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2 0 1 2 H C I C 1 H2 Chemistry
10.
Atoms, Molecules, Stoichiometry and Redox
When a sample of green potassium manganate(VI), K M n 0 , is added to aqueous sodium
hydroxide, a brown black solid A and a purple solution of B are obtained.
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(i)
A contains 63.8% by mass of manganese and 36.2% by mass of oxygen.
Determine the empirical formula of A.
(ii)
Suggest the identity of B and state the type of reaction that has occurred.
(iii)
Construct a balanced ionic equation for this reaction.
(iv)
2.00 g of impure K M n 0 was added to excess aqueous sodium hydroxide and the
brown black solid A was filtered off. The remaining solution was made up to a
volume of 500 c m with distilled water. 25.0 c m of this solution was pipetted out
and acidified with dilute H S 0 . The acidified solution requires 5.60 c m of
0.200 mol d m " aqueous iron(ll) sulfate for titration. Given that 1 mol of B reacts
with 5 mol of F e , calculate the percentage purity of K M n 0 in the original sample.
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Application of oxidation number and electron transfer in redox reaction
11.
Sodium nitrate(lll), N a N 0 , is used as a preservative in meats and fish. It is very soluble in
water and is slowly oxidized by oxygen in the air to sodium nitrate(V), N a N 0 . N a N 0 can
also be converted to H N 0 by the action of dilute sulfuric acid. H N 0 is unstable and
decomposes into N 0 , H N 0 and H 0 .
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A student was required to show that N a N 0 as a preservative is unstable and is oxidised to
N a N 0 . He carried out an experiment as described below.
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1. Fill the burette with an aqueous solution containing 6.64 x 10" mol d m N a N 0 .
2. Using a pipette, measure out 25.0 c m of a 2.00 x 10" mol d m " K M n 0 into a conical
flask.
3. Add 10 c m of sulfuric acid into the conical flask.
4. Titrate this solution against the N a N 0 solution in the burette.
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He found that 18.80 c m of the N a N 0 solution was required to reach the end-point of the
titration.
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(i)
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Calculate the number of moles of N a N 0 and K M n 0 used in the titration. Hence
show that N a N 0 is oxidised to N a N 0 .
Why was N a N 0 placed in the burette instead of the conical flask for titration in this
experiment?
Why was sulfuric acid used in step 3 instead of hydrochloric acid?
Give the colour change at the end-point of the titration.
Write a balanced equation for the reaction between N a N 0 and K M n 0 .
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(ii)
(iii)
(iv)
(v)
12.
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An acidified solution of the salt KC/O (x = 1, 2 or 3) will oxidise Fe (aq) to Fe (aq)
quantitatively, the chlorine being reduced to C/~(aq). When 0.150 g of the salt KC/O was
reacted with 0.500 mol d m " Fe (aq) in the presence of H (aq), 11.3 c m of Fe (aq) was
needed for complete reaction.
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Calculate the value of x and construct an equation for the reaction between Fe (aq) and
acidified KC/O (aq).
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x