1. Stoichiometry (SL+HL) Flashcards
mole
the SI unit of amount of substance. One mole contains exactly the number of elementary entities given by the Avogadro constant: 6.02 x 10^23
*international standard of measurement
elementary enities
any chemical particle such as atoms, molecules, ions or electrons
the unit for Avogadro constant
mol^-1
relative atomic mass
the weighted average mass of an atom compared to 1/12 the mass of an atom of carbon-12
relative formula mass, Mr
the mass of a compound relative to 1/12 the mass of an atom of carbon-12
includes both ionic and molecular compounds
molar mass, M
the mass of one mole of a substance
the unit for molar mass M
g mol^-1
formula for moles
n = m / M
moles = mass / Mr
relative isotopic mass
mass of an isotope of an atom relative to 1/12 of a carbon-12 atom
mass spectrometer
a tool to measure the abundance of different isotopes in an atom
Law of Definite Proportions
the fixed ratio of constituent elements of a given compound eg. H20
emperical formula
gives the simplest ratio of atoms of each element present in a compound
molecular formula
gives the actual number of atoms of each element present in a molecule
what is Avogadro’s law?
it states that equal volumes of all gases measured under the same conditions of temperature and pressure contain equal numbers of molecules
formula for molar concentration
c = n / V
c = concentration (molarity), g/dm
n = moles
V = volume, dm^3
factors affecting gases
the number of gas particles that can fit inside a closed container
- volume of the container
- temperature of the gas
- pressure of the gas -> caused by a difference in the number of particles
calculating percentage composition
- calculate the molar mass of the compound
- calculate the mass that each element contributes individually to the molar mass of the compound
- divide the mass of each element by the molar mass of the compound and multiply by 100 to get the percentage
make sure to use the appropriate units and the number of significant numbers
formula for molecular formula
Mr / EF
Relative formula mass / emperical formula
formula for Atom Economy
= (Mr of desired/useful product / Mr of all products) x 100
formula for Percentage Yield
(actual yield / theoretical yield) x 100
yield = amount of product made
actual yield - what is made
theoretical yield - assumes a complete reaction
reasons why actual yield is not the same as theoretical yield
- incomplete reaction
- reactants were inpure
- reactants/products were left behind on equipment (transfer losses)
Ideal Gas equation
PV = nrt
P = pressure, Pascals Pa
V = volume, metres cubed m^3
n = moles
R = the gas constant, 8.13 J mol^-1 K^1
T = temperature, Kelvin K
C° -> K = +273
KPa -> Pa = / 1000
cm^3 -> m^3 = / 1000
number of dm^3 in 1 mole of any gas
24 dm^3 (at room temperature)
temperature, T
a measure of the average kinetic energy (Ek) of particles. In Kelvin (K)
ideal gas model
An ideal gas consists of moving particles with negligible volume and no intermolecular forces. All collisions between particles are considered elastic (no energy is lost).
what happens to gases at low temperature and high pressure?
at high pressure - gas particles are closer together and are influenced by forces of attraction.
at low temperature - particles move less rapidly meaning there is a greater opportunity for intermolecular forces to have an effect.
For both conditions the gas no longer behaves as an ideal gas and behaves as a real gas
*deviates from the ideal gas model
under which conditions do real gases deviate from the ideal gas model?
at high pressure and low temperature
which gases show the most ideal behaviour?
those with a low molar mass and weakest intermolecular forces
when is the molar volume of an ideal gas constant
*volume of 1 mole
under standard conditions of temperature and pressure
formula for combined gas law
P1V1 / T1 = P2V2 / T2
P = pressure, KPa
V = volume, dm^3
T = Temperature, K
limitations of the ideal gas model
gas laws only apply to ideal gases. With real gases, some of the assumptions made about ideal gases no longer apply under certain conditions.
Titration
a practical technique that is used to calculate unknown things about a solution, usually its concentration in mol dm^-3
Standard Solution practical
carrying out a titration
To carry out tirtration we use a standard solution. This is an acid or base that we know the precise concentration of. We then measure the V of that solution used and calculate the number of moles. The end result should be very pale pink
Standard Solution practical
Explain why the solution must be allowed to cool before making it up to the final volume
The solution got hot whilst dissolving. The water will expand and the volume will be increased. The water will cool and contract and ∴ the volume will decrease until the final conc. will be higher than it should be.
formula for percentage uncertainty
(ununcertainty x number of measurements) x 100 final measurement value
Total uncertainty = sum of all uncertainties used in the investigation
nuetralisation reaction
acid + base -> soluble salt + water
nuetralisation reaction of water
H+ (aq) + OH- (aq) -> H2O
Acid-Base Titration practical
improving the titration method
regularly pausing the titration to rinse the inside of the conical flask with small amounts of water. This helps to return any unreacted substances to the reaction mixture improving the accuracy of the results.
Acid-Base Titration practical
state the sources of error in the investigation
- paradox error
- not filling the space under the top of the burette (volume lower than measured)
characteristics of a Primary Standard
must be:
- a pure compound
- must not lose or gain mass when exposed to air
- must be soluble in water
formula for molecular formula
using molar mass
*=Mr
- calculate the emperical formula’s molar mass
- divide the compounds molar mass by the emperical formula’s molar mass to find the ratio
- multiply the emperical formula by the ratio
calculating back Titration
- calculate the moles of acid that reacted
- calculate the number of moles of pure substance the impure sample contains
- convert moles to grams and work out percentage purity
