chem core practicals

method to determine molar volume of a gas using ethanoic acid and CaCO3

1. place excess ethanoic acid in a conical flask

2. add pre-weighed marble chips and quickly place the bung on

3. collect the gas produced in a gas cylinder using a delivery tube

4. record the volume of gas produced

repeat, increasing the mass of marble chips added by 0.05g each time

equation for reaction between ethanoic acid and calcium carbonate

2CH3COOH + CaCO3 -> Ca(CH3COO)2 + CO2 + H2O

why is it important not to use too much calcium carbonate?

make sure all the gas produced fits in the gas syringe

why is dilute rather than concentrated ethanoic acid used?

rate of reaction is slower so less gas is lost before bung is put on, reducing error

causes of error in this practical and how they could be reduced

some CO2 may dissolve in the solution

• reduce by adding large spatula of calcium carbonate before adding weighed mass with bung to saturate solution

some gas escapes before bung is placed on

• reduce by using small test tube containing calcium carbonate and tipping tube after placing bung on

may be leak because bung does not fit tightly enough

errors upon transferring solid

• weigh based on difference between original mass and mass after adding calcium carbonate to the reaction mixture

burette safety precautions

fill when top of burette is below eye level

procedure to create a standard solution

weigh volume of solute

dissolve in solvent in a beaker

transfer solution including washings to a volumetric flask, using a funnel

fill the flask up to the mark with solvent (bottom of meniscus on line)

put stopper on and invert several times to mix thoroughly

how to read a pipette accurately

read from eye level

bottom of meniscus should be on the line

why should the burette be rinsed with distilled water and then sodium hydroxide before it is filled?

rinsing with distilled water removes any soluble impurities from the inside of the burette

then rinsing with sodium hydroxide means the concentration of the solution in the burette will be exactly what it is supposed to be, preventing dilution from the distilled water

why is there no need to dry the conical flask after washing it out between trials?

the number of moles of chemical inside the conical flask will be the same regardless of if there is also some distilled water in it

why is it better to have a larger titre?

the larger the titre, the smaller the effect of error due to absolute uncertainty

concordant titres

titres close to each other (usually within 0.2cm³ of each other)

why should the acid have a large molar mass?

reduce weighing errors

what other properties should the acid have?

should not react with air components e.g. water or CO2

how could dilution improve accuracy of a titration?

larger titre reduces percentage error

why is ethanol added to halogenoalkanes before adding silver nitrate?

acts as a solvent

mixture of ethanol and water will dissolve everything involved

describe a procedure to oxidise ethanol

add potassium dichromate to ethanol and add anti-bumping granules to prevent the formation of bubbles that cause violent boiling

reflux using water bath

then distill mixture to extract ethanoic acid

colour change orange (Cr2O72-) to green (Cr3+)

describe a procedure for chlorination of 2-methylpropan-2-ol

1. add the alcohol and concentrated HCl to a flask

2. put a bung on and swirl gently, removing the bung to release gas pressure every few minutes. Do this for twenty minutes

3. add anhydrous CaCl2 and shake, creating two layers. The desired product is in the upper organic layer.

4. use a separating funnel to drain off the lower aqueous layer. Close the tap to keep the upper organic layer

5. add NaHCO3 to remove any unreacted HCl, swirl and invert (add stopper!), opening tap to release CO2 formed

6. remove stopper and run off aqueous layer

7. run organic layer into conical flask and add Na2SO4 to act as a drying agent

8. filter

9. distill the liquid

produces 2-chloro-2-methylpropane

why is a weak base used instead of NaOH, for example?

to prevent the OH- substituting the halogen on the haloalkane

tests for inorganic unknowns

acid or base character using indicator paper

flame test

CO32-/HCO3-

NH4+ ions

SO42- ions

Cu2+/Fe2+/Fe3+

halide ions

test for carbonate/hydrogencarbonate ions

add aqueous acid

bubbles of CO2 gas produced (turns limewater cloudy)

test for NH4+ ions

add aqueous NaOH and warm

gas produced will turn damp red indicator paper blue and has a disgusting, choking smell

test for sulfate ions

acidified (HCl) barium chloride

acid removes any carbonate ions

produces white precipitate

test for Cu2+/Fe2+/Fe3+ ions

aqueous NaOH

Cu2+: blue precipitate

Fe2+: green/white precipitate, slowly turns brown as oxidised in air to Fe3+

Fe3+: brown precipitate

halide ions test

acidified silver nitrate

then ammonia

chloride: dissolves in both dilute and concentrated

bromide: dissolves in concentrated

iodide: dissolves in neither