practical techniques

Question 1

rules for reading burettes and measuring cylinders

Answer 1

• read to bottom of the meniscus
• read to half the unit marked

Question 2

rule for reading thermometers

Answer 2

read to half the smallest unit marked

Question 3

method for weighing out substance using a weigh boat

Answer 3

1. weigh boat + contents
2. tip out contents
3. weigh boat again

Question 4

plotting graphs

Answer 4

plot the independent variable (the one you change) on the x axis, and the dependent variable on the y axis

Question 5

how to find the gradient

Answer 5

change in y divided by the change in x

Question 6

notes on colour observations

Answer 6

• if there is a colour change, state the starting colour and the finishing colour
• no colour = you need to say colourless, not clear

Question 7

significant figures in calculations

Answer 7

• in the intermediate answers, keep at least one more significant figure than you will need in the end
• in the end answer, use the same sig figs as the smallest number of significant figures in the data given

Question 8

data that needs to be given in a final answer

Answer 8

• sign (plus or minus)
• units
• significant figures

Question 9

purpose of a titration

Answer 9

to find the volumes of solutions that react

Question 10

calculating concentrations in titrations

Answer 10

1. write a chemical equation for the reaction (or look at given equation)
2. work out the number of moles of one of the reagents
3. use the chemical equation to work out the number of moles of the reagent that reacts with reagent one
4. calculate concentration of reagent 2

Question 11

working out uncertainty

Answer 11

uncertainty can be taken to be the same size as the smallest unit that can be measured on the instrument (i.e. half the smallest division marked on the instrument)

Question 12

working out percentage uncertainty

Answer 12

( uncertainty / reading ) x 100

Question 13

rule when multiplying and dividing quantities with uncertainties

Answer 13

add the percentage uncertainties

Question 14

rule when adding and subtracting quantities with uncertainties

Answer 14

add the actual uncertainties, not the percentage uncertainties

Question 15

what is precision in chemistry

Answer 15

the repeatability of results

Question 16

calculating percentage error

Answer 16

( difference between experimental result and theory results / theory result) x 100

Question 17

identifying a systematic error

Answer 17

if the experiment is not accurate, the deviation from the theory value must be due to a systematic error

Question 18

types of systematic error

Answer 18

• operator error
• apparatus error
• method error

Question 19

potential causes of systematic error and their ways of minimising

Answer 19

• heat loss from the apparatus ~ cover the top of the beaker
• heat loss to the beaker ~ use metal calorimeter
• evaporation of fuel ~ place cap on burner

Question 20

method to make up solutions

Answer 20

1. calculate the mass of substance you need to weigh out
2. dissolve substance in 100cm3 of distilled water
3. pour solution into volumetric flask
4. rinse beaker and funnel with distilled water into volumetric flask
5. add distilled water to flask until it reaches level line
6. stopper the flask and invert 10 times for a homogenous solution

Question 21

method for dilution solutions

Answer 21

1. estimate volume of diluted solution needed
2. calculate volume of undiluted solution you will need using (new volume x new concentration = old volume x old concentration)
3. measure out the volume of undiluted solution in the most precise way you can
4. run this volume into volumetric flask
5. make flask up to mark with distilled water and invert 10 times for a homogenous solution

Question 22

apparatus for titrations

Answer 22

• conical flasks
• beakers
• burettes
• pipettes
• pipette filler

Question 23

procedure for titrations

Answer 23

1. set up apparatus
2. add reagent one in burette, add reagent two into conical flask on white tile below the burette
3. add 5 drops of indicator (if needed) into conical flask
4. record initial burette reading (miniscus at eye level)
5. run solution from burette into titration flask until indicator just changes colour (add drop by drop at end point)
6. record burette reading, calculate titre
7. rinse out titration flask with distilled water and repeat to until you have three concordant results
8. calculate an average titre

Question 24

what are concordant results

Answer 24

results that are within 0.1cm3 of each other

Question 25

what is heating under reflux

Answer 25

using a water condenser mounted vertically above a heated flask, top of the flask is not stoppered

Question 26

uses of heating under reflux

Answer 26

useful for slow reactions with volatile reactants and products that would otherwise escape also useful because many organic liquids are flammable

Question 27

use of distillation

Answer 27

to remove one liquid from a mixture of liquids

Question 28

method for purifying a liquid

Answer 28

1. shake liquid with various substances in a separating funnel 2. if any gases given off, the funnel must be held upside down after shaking and pressure released through the tap 3. allow funnel to stand until layers have separated, run off a layer and collect it 4. if an organic layer has water ass the main impurity, it can be dried by leaving it to stand over anhydrous sodium sulfate in a flask with a stopper, then filter through a mineral wool plug 5. distill to remove any remaining impurities

Question 29

method for purifying a solid

Answer 29

vacuum filtration - a moistened filter paper of the correct size is placed in buchner funnel vacuum pump (tap) is turned on and a suspension of the solid required is poured in when liquid is drained, solid is left on filter paper, which can then be recrystallised

Question 30

method for recrystallisation

Answer 30

1. set up a hot water bath, containing two boiling tubes one containing solvent (e.g. ethanol) and the other containing the solid 2. add hot solvent to solid in small proportions and stir the mixture each time. stop adding immediately after a clear solution is formed 3. leave solution to cool and recrystallise 4. use vacuum filtration, filter off the solid, then wash it with a little cold solvent and allow it to dry

Question 31

calculating percentage yield

Answer 31

(actual mass of product / theoretical mass) x 100 - based off number of mole of limiting reactant

Question 32

use of colorimetry

Answer 32

used to measure the concentration of a coloured substance in a solution

Question 33

steps for colorimetry

Answer 33

1. select filter (complementary colour of the tested solution) 2. zero colorimeter with a cuvette of distilled water 3. make up several standard solutions of the coloured substance 4. measure absorbance for each of these solutions 5. plot a graph of absorbance against concentration 6. plot line of best fit to get calibration curve 7. find absorbance for solution of unknown concentration, read off the concentration of of the unknown from calibration curve

Question 34

thin layer chromatography procedure

Answer 34

1. use a pencil, draw a base line on a thin layer plate, making it above the level of the solvent 2. use capillary tube, place spots of known and unknown substances on baseline 3. place the plate in a solvent in a beaker 4. cover beaker with clingfilm 5. when solvent is near top of the plate, remove plate and mark solvent level 6. allow plate to dry and then spray with ninhydrin solution to see spots 6. calculate Rf value and compare substances

Question 35

calculating Rf value

Answer 35

Rf = distance moved by spot / distance moved by solvent front