random practical questions

outline steps to determine the solubility of calcium hydroxide in a saturated sample

1. take a known volume of the saturated solution

2. evaporate until its dry

3. weigh the residue

whats the type of reaction in step 3

hydrolysis

why can an XS of KMnO4 be undesirable when treating water

unpleasant taste

• decrease SA

• lower ROR

practical reason why mass of MgO obtained is slightly less than expected

some solid is blown away with the gas

safety and hazards - irritant, corrosive, flammable, toxic and oxidising

• irritant - dilute acids and alkalis - wear gloves

• corrosive - stronger acids and alkalis - wear goggles

• flammable - keep away from naked flames

• toxic - wear gloves, avoid skin contact, wash hands after

• oxidising - keep away from flammable/easily oxidised materials

heating a crucible - whens it used

• measuring mass loss in decomposition reactions

• measuring mass gain when reacting Mg in Oxygen

heating a crucible - water of crystallisation in calcium sulfate crystals

1. weigh empty dry crucible and lid

2. add hydrated calcium sulfate to crucible and weigh again

3. heat with bunsen burner

4. allow to cool

5. weigh crucible and contents again

6. heat again and reweigh until constant mass

heating a crucible - why should large amounts of calcium sulfate not be used

the decomposition is likely to be incomplete

heating a crucible - why does the crucible need to be dry

a wet crucible would give an inaccurate result

• mass lost would be larger as the water would be lost when heating

heating a crucible - purpose of lid

improves accuracy

• prevents loss of solid from crucible

• but should be loose fitting to allow gas to escape

heating a crucible - why should small amounts of solid not be used

the % uncertainties will be too high

when to use gravitational vs vacuum filtration

spectrometry method

1. add appropriate ligand to intensify colour

2. make up solutions of known conc

3. measure absorption

4. plot graph absorption vs conc

5. measure absorption of unknown and compare

spectrometer filter colour

colour chosen to only allow the wavelengths of light through that would be most strongly absorbed by the coloured solution

why cant the mass of water in the test tube be relied on to determine the mass of water for water of cystallisation

• mass may be lower due to evaporation

• mass may be higher due to condensation on test tube outside

question ii

• function groups are alkene and benzene ring

• means can undergo addition polymerisation

homolytic fission of the O-O bond forming two radicals

whats the assumption made

at the boiling point of a liquid, equilibrium is reached between liquid and vapour so that delta G = 0

describe the basic steps to determine the order of reaction with respect to bromine (or anything) using colorimetry

1. measure absorbance of a set of standard solutions of bromine to obtain a calibration curve

2. measure absorbance of the reaction mixture over time

3. ploy graph of conc bromine against time

4. determine order with respect to bromine from shape

in a redox titration why is iron dissolved in an XS of H2SO4 and not CH3COOH

• XS - ensures all MNO4- reacts to form Mn2+

• ethanoic acid is a weak acid and so does not supply sufficient H+

outline the mechanism to show how an aqueous Al3+ ion can act as a bronsted lowry acid

question ii

EQ shifts RHS to increase the ester conc

• round bottomed flask - NO - just needs container that can hold minimum 110cm3

• dropwise - YES - reaction is exothermic

• reflux - NO - reaction can occur under distillation

• electric heating mantle - YES - stops flammable vapors igniting

two equations incl state symbols to show the laboratory formation of tollens reagent

for testing tollens why should only clean and dry glassware be used

• if its contaminated it prevents the glass from being uniformly wetted

• so the silver mirror will not be able to adhere onto the glass surface

how can an airtight seal be formed between the reflux condenser and a round bottomed flask

greasing/using petroleum jelly on connecting parts

method for nitration of methylbenzoate

1. prepare nitrating mixture - dissolve small amount MB in conc H2SO4 then cool in an ice bath

2. prepare acid mixture - separate test tube mix equal vols of conc H2SO4 and HNO3 then cool in an ice bath

3. add acids - use dropping pipette to add to MB

4. monitor temp - stir continuously and ensure temp stays below 10 degrees

5. allow to stand at room temp so reaction goes to completion

6. precipitate the product - pour mixture over crushed ice

7. filter under reduced pressure, wash with cold water

safety considerations for nitration of methylbenzoate

• conc acids are corrosive - goggles and gloves, fume cupboard

• temp control - reaction is very exothermic so keep below 10 degrees to prevent splashing and to stop further nitration

• methylbenzoate - flammable and an irritant, keep away from naked flames

what is methyl benzoate

describe how you would safely and accurately measure out and place 25cm3 of a solution in a conical flask

• rinse pipette with the solution

• using a pipette and pipette filler, transfer the solution

whats a pipette and pipette filler

a student carried out experiment to determine which of a chloro/bromo/iodoalkane hydrolysed the fastest when heated with silver nitrate in ethanol - describe an experimental method he could use. ensure that it is a fair test

fair test - haloalkanes with same chain length, all tubes same temp, equal vols of everything

• place equal volumes of each haloalkane in separate test tubes and place in water bath

• heat ethanol and silver nitrate in same water bath

• when all reached same temp start clock

• record time for each ppt to form

you have chloro/bromo/iodoethane - which has highest density and if all three were present in a container at room temp how would you separate

1. iodo highest as highest halogen mass

2. chloroethane is a gas and so must be removed via fractional distillation

if you add a substance to water and one layer forms what can be deduced

the substance is miscible in water - it can hydrogen bond

why can colorimetry be used to investigate this RoR

the orange colour of bromide fades as the reaction proceeds and colourless bromide ions are produced

explain the purpose of adding a large excess of methanoic acid

the conc of it is constant - so the effect of changing the conc of bromine on the rate can be determined

how does boiling water and then allowing it to cool to room temp increases accuracy

boiling it removed CO2 - which is acidic and so affects the pH

how do the conditions for nitration of methyl benzoate differ from those of nitration of benzene

• benzene heated to 50 degrees

• methyl benzoate cooled to less than 10 degrees

safety precaution using methanal

fume cupboard - irritates the nose

benefits of buchner filtration over normal

faster and produces drier crystals

explain why poly(cyclohexene) is a solid at room temp but the monomer cyclohexene is a liquid

• poly(cyclohexene) is a larger molecule

• so has more VDW forces between molecules and so more energy to overcome

polarimeter diagram