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