Calculations Flashcards Preview

A Level Chemistry > Calculations > Flashcards

Flashcards in Calculations Deck (42)
Loading 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