Inorganic I

During a practical session, a good 2nd year Inorganic I student prepared a good quality alum double salt
with the general formula, M2SO4.M’2(SO4)3.24H2O, where M is +1 and M’ is +3 cations. This was
carried out by weighing exactly 1.7418g of potassium sulphate, which was dissolved in 10mL distilled
water (dH2O). Similarly, 6.6644g of Al2(SO4)3.18H2O in the exact same fashion. The dissolved clear
solutions was mixed in a clean evaporating dish and allowed to crystallize out overnight. The crystals
were filtered off, washed with cold water and dried between filter papers. After complete drying the
experimental yield was determined to be 9.0500g with a NANI AnalyticaR analytical balance.
1.1 Answer all questions below after completing and balancing the following chemical equation:
a) K2SO4 + Al2(SO4)3·18H2O + __H2O → K__SO4·Al__(SO4)3·__H2O [4]
b) What is the theoretical yield of the reaction in the experiment narrated above? [7]
c) Determine the percentage yield of this experimental product. [2]
d) Give the chemical formula of an alum without the amphoteric Al trivalent ion, Al3+
. [2]
e) When limited amount of 0.1M NaOH solution is added to 0.1M of the alum product give
precipitate.
(i) What is the identity of the precipitate formed in this reaction? [1]
(ii) Give an explanation for the product in (i) in the form of a balanced equation. [2]
f) In a similar fashion as in e) above, Al2(SO4)3 solution reacts with the hydroxide solution to
produce a precipitate. Give the product by balancing the net ionic equation. [2]
g) To the product in e) above if one adds an excess of the base something peculiar happens. What
do you expect to happen? Explain with a balanced equation! Hint: Al is amphoteric! [2]
1.2 Why is it almost impossible for Al metal to react with water at normal ambient conditions? Give the
chemical formula of the protective coat that forms on the surface of the metal. [4]
1.3 In an experiment 0.3000 g of Al (Mr = 26.982 g/mol) is reacted with 10.00mL of 4.0 M KOH under
ambient conditions; determine the:
(i) limiting reagent [4]
(ii) theoretical yield [2]
Question 2 [16]
Determining the Formula of an Unknown Alkaline Earth Carbonate:
In an experiment of Inorganic I a student weighed accurately 0.2200 g of an unknown group IIA
carbonate (MCO3) sample on the analytical balance into a 250mL Erlenmeyer flask. Pipetted 10.00 mL
of standardized 0.4999M HCl into the flask, swirled the mixture until the reaction until the fizzing
ceased and the sample was completely dissolved. Added 3 drops of bromophenol blue indicator and
titrated with the standardized 0.2000M NaOH solution. Repeated the analysis with two additional
samples to give two more determinations. The average titre volumes for each of the three determinations
was found to be exactly 10.10 mL.
2.1 Balance step-wise with physical states the two respective chemical equations involved in this back-
titration technique reaction. [6]
2.2 Calculate the molar mass of the unknown carbonate, MCO3, identify the M in the sample. [10]
Question 3 [12]
3.1 Assume you carried out an experiment of Na and K metals reacting with water; report briefly what is
likely to happen in both cases. Also, provide the corresponding reaction equations that describe those
reactions. Hint: Consult your Experiment 3b! [4]
3.2 For the two metals above, which is more reactive? How does this trend depict the reactivity of the
elements within the group? Rank them in order of increasing reactivity. [4]
3.3 Write the balanced chemical equation for the reaction of oxygen with sodium to produce the resultant
peroxide? [2]
3.4 With physical states provide the general equation for the reaction between elements of Group VII
(i.e. the halogens) with those of Group I?

order now