Calculations Flashcards
Equation for percentage yield?
Percentage yield = actual moles/theoretical moles x 100
What is Avogadro’s Law?
Equal volumes of all gases, under the same conditions of temperature and pressure contain equal numbers of molecules (or equal numbers of moles of molecules)
What are the conditions of the use of Avogadro’s Law?
1) conditions of temperature and pressure before and after the reaction must be the same
2) the law can only be used with gases.
pV = nRT
p= pressure in Pa V= volume in m^3 n= number of moles R= gas constant (8.31 Jmol^-1K^-1) T= temperature in Kelvin
cm^3 -> dm^3
Divide by 1000
Units for concentration?
mol dm^-3 (moles of solute in 1 dm^3 of solution)
g dm^-3 (mass of solute in 1 dm^3 of solution)
Units of very small quantities?
mmol dm^-3 (number of millimoles in 1 dm^3 of solution) - can go to micromoles etc
ppm (number of parts of solute to 1 million parts of solution) - can go to ppb (parts per billion)
Equation for molar concentration?
(moles x 1000) / vol (cm^3)
Equation for gram concentration?
(mass of solute x 1000) / vol (cm^3)
Equation for parts per million?
(mass of solute (g) x 1,000,000) / mass of solution (or gas mixture) (g)
Equation for number of particles?
Moles of substance x Avogadro’s constant
Degrees celsius -> Kelvin
+ 273
Definition of empirical formula
The empirical formula is the simplest ratio of atoms of each element in the compound
Definition of molecular formula
The molecular formula is the actual number of atoms of each element in the compound
Heating in a crucible to find water of crystallisaton (example: hydrated calcium sulfate CaSO4.XH2O)
1) weigh an empty, clean and dry crucible and lid’
2) Add 2g of hydrated calcium sulfate to the crucible and weigh again
3) Heat strongly with a Bunsen for a couple of minutes and allow to cool
4) Weigh crucible and contents again
5) Heat crucible again and reweigh until you reach a constant mass
- A lid is used to stop loss of solid and make experiment more accurate
- A reasonable amount of solid needs to be used, if it is too small (e.g 0.1g) then the percentage error would be too high, but a large amount (e.g. 50g) would mean that the decomposition would probably be incomplete
1 mole of gas at room temperature and pressure…
will always have a volume of 24dm^3
Potential errors of a gas syringe
- gas escapes before bung inserted
- the syringe sticks
- some gases are soluble in water so the true amount is not measured
- there may be gaps where the gas could escape (be sure not to leave gaps if drawing a diagram)
Making up a standard solution from a solid
1) Weigh required mass of solid in a weighing bottle
2) Tip contents into a beaker and add 100cm^3 of distilled water. Use a glass rod to stir and help dissolve the solid. The beaker and its contents could be heated gently if the solid takes a long time to dissolve
3) Pour solution into a 250cm^3 volumetric flask via a funnel. Rinse the beaker and funnel and add the washings from the beaker and glass rod to the volumetric flask
4) Make up to the mark on the volumetric flask with distilled water (a pipette can be used for the last few drops)
5) Invert flask several times to ensure uniform solution
Diluting a solution
1) Pipette 25 cm^3 of original solution into a volumetric flask
2) Make up to the mark with distilled water
3) Pipette the last few drops
4) Invert flask several times to ensure uniform solution
How to avoid errors when making up/diluting solution
- Shake volumetric flask thoroughly to ensure uniform concentration
- Use a pipette rather than a measuring cylinder because it has a smaller sensitivity error
- Use a teat pipette to make up to the mark in a volumetric flask to ensure the volume of solution is accurate and doesn’t go over the marked line
How to carry out a titration
1) Rinse equipment
2) Pipette 25cm^3 alkali into conical flask
3) Add acid solution to the burette
4) Make sure the jet space in the burette is filled with acid
5) Add a few drops of indicator (Use phenolphthalein if NaOH is used, and methyl orange if HCl is used)
6) Use a white tile underneath the flask to help observe the colour change
7) Add acid to alkali whilst swirling the mixture and add acid dropwise at the end point
8) Note burette reading before and after addition of acid
9) Repeat titration until at least 2 concordant results are achieved
What substances should be used to rinse the equipment in titrations?
Burette and pipette with acid or alkali, depending on which way the titration goes
Conical flask with distilled water
Why is a conical flask used in a titration?
It is easier to swirl the mixture without spilling the contents
Why is distilled water sometimes added during a titration?
To wash the sides of the flask so all the acid on the side is washed into the reaction mixture
It can be added as it will not affect the titration reading as water will not react with either reagent or change the number of moles added
Notes about pipette accuracy
The small amount of liquid left in the pipette does not need to be forced out. The pipette is calibrated to allow for it
What happens if the jet space is not filled in the burette?
It will lead to errors in the titration, as it will fill during the titration, leading to a larger expected titre reading
How should results be recorded?
- Clearly in a table
- Recorded in full
- Should be recorded to 2dp (0.05)
- Averages should only be calculated with concordant results
- If 2 or 3 values are within 0.1 then the results are accurate and reproducible, and the titration technique is good
- A test titration could be carried out
What is the sensitivity error of a mass balance?
± 0.001g
What is the sensitivity error of a volumetric flask?
± 0.1g
What is the sensitivity error of a 25cm^3 pipette?
± 0.1 cm^3
What is the sensitivity error of a burette?
± 0.05 cm^3
How is percentage error calculated?
sensitivity / measurement made on apparatus x 100
To calculate maximum percentage error in the final result add all the individual equipment errors together
How can apparatus errors be decreased?
- Use apparatus with a greater resolution/finer scale divisions
- Increase the size of the measurement made
How can titration errors be reduced?
- Replacing measuring cylinders with pipettes or burettes which have a lower sensitivity errir
- The error in a burette can be reduced by making a larger titre - this can be done by increasing the volume and concentration of the substance in the conical flask or by decreasing the concentration of the substance in the burette
How can errors be reduced in measuring mass?
- Use a more accurate balance
- Use a larger mass
- Weighing sample before and after addition of substance and calculating the difference will ensure an accurate measurement of how much is used
If % error < % difference between actual value and calculated value…
then the discrepancy in the result is due to other errors
If % error > % difference between actual value and calculated value…
then there is no discrepancy and all errors can be explained by sensitivity of the equipment
Equation for % atom economy?
mass of useful products / mass of all reactants x 100
(take into account balancing numbers)
- Reactions with only 1 product have 100% atom economy
- Sustainable chemistry requires processes with high atom economy and low production of waste products, economics of the process can be used by selling the bi-product for other uses
Acid + metal ionic equation
HCl and Mg
2H+(aq) + Mg (s) -> Mg^2+ (aq) + H2 (g)
Acid + alkali ionic equation
H+ + OH- -> H2O
Acid + carbonate ionic equation
2H+ + CO3^2- -> H2O + CO2
How to write ionic equations of precipitation reactions
- Leave out spectator ions which do not change state or oxidation number
- put together the 2 ions that make the ppt
