Amount Of Substance Flashcards
what is relative atomic mass
relative atomic mass is the average mass of an atom of an element compared to one twelfth the mass of an atom of carbon-12
definition of Ar
average mass of an atom of an element/1/12 mass of one atom of carbon-12
what is the molecular mass
the average mass of a molecule compared to one-twelfth the mass of a carbon-12 atom
why should ionic compounds not be referred to as molecules
they have a giant lattice structure
find the mr of H2SO4
-hydrogen = 1 so H2 = 2
-sulphur = 32.1
-oxygen (4x16)= 64
add together = 98.1
what is the standard unit of measurement for chemists
the mole
what is the mole
6.022x10^23
what is the mole also referred to
avogadro constant
why can we not use volume for standard SI
equal masses of substances are unlikely to contain the same number of particles
what is one mole equivalent to
the ar or mr in grams
formula for no. of particles
moles x 6.022x10^23
formula for moles-in terms of avagadro
no. particles/ 6.022x10^23
formula for moles using mass
mass/MR
formula for mass
moles x MR
what must every mass be in
grams
how to convert
leave in x10 form
mg = 10^-3
microgram = 10^-6
1 tonne = x10^6
formula for density
mass= density x volume
monatomic
exist as one, cannot form a compund
formula for nitric acid
HNO3
phosphoric acid
H3PO4
where to look for ions
group number
silver
Ag^+
zinc
Zn2+
carbonate
CO3^2-
how to form formula
cross over rule
why do some form positive ions - in ion detection
gain a H^+ ion which is a proton
What do metals form when mixed
they are alloys which is a mixture
how many moles are in 72.0g of Mg
mass/MR= 72.0/24.3
calculate moles of an ion in a compound
take total moles of compound and times by number of atoms
what is the empirical formulae
the simplest whole number ratio of atoms of each element present in each compound
which formula is true for ionic compounds
it is always the empirical formula
what is the molecular formula
the true number of atoms of each element in each molecule
how to work out empirical formulae
-moles =mass/mr
-the find closet ratio
-specific ratio corresponds to the formula
how do you find the molecular formula from empirical
find original mr and see if it matches
then divide mr of molecular by mr of empirical
what is a hydrated salt
Water of crystallisation is when some compounds can form crystals which have water as part of their structure
what is a anhydrous salt
when water is driven off via heating and leaves a solid compound and one that does not contain water of crystallisation
what do you do to find the formula of a hydrated salt
weigh the crucible
weigh out 2g of hydrated magnesium sulfate or any form of sulfate into the crucible and weigh
heat without lid gently using a bunsen
allow to cool and re-weigh
heat again for 2 min and re-weigh
repeat this until results are at a constant mass
what results should you collect- hydrated salt
-exact mass of hydrated salt- mass of c and mgso4 - empty crucible
-exact mass of anhydrous-
-exact mass of water- first heating minus 2nd heating
what is an equation to show the formation hydrated salt -magnesium
MgSO4*5H2O ⇌ MgSO4 + 5H2O
how do you calculate percentage error
uncertainty / measurement(g) x100
what are some sources of error-mass of hydrated salt
-convection current
-spitting - these will make the mass appear greater than expected
-not fully reacted-some water remains - x is less than expected
-anhydrous could decompose- eg sulfer dioxide produced which could decompose- x is greater than
-busen flame- incomplete combustion-soot forms - x is less tan expected
how would you improve these factors-mass of hydrated
-make uncertainty smaller
-make measurement larger
give some improvements to the experiment-formula of hydrated salt
-use a lid to prevent spitting
-heat to a constant mass
to calculate number of moles of water which combine with one mole of anhydrous salt
mass of moles (water) =mass/mr
moles of salt = mass/mr
divide by smallest number
place in ratio
water number is number of moles of water eg number in front of H20 and behind the dot
how to calculate percentage error using given value
look at how many calcs have been used to get mass and times the given uncertainty in question by this number
eg 0.005g uncertainty
find hydrated salt - 2 calcs before - e,pty crucible and crucible and hydrated salt
so times 0.005g by 2 and divide by mass
= 0.01/2.05= 0.49%
very small error
what do you do in a reacting mass calc
-find moles of the thing you know mass of
-then look at ratio - if 2 : 1 then divide moles by 2
-then find mass via moles x mr
what do you do if given two reactants-reacting mass calc
work out which one is in excess and which one is the limiting reagent
how would you find the limiting reagent
-find moles of both
-then smaller one is the limiting
-larger is in excess
-you would then use the limiting reactant for the calc
how to find LR and IE if one is a 2 or 3 due to reacting ratio
you would x the one that is singular by value of other one
-then see if it is larger
-if not then the smaller is limiting
-if it is then multiple is the limiting
example
Step 1: The symbol equation is:
2Mg (s) + O2 (g) → 2MgO (s)
Step 2: The relative formula masses are:
Magnesium : 24 Oxygen : 32 Magnesium Oxide : 40
Step 3: Calculate the moles of magnesium used in reaction
Step 4: Find the ratio of magnesium to magnesium oxide using the balanced chemical equation
Therefore, 0.25 mol of MgO is formed
Step 5: Find the mass of magnesium oxide
mass = mol x Mr
mass = 0.25 mol x 40 g mol-1
mass = 10 g
Therefore, the mass of magnesium oxide produced is 10 g
for reacting mass of volume and conc
moles=concxvol
moles=vol(dm3)/24 - if in g then 24000
what makes up a solution
a solute(solid) and solvent(liquid)
how to calculate conc from an equation
-calculate moles by mass/mr
-then use equation to see ratio for unknown
-the use conc= mol/vol
what is the definition of titration
Volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a standard solution) to determine the concentration of another unknown solution
what equipment is used-for a titration
a volumetric or graduated pipette and a burette
what is the reaction that occurs
neutralisation
what is the precision of a burette
Burettes are usually marked to a precision of 0.10 cm3
Since they are analogue instruments, the uncertainty is recorded to half the smallest marking, in other words to ±0.05 cm3
The stoichiometric point or equivalence point occurs when the two solutions have reacted completely and is shown with the use of an indicator
what is the method that is used
- Pour approximately 100cm3
of the standard
solution of known concentration into a beaker.
2.Fill the burette with the standard solution of
known concentration. - Pour approximately 100cm3
of the solution with
unknown concentration into a second beaker. - Using a pipette filler and pipette to transfer exactly 25cm3 of solution into a 250cm3 conical flask.
- Add two to three drops of phenolphthalein indicator to the solution in the conical flask and
note the initial colour of the indicator. - Record the initial burette reading.
- Titrate the contents of the conical flask by adding solution to it from the burette until the indicator undergoes a definite, permanent colour change. Record the final burette reading
in your table of results. Calculate the titre volume (change in volume in the burette). - Repeat, calculate and record the volume of
solution used in the titration in a table (titre volume). Repeat until two concordant results are obtained. Record all of the results that you obtain
why is acid used to wash the burette
to make sure it is clean so conc/moles arent affected
why should u not keep the funnel in
it can cause the measurement to be less than it should be due to drips
why should u make sure there is no air bubbles
can cause the titre to be more than it actually was
titration calc example
Example 1: Calculations from titration results
In a titration, 25.00 cm3 of 0.05 mol dm-3 hydrochloric acid was neutralised by 8.50 cm3 of sodium hydroxide solution. Calculate the concentration of the sodium hydroxide solution.
Answer
Step 1: Find the number of moles of acid
moles of acid = concentration x volume in dm3
moles of acid = 0.05 x 25/1000 = 1.25 x 10-3 mol
Step 2: Deduce the number of moles of alkali
The equation for the reaction shows the mole ratio is 1:1
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
∴ moles of alkali = 1.25 x 10-3 mol
Step 3: Work out the concentration of the alkali
concentration = moles/volume in dm3
concentration = 1.25 x 10-3/0.0085 = 0.15 mol dm-3
Titration calc
find moles of things that you know off
How to find percentage yield
Actual/theoretical x 100
Method for preparing solution from mass of a solid
Weigh out accurate mass if solid in weighing boat and dissolve in a suitable vol of deionised water in a beaker
Stir with glass rod and rinse the weighing boat into beaker with deionised water
Rinse glass rod into beaker
Place glass funnel into top of volumetric flask and pour prepared solution down glass rod into beaker
Rinse glass rod again
Rinse funnel
Remove funnel and add deionised water to the volumetric flask until just below line
Add deionised water using pipette up to the line so meniscus is on
Place stopper in and invert 10x
What is a back titration
Reacting insoluble solid with known excess of HCl
Then titrate excess’s did with alkali eg NaOH
Colourless to pink
Gas calc
No of moles = gas vol / 24 dm3
What is the ideal gas equation
pV=nRT
R is always 8.31 kJ
P=Pa
V = m3
N = no of moles
T= kelvin
Converting from degrees to kelvin
+273
Convert from cm3 to m3
Divide by 1000000
