Amount of Substance Flashcards
Relative atomic mass
Average mass of one atom compared to 1/12th of the mass of one atom of carbon 12
Relative molecular mass
the average mass of a molecule to 1/12th the mass of one atom of carbon 12
What is the definition of Avogadro’s constant
Number of particles in a mole
equation relating mass, moles and Mr
moles = mass / Mr
Ideal gas equation
PV = nRT
equation for number of moles in an aqeuous solution
moles = concentration x volume
Unit for pressure
Pa
Unit for volume
m^3
cm^3 to m^3
n/1000000
cm^3 to dm^3
n/1000
dm^3 to m^3
n/1000
unit of temperature
K
degrees celsius to kelvin
+ 273
1 tonne =
1000kg
Number of atoms in 1 mole of copper atoms
6.022 x 10^23
number of molecules in 1 mole of carbon dioxide molecules
6.022 x 10^23
Number if ions in 1 mole of sodium ions
6.022 x 10^23
Number of particles =
moles x Avogadro’s constant
density =
mass / volume
Unit of density
g cm^-3
Empirical formula
Empirical formula is the simplest whole number ratio of atoms of each element in a compound.
How do you find the empirical formula
- mass/Mr
- moles / smallest amount
- convert to whole numbers
Molecular formula
Molecular formula is the actual number of atoms of each element in a compound.
Relationship between empirical formula and molecular formula
From the Mr work out how many times the mass of the empirical formula fits into the Mr
n x Mr = relative formula mass
n = relative formula mass / Mr
How to find the Mr from a mass spectrometer
The molecular ion (peak with highest m/z) = Mr
Percentage atom economy =
Mr of all desired products / sum of Mr of all reactants x 100
Equation linking mass, concentration and volume
mass = concentration x volume
Why is the actual yield less than the theoretical yield
Reaction does not go to completion
There were unwanted side reactions competing with the main reaction
Percentage yield =
Actual yield / theoretical yield x 100
What is the actual yield and theoretical yield
The actual yield is the number of moles or mass of product obtained experimentally
The predicted yield is the number of moles or mass obtained by calculation
How to determine which reactant is the limiting reagent
Calculate the numbers of moles, with the ratios taken into account
The reactant left over is in excess so the other is the limiting reactant
Why is atom economy in addition reactions always 100%
Only one product is formed
What is water of crystallisation
when some compounds can form crystals which have water as part of their structure
concentration in g dm^-3 =
concentration in mol dm^-3 x Mr
new diluted concentration =
original concentration x (original volume / new diluted volume)
safety precautions for irritants
wear goggles
safety precautions for corrosive
wear goggles
safety precautions for flammable
avoid naked flames by using water baths or electric heaters
safety precautions for toxic
wear gloves, avoid skin contact and wash hands after use
safety precautions for oxidising
keep away from flammable and easily oxidising materials
How do you find the average titre results
By averaging only the concordant results
Why is a conical flask used in titrations instead of beakers
It is easier to swirl a conical flask without spilling the contents
Why is only a few drops of indicator used
They are generally weak acids and if too much is added it will affect the titration result
Concordant
within 0.10cm^3
Method for making a volumetric solution
- Use a balance to measure out a precise mass of the solid
- Measure out a small volume of water and add to a beaker and pre-dissolve the solid using a glass rod
3.Transfer to a volumetric flask through a funnel - Rinse the beaker with distilled water and add to the flask
- Add more water until the bottom of the meniscus
- Add the stopper/bung and mix the contents
What equipment is used when making a volumetric solution to reduce impact of measurement uncertainties
3 decimal place balances and volumetric flasks
What is volumetric analysis
a process that uses the volume and concentration of one chemical reactant (volumetric solution) to determine the concentration of another unknown solution
What precise equipment is used to measure volume
Volumetric or graduate pipette and a burette
What is the uncertainty of a burette measured to
half the smallest marking, usually 0.05
Why are white tiles used in titrations
To make it easier to see colour change
What are the steps in a titration
*rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)
*pipette 25 cm3 of alkali into conical flask
*touch surface of alkali with pipette
*adds acid solution from burette
*make sure the jet space in the burette is filled with acid
*add a few drops of indicator and refer to colour change at end point
*use a white tile underneath the flask to help observe the colour change
*add acid to alkali whilst swirling the mixture and add acid dropwise at end point
*note burette reading before and after addition of acid
*repeats titration until at least 2 concordant results are obtained
What is the colour change when using phenolphthalein
pink in alkali to colourless in acid.
End point is colourless
What is the colour change when using methyl orange
yellow in alkali and red in acids
The end point is orange
What uncertainty is the tire calculated and recorded to
double the uncertainty because 2 burette reading are made to obtain titre, initial and final
A student rinses a burette with deionised water before filling with sodium hydroxide solution. State and explain the effect, if any, that this rising will have on the value of the titre.
Titre value would increase
Because the sodium hydroxide solution would be more dilute
A student notices some of the liquid injected into the gas syringe did not vaporise. What effect would this have on the Mr calculated by the student.
Calculated Mr would be greater than the actual value
A lower volume would have been recorded / mass recorded would be too high / mass evaporated less than mass of liquid
Importance of percentage yield
Idea of getting as much products as possible in the reaction / efficient conversion of reactants to products
Importance of percentage atom economy
Idea of maximising the mass of reactants / atoms that ends up in desired product and minimising amount of by products
State why the student should use NaOH to rinse a burette which is about to be filled with NaOH rather than water for the final rinse of the burette
Use of water would dilute the NaOH or would change the conc.
Why did using deionised water when approaching end point to rinse the inside of the conical flask improve the accuracy of the titration
It ensured all the alkali / reactants are in the mixture
Give the meaning of concordant titres
Titres that are within 0.1 cm^3 of each other
Suggest why other than apparatus inaccuracy why the Mr value determined from the experimental results differs from the actual Mr
Volume in gas syringe is greater than true volume.
Use pv=nrt equation to prove
Suggest the safety precaution and reason a student using toxic chloroalkane should use
Carry out experiment in a fume cupboard to avoid toxic vapour.
