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Chemistry
Paper2
Leaving
Certificate Chemistry - 1999
Question
A mass of 2.52g of ethanedioic acid crystals, 
, was dissolved
in deionised water and the solutions was made up accurately
to 500 cm3 in a volumetric flask. Some of
the solutions was poured into a clean, dry beaker. A
pipette, fitted with a pipette filler (see diagram), was
used to transfer 25.0 cm3 of the solution from
the beaker to a conical flask and about 20 cm3
of dilute sulphuric acid were added. A thermometer was
placed in the conical flask, and the flask and contents
were heated to about 70°C. The flask was removed from
the heat and the thermometer was removed from the solutions.
It was found by titration that the acidified solution
of ethanedioic acid required 20.0 cm3 of a
0.020 mol dm-3 solutions of potassium manganate(VII)
for complete oxidation.
The equation for the reaction involved
in the titration is
i)
In preparing the pipette for the titration
it was rinsed with deionised water and then the water
was removed by rinsing the pipette with the solution to
be transferred. Explain why it was important to remove
the water. Why was the conical flask not rinsed
with the solution? (12)
ii)
Pipette fillers are used for safety reasons.
What property of ethanedioic acid makes the use of a pipette
filler essential? (6)
iii)
What problem was encountered when removing
the thermometer from the solution in the conical flask?
How could this problem have been overcome?(9)
iv)
Why was it necessary to heat the contents
for the conical flask immediately before carrying out
the titration? Explain why it was not necessary to continue
heating during the titration. (9)
v)
What colour change was observed as the potassium
manganate (VII) solution reacted with the acidified ethanedioic
acid solution? How as the end-point of the titration
identified? (12)
vi)
Calculate the concentration of the ethanedioic
acid solution in the volumetric flask (a) in g dm-3
, (b) in mol dm-3 . Hence find the percentage
water in the ethanedioic acid crystals and the value of
x in the formula. (18)
Solutions
i)
If drops of water remain in pipette (or
burette), this would dilute the solutions they are going
to contain. However, if the conical flask is rinsed with
the solution, drops of solution would remain which would
increase the amount of the reactant.
ii)
Pipette fillers are used for health safety
reasons as they prevent intake of dangerous solutions
through the mouth. Ethanedioic acid is, actually poisonous.
iii)
Drops of the flask solution would be removed
on the thermometer. If this happens calculations are
inaccurate. The thermometer should be rinsed with deionised
water before taking it out of the solution. (The deionised
water will not alter the moles of reactant in the flask.)
iv)
Heat increases the rate of a chemical reaction
and also breaks bonds of the reactants. In this case
the C to C bonds in 
must be
broken. One of the products in this reaction is 
which will
act as a catalyst for the reaction. The catalyst will
speed up the reaction without more heat.
v)
A purple to colourless change was noted.
The end point occurs when one further drop won't go colourless.
vi)
Parts a and b first:
a.
Note that the flask solution was made up
by mixing 2.52g of crystals per 500
[
. This
is the same concentration as 
.
b.
To change g 
to moles
we divide
by the molecular . But, we don't know the molecular weight
as x is involved.
(1)
(2) 0.04 moles = 5.04g 
1 mole
=
(3)
%Water = 
