Unit 1: Section 2 - Amount of substance CDS * Flashcards
the mole equations and calculations titrations formulas, yield and atom economy
what is the amount of a substance measured in?
moles
how many particles are in 1 mole of a substance?
Avogadro’s constant (6.02 x 10^23)
how many penguins are in 1 mole of penguins?
6.02 x 10^23 penguins
how to find out the number of moles in a solid?
mass of substance(g) / Mr
wat is the concentration of a solution?
how many moles are dissolved per 1 dm^3 of solution
mol/dm^3
how to find out the number of moles in a solution?
concentration (mol/dm^3) X volume (dm^3)
what is 1 dm^3 equal to?
1 dm^3 = 1 L = 1000cm^3
how to find out the number of moles in a gas?
PV=nRT
pressure(Pa) X volume(m^3) = number of moles X gas constant X temperature(kelvins)
what are ionic equations?
equations where only the reacting particles and the product they form are included.
an ion present in the reaction mixture but doesn’t get involved in the reaction is called a spectator ion
example of an ionic equation:
HNO3 + NaOH –> NaNO3 + H2O
H+ +NO3- + Na+ +OH- –> Na+ + N03- +H2O
cross out any ions that appear on both sides of the equation
H+ + OH- –> H2O
if the charges don’t balance the equation is wrong
what are the state symbols?
solid = s
liquid = l
gas = g
aqueous =aq
how can you work out the mass of a reactant from the mass of a product?
find moles by doing mass over Mr
use the balanced equation to find the ratio then use it to find the moles of the reactant
multiply moles by Mr to get the mass
how to use a balanced equation to find volume of a gas?
work out number of moles then use ideal gas equation to find volume of gas
the same number of moles of any gas occupy the same volume at the same pressure and temperature
what is a standard solution?
any solution you know the exact concentration of. aka volumetric solution
how to work out how much solid is needed to make a volumetric solution?
e.g. 250cm^3 of 2mol/dm^3 solution of NaOH
1. find the number of moles of acid/alkali you need moles = conc X volume 2mol/dm^3 X 0.250dm^3= 0.5 moles 2. find how many grams you need mass=moles X Mr 0.5 X 40 = 20g
steps of how to make a volumetric solution?
- weighing the mass of the solid
- dissolving the solid
- knowing the volume of the liquid
- calculating concentration
what is involved in weighing the mass of the solid for a volumetric solution?
- tare/ zero mass balance
- place weighing boat on digital balance and add rough amount of solid needed
- tip into beaker
- re-weigh the boat, which may contain residue
- subtract weight of boat and solid from weight of boat and residue to find exact mass of solid used
what is involved in dissolving the solid for a volumetric solution?
add roughly 50cm^3 of distilled water to the beaker containing the solid and stir until fully dissolved
what is involved in knowing the volume of the liquid for a volumetric solution?
- use funnel to transfer solution into a volumetric flask that’s the volume you are trying to make e.g. 250cm^3
- rinse beaker and stirring rod with distilled water and add that to the flask
- top up the flask with distilled water until near calibration mark
- use dropper to add distilled water until the meniscus reaches the line
- stopper the flask and invert 10 times to thoroughly mix
what is involved in calculating concentration for a volumetric solution?
use exact mass of solid that was found to find moles
moles = mass/Mr
use the number of moles to find the exact concentration
concentration = moles/volume
include uncertainty by adding the uncertainties of each piece of equipment every time it is used
what does a titration allow you to do?
find out exactly how much acid is needed to neutralise a measured quantity of alkali or vice versa.
this can be used to find the concentration of the alkali
what to do before titration?
use pipette to measure out set volume of solution you want concentration of, put in flask
add indicator to flask
use funnel to fill burette with standard solution, that you know concentration of
steps to carry out a titration?
- do rough titration
- do accurate titration
- work out amount of acid to neutralise alkali
- repeat until concordant results are achieved
- find mean volume of acid used (remove anomalous results)
how to do a rough titration?
take an initial reading from burette then add acid whilst swirling conical flask until the indicator changes colour, take another reading from the flask
take these 2 readings away from each other, this gives the rough amount of acid needed to neutralise the solution
how to do an accurate titration?
take initial reading to find how much acid in burette
run acid to within 2cm^3 of rough titre
then add acid dropwise until the indicator changes
colour
take another reading from burette
how to work out amount of acid to neutralise alkali in a titrations?
subtract the final reading from the initial reading. this volume is known as the titre
what are concordant results?
results which are within 0.1 of one another
what are the main indicators used for acid/alkali reactions (titrations)?
methyl orange - red in acid and yellow in alkali
phenolphthalein - colourless in acid and pink in alkali
what is done with the mean volume of acid found from a titration?
the known concentration and mean volume of the acid is used to find the moles.
the ratio in the balanced equation is used to find the moles of the alkali which is used with the known volume to find the concentration of the alkali (in conical flask)
what is the empirical formula?
this gives the simplest whole number ratio of atoms of each element in a compound
what is the molecular formula?
this gives the actual numbers of atoms of each element in a compound
how to find the molecular formula from the empirical formula and the Mr?
find the Mr of the empirical formula then divide the Mr by this number to find how many times it goes into it
that number is how many times bigger the molecular formula is than the empirical formula, so multiply it by the simplest ratio of the number of each number
how to find the empirical formula of a compound from the percentages of the different elements it contains?
divide each elements percentage by its Mr for every element in the compound. then divide every number by the smallest number to find their ratio when the smallest is 1
use this to find the simplest ratio of those numbers in whole numbers, this is the the number of atoms for each element
how to find the empirical formula of reactants based on weights of products?
e.g. empirical formula of hydrocarbon burned in excess of O2 to make 4.4g of CO2 and 1.8g of H2O
e.g. empirical formula of hydrocarbon burned in excess of O2 to make 4.4g of CO2 and 1.8g of H2O
find the Mr of the wanted element as a fraction of the Mr of the compound, then multiply by mass to find the mass of the wanted element.
12/44 X 4.4 = 1.2g of C
2/18 X 1.8 = 0.2 g of H
this is how much carbon and hydrogen there is in the hydrocarbon.
find the number of moles of element by dividing by Ar
1.2/12 = 0.1 moles of C
0.2/1 = 0.2 moles of H
ratio = 1:2
empirical formula = CH2
what is the theoretical yield?
the mass of product that should be formed in a reaction, assuming no chemicals are lost in process.
the theoretical yield is the mass of product calculated using the equation
why might the actual yield be less than the theoretical yield?
not all reactants react fully, reversible reaction
some chemicals may be lost whilst being transferred, e.g. on filter paper or lost in transferring between containers
some reactants react differently than expected and make other products
how to work out percentage yield?
percentage yield = actual yield/theoretical yield X100
what does percentage yield tell you?
how wasteful a process is, based on how much product is lost.
it doesn’t tell you how wasteful a reaction is
what is atom economy?
a measure of the proportion of reactant atoms that become part of the desired product, in the balanced chemical equation
what is the formula for % atom economy?
% atom economy = molecular mass of desired product/sum of molecular masses of all reactants X100
what is the molecular mass in % atom economy?
the Mr of the product/reactants including the number of moles e.g. 2CO2 2 x ( (12 x 1)+(16 x 2) )
advantages of high atom economy?
better for environment - produce less waste that needs to be disposed of
sustainable - make more efficient use of raw materials
less expensive - spend less money on reactants and on separating desired product and treating waste
what’s the method for find the water of crystallisation of a hydrated solid?
- find mass of empty boiling tube
- place about 2cm^3 depth of hydrated solid and record mass
- heat the tube for 1 min (using test tube holders)
- record mass of tube
- repeat 3 and 4 until mass remains constant. if steam is still being given off there is still water of crystallisation to be lost
how to find the value of x in MgSO4. xH2O
use water of crystallisation experiment
subtract mass of boiling tube from boiling tube + hydrated solid, to find the mass of MgSO4. xH2O
subtract mass of boiling tube from boiling tube + anhydrous solid, to find the mass of MgSO4
subtract these 2 masses to find the mass of water lost
divide masses of anhydrous solid and water from their Mr’s to find the ratio
make it a 1 to x ratio and x is the number in front of H20
What is the end-point in a titration?
When the acid has neutralised the alkali or vice versa
Why is it important to wash out the inside of the conical flask with distilled water as the end-point is reached?
To make sure all of the solutions for both the acid and the alkali are in the flask to react
What can be done to make a titration more accurate?
Rinse sides of conical flask so all reagents react
Don’t leave funnel in burette so that extra acid doesn’t drip in and ruin titre
Run some acid out of the tap before starting to make sure there’s no air bubbles
