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chemistry a level > practical skills > Flashcards

practical skills Flashcards

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1
Q

how do we make a solution

A
  1. weigh the sample bottle containing the required mass of solid on a 2 dp balance
  2. transfer to a breaker and reweigh sample bottle
  3. record the difference mass
  4. add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid
  5. pour the solution into a 250cm3 graduated flask via a funnel
  6. rinse beaker and add washings from the beaker and glass rod to the volumetric flask
  7. makeup to the mark with distilled water using a dropping pipette for the last few drops
  8. invert flask several times to ensure uniform solution
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2
Q

what do we do to the solution if it does not dissolve in cold water

A

the beaker and its contents could be heated gently until all the solid had dissolved

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3
Q

how do we dilute s solution

A
  1. pipette 25cm3 of original solution into a 250cm3 volumetric flask
  2. make up to the mark with distilled water using a dropping pipette for the last drops
  3. invert flask several times to ensure uniform solution
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4
Q

why is using a volumetric pipette more accurate

A

using a volumetric pipette is more accurate than a measuring cylinder because it has a smaller uncertainty

use a teat pipette to make up to the mark in volumetric flak to ensure volume of solution accurately measured and one doesn’t go over the line

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5
Q

how do we calculate dilutions

A

diluting a solution will not change the amount of solute present but increase the volume of solution and hence the concentration will be lower

moles =volume x concentration

if the amount of moles does not change then

the original volume x original concentration = new diluted volume x new diluted concentration

the new diluted volume will be equal to the original volume of solution + the volume of water

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6
Q

what are the safety and hazards of making dilutions

A

irritant - dilute acid alkalis - wear goggles

corrosive - stronger acids and alkalis wear goggles

flammable - keep away from naked flames

toxic - wear gloved - avoid skin contact - wash hands after use

oxidising - keep away from flammable/ easily oxidised materials

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7
Q

when do we use gas syringes

A

gas syringes can be used for a variety of experiments where the volume of a gas is measured, possibly to work out moles of gas or to follow reaction rates

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8
Q

what does the volume of gas in gas syringes depend on

A

the volume of a gas depends on pressure and temperature, so when recording volume it is important to note down the temperature and pressure of the room

  • the moles of the gas can then be calculated from gas volume
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9
Q

what are the potential errors using a gas syringe

A
  1. gas escape before bung inserted
  2. syringe sticks
  3. some gases like carbon dioxide are soluble in water so the true amount of gas is not measured
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10
Q

how do we carry out titrations

A

rinse equipment (burrette with acid,pipette with alkali, conical flask with distilled)

pipette 25cm3 of alkali into a conical flask

touch surface of alkali with a pipette ( to ensure the correct amount is added)

adds acid solution from the burette

make sure the jet space in the burette is filled with acid

add a few drops of indicator and refer to a colour change at the endpoint

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 endpoint

note burette reading before and after addition of acid
repeat titration until at least 2 concordant results are obtained - two reading within 0.1 each other

repats titration until at least 2 concordant results are obtained two - readings within 0.1 of each other

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11
Q

how do we work out average titre results

A

we only make an average of the concordant titre results

if 2 or 3 values are 0.10cm3 and therefore concordant or close then we can say results are accurate and repeatable and the titration techniques is good/consistent

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12
Q

how do we record results

A

results should be clearly recorded in a table

results should be recorded in full ( i.e. both initial and final readings)

record titre volumes to 2.d.p (0.05 cm3)

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13
Q

what are the safety precautions we must take when carrying out a titration

A

Acids and alkalis are corrosive ( at low concentrations acids are irritants)

Wear eye protection and gloves
If spilled immediately wash affected parts after spillage

If the substance is unknown treat it as potentially toxic and wears gloves

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14
Q

what happens when the jet space is not filled

A

is the jet space not filled properly prior to commencing the titration it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading

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15
Q

why do we use a conical flask

A

it is used in preference to a beaker because it is easier to swirl the mixture in a conical flask without spilling its contents

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16
Q

why must we add more than a few drops of indicator

A

indicators are generally weak acids so only add a few drops of them

If too much is added it will affect the titration results

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17
Q

why do we add distilled water to the conical flask during a titration

A
  • we do it to wash both sides of the flask so that all the acid on the side is washed into the reaction mixture to react with alkali

It does not affect the titration reading as water does not react with the reagents or change the number of the moles of acid added

18
Q

how do we record the results in a titration

A

results should be clearly written in a table

results should be recorded in full (i.e. both initial and final readings

record titre volumes to 2.d.p.

19
Q

how do we titrate mixtures

A

if titrating a mixture to work out the concentration of an active ingredient

it is necessary to consider if the mixture contains other substances that have acid base properties

if they don’t have acid base properties we can titrate with confidence

20
Q

how do we deal with excess acid

A

sodium hydrogen carbonate (NaHCO3) and calcium carbonate (Caco3) are good for neutralising excess acid in the stomach or acid spills because they are not corrosive and will not cause a hazard if used in excess

They also have no toxicity if used for indigestion remedies but the CO2 produced can cause wind

Magnesium hydroxide is also suitable for dealing with excess stomach acid as it has low solubility in water and is only weakly alkaline so not corrosive or dangerous to drink (unlike the strong alkali sodium hydroxide).
It will also not produce any carbon dioxide gas

21
Q

what is the uncertainty of a reading

A

the uncertainty of a reading ( one judgement) is at least +/- 0.5 of the smallest scale reading

22
Q

what is the uncertainty of a measurement

A

the uncertainty of a measurement ( two judgement) is at least +/- 1 of the smallest scale reading

23
Q

what are readings

A

the value found from a single judgement when using a piece of equipment

24
Q

what is a measurement

A

the values are taken as the difference between the judgements of two values

(e.g. using a burette in a titration)

25
Q

how do we calculate apparatus uncertainties

A

each type of apparatus has a sensitivity uncertainty:

  • balance +/- 0.001g
  • volumetric flask +/- 0.1cm3
  • 25 cm3 pipette +/- 0.1cm3

burette (start and readings and end point) +/- 0.15

26
Q

how to we calculate the percentage error for each piece of each equipment used by

A

% uncertainty = +/- uncertainty/ measurement made on apparatus x 100

e.g. for burette
% uncertainty = 0.15/ average titre result x 100

to calculate the maximum total percentage apparatus uncertainty in the final result add all the individual equipment uncertainties together

27
Q

how do we reduce uncertainties in a titration

A

replacing measuring cylinders with pipettes which have lower apparatus uncertainty will lower the % uncertainty

to reduce the % in a burette reading it is necessary to make the titre a larger volume
This could 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

28
Q

how do reduce uncertanties in measuring mass

A

using a balance that measures to more decimal places or using a larger mass will reduce the % uncertainty in weighing a solid

weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added

29
Q

how do you decrease uncertainty

A

to decrease the apparatus uncertainties you can either decrease the sensitivity uncertainty by using apparatus with a greater resolution (finer scale divisions) or you can increase the size of the measurements made

30
Q

uncertainty of a measurement using a burette

A

if the burette used in the titration had an uncertainty for each reading of +/- 0.05 cm3 then during a titration two readings would be taken so the uncertainty on the titre volume would bee +/ 0.10cm3

Then often another 0.05 is added on because of uncertainty identifying the end point colour change

31
Q

how do we remove the water in the crystallisation of calcium sulfate

HEATING IN A CRUCIBLE

A
  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 burner for a couple of minutes
  4. allow to cool
  5. weigh the crucible and contents again
  6. heat crucible again and reweigh until you reach a constant mass (do this to ensure reaction is complete)
32
Q

why can we not use large amounts of calcium sulfate when we are heating in a crucible

A

large amounts of hydrated calcium sulfate, such as 50g should not be used in this experiment as the decomposition is likely to be incomplete

33
Q

why must the crucible be dry

A

the crucible needs to be dry otherwise a wet crucible would give an inaccurate result

It would cause mass loss to be too large as the water would be lost when heating

34
Q

why can’t we use small amounts of the solid when we are heating in a crucible

A

small amounts of the solid should not be used in this experiment as the percentage uncertainties in weighing will be too high

35
Q

why should we use a lid when we are heating in a crucible

A

the lid improves the accuracy of the experiments as it prevents loss of solid from the crucible

the lid should be loose enough to allow gas to escape though (gas as in the evaporated water)

36
Q

practical 10

preparation of a pure organic solid

A

preparation and separation of the crude product

purification and drying

checking the purity of an organic solid

percentage yield

37
Q

preparation and separation of the crude product

A

Preparation: react suitable quantities of the reactants to produce the product

separation of the crude (impure) product: the solid is separated from the reaction mixture by suction filtration

Purification of the product: removed impurities and is carried out by recrystallisation

Drying the product: this is carried out b sucking air over the solid in the Buchner flask and in a lower temperature oven

Check the purity: by carrying out a melting point determination

38
Q

how do we purify a product

A

to obtain a purified solid product, recrystallisation is usually required:

  1. dissolve the impure crystals in the minimum volume of hot solvent
    (This ensures that the hot solution is saturated so that crystals form on cooling)
  2. filter the hot solution by gravity filtration using a hot funnel and fluted filter paper, to remove any insoluble impurities
    (filtering through a hot filter funnel and using fluted paper prevents precipitation of the solid)

3, allow the solution to cool and crystallise
(Reducing the temperature reduces the solubility of the crystals. The impurities will remain in the solution).

Sometimes scratching the side of the flask with a glass rod or adding a small seed crystal can aid crystal formation

  1. filter off the crystal using suction filtration
  2. Wash by putting over some ICE - COLD SOLVENT, which removes any aqueous impurities ( the solvent is cold to prevent the crystals from dissolving)
  3. dry by sucking air over the crystals in the Buchner flask and then in a low - temperature oven. Alternative methods of drying include placing in a desiccator with a drying oven
39
Q

how do we check the purity of an organic solid

A

the melting point of a substance is not the exact point at which it melts but rather the range of temperatures from when the sample starts to melt until it has complementarily melted

The greater the range the more impurities - it melts over a range of temperatures and at a lower temperature than the pure substances

40
Q

how do we check for purity

A
  1. place some of the solid in a melting point tube sealed at one end
  2. place in melting point apparatus and heat slowly
  3. record the temperature at which the solid starts to melt and the temperature at which it finishes melting
  4. repeat and average the temperatures
  5. compare the metling point with known values in a data book

chemistry a level (24 decks)

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