6.02×10²³mol^-1

→ mol^-1 = per mole

represents the number of particles in one mole

Mole Calculations

n=N/NA → particles to moles

n=number of moles

N=number of particles

NA=Avogadro`s constant

N=nxNA → moles to particles

Molar Mass

mass of one mole of a substance, measured in g mol^-1

how to calculate moles from mass

n=m/M

n=number of moles

m=mass(g)

M=molar mass(g mol^-1)

Molar Volume of Gases

at RTP (room temperature and pressure): one mole of any gas occupies 24dm³

at STP (standard temperature and pressure): one mole of any gas occupies 22.4dm³

Gas Volume Equation

n=V/24

n=moles

V=gas volume in dm³

divided by 24 because gas at RTP is always 24dm³

Composition of Substances

compounds contain elements chemically bonded together in fixed ratios by mass

Law of Conservation of Mass

Mass cannot be created or destroyed → total mass of reactants = total mass of products

Rules for balancing chemical equations

1. only coefficients can change

2. subscripts must never be changed

3. equations must balance for atoms and overall charge

Mole Ratios

stoichiometry uses balanced equations to determine quantities of reactants and quantities of products, the coefficients in an equation represent mole ratios

Limiting Reactants

limiting reactant is completely used up first and limits the amount of product formed

theoretical yield

maximum possible product predicted from stoichiometry

actual yield

amount of product actually obtained

percentage yield

real reactions rarely produce 100% yield

equation: %yield=actual yield/theoretical yieldx100

Concentration

measures amount of solute dissolved in a volume of solution → standard unit: mol dm³

Concentration Equation

c=n/V

c= concentration (mol dm³)

n=moles

V=Volume in dm³

Percentage by Mass

mass of solute in total mass of solution

%by mass=(mass of solute/total mass of solution)x100

%element=(mass of element in compound/molar mass of compound)x100

Dilution Calculations

moles remain constant, concentration decreases, volume increases

Dilution Equation

C1V1=C2V2

C1= initial concentration

V1= initial volume

C2= final concentration

V2= final volume

Empirical Formula

simplest whole number ratio of atoms

Molecular Formula

actual number of atoms in a molecule

Significant Figures

reflect precision of measurements

→ non-zero digits are always significant

→ leading zeros are not significant

→ trailing zeros after decimal are always significant

the final answer should usually match the least precise measurement

Uncertainty

all measurements contain uncertainty because of

• instrumential limitations

• human error

• environmental factors

absolute uncertainty

absolute uncertainty = instrument limit (+- value)

Percentage Uncertainty

%uncertainty=(absolute uncertainty/measured value)x100

→ helps compare measurements fairly

chemists analyze data to

• identify patterns

• determine reliabilty

• evaluate precision and accuracy

accuracy

how close results are to the true value

precision

how close repeated measurements are to each other

common sources of experimental error

• heat loss

• incomplete reactions

• reading errors

• contamination

• instrument calibration errors