topic 5: formulae, equations and amounts of substance Flashcards
mole is
unit for amount of a substance
same no. of particles as there are atoms in 12g of 12C
avogadros constant
6.02 x 10^23 mol-1
molar mass
mass of 1 mole of substance in g/mol
ideal gas equation
pV = nRT
pressure = pa
V = m3
n= mol
R= gas constant (data book)
T = kelvin
conc =
mol/vol
(dm3)vol (gas)=
mol/24
ROOP TEMP AND PRESSURE
molecular formula
shows number and type of each atom in a. molecule
empirical formula
simplest whole number ratio of the elements present in one molecule of a compound
celcius to kelvin
add 273
relative atomic mass
average mass of one atom compared to the mass of 1/12 of c-12 atom
relative isotopic mass
mass of an isotope compared to the mass of 1/12 of c-12 atom
mol, mass , mr
mol = mass/mr
1 tonne
1000kg
water of crystallisation experiment is done by …
heating in a crucible
heating in a crucible experiment method
- weigh an empty can dry crucible + lid
- add 2g of hydrated and reweigh
- heat strongly
- cool
- reweigh
- heat and reweigh until a constant mass
what happens if the crucible is wet
- mass loss too large as water lost when heating
purpose of lid in crucible
- prevents loss of solid but still lets gas out
why not use tiny masses of solid in crucible experiment
errors in weighing too high
calculation from the crucible experiment
- moles anhydrous salt
- moles water
- divide by smallest and find ratio
no. particles, mol, AC
particles = mol*AC
density equation
mass/ vol
g/cm3
M unit of conc
mol/dm3
how to go from moldm-3 to gdm-3
multiply by the Mr
making a standard solution
- weigh mass by difference
- transfer mass to beaker
- add distilled water, use a glass rod to store
- pour through funnel into volumetric flask
- washings
- make up to the mark and invert so its UNIFORM
- bottom of meniscus
what does diluting change
NOT moles of solute but increases the vol so conc is lower
how to calc diluted conc
CONSTANT MOLES SO
og conc * og vol = new conc * new vol
what to remember with volumes in concentrations
add / subtract the ORIGINAL when they’re asking how much new
mixture of gases moles with ideal gas equation
n is the total moles of all gases in the mixture
errors in gas syringe (3)
- friction - use a lubricant
- gas escapes before bung inserted - use the vial
- some gases are soluble in water so true amount isn’t measured
reacting volumes of gases
- same at room temp
reading
SINGLE judgement- one reading
measurement
DIFFERENCE between 2 readings
uncertainty is
at least +- 0.5 of the smallest scale reading
how to decrease uncertainty
- use apparatus with a greater resolution
- increase the size of the measurement made
balance uncertainty
0.001
volumetric flask uncertainty
0.1
25cm3 pipette unceratinty
0.1
burette uncertainty
0.05
% uncertainty
uncertainty/measurement * 100
% uncertainty vs percentage diff in the measured and real value
- if % of equipment is less than the measured and real, there are other errors
- if % of equipment is greater than measured and real, no other errors
% yield
actual yield/theoretical yield * 100
factors lowering percentage yield
- incompleet reactions
- side reactions
- losses during transfer
atom economy
mr of useful products / mr of all reactants * 100
USE REACTING RATIO
why might economy be lower
reaction not gone to completion
reactions with one product has …
100% atom economy
observations with acid + carbonate
- effervescent (CO2)
- carboante dissolves
- TEMP INCREASE
spectator ions
- same state
- same oxidation number
why must gas be dry
- water vapour affects molar mass
reason for vol of gas lower
- gas lost when replacing bung (put mg in a vial so reacts after bung replaced)
- magnesium coated in oxide, so water produced instead of H2 (clean it before)
- co2 soluble in water
- temp lower than room temp (if u used 24)
how to know if gas syringe leaky
- plunger doesnt return to 0 when released
- mr decreases because air has a lower mr
why deos phenolphthalein turn pink in alkali
forms a pink anion by loss of H+ ions
