Chemistry Test I
Secondary Six
29-10-2010
(Time allowed 60 minutes)
1.
At 359 K and 101 kPa, 0.160 g of a volatile liquid changes completely into its vapour which occupies 80.0 cm 3.
Assuming that the vapour behaves ideally. Calculate the molar mass of the liquid.
2.
(2 marks)
Car bumpers made of steel are usually plated with chromium. A chromium-plating bath consists of an aqueous
solution of CrO3 and H2SO4.
(a)
Draw a labelled diagram to show the set-up of chromium-plating car bumpers.
(b)
Write a half equation for the reaction involved at the cathode.
(c)
A car bumper with a total surface area of 3.0 x 10 3 cm2 is chromium-plated using a current of 35 A.
Assuming 100 % current efficiency, calculate the time required for depositing a layer of chromium
3.0 x 104 cm thick onto the bumper.
(Density of chromium = 7.2 g cm–3)
(d)
If a current much larger than 35 A is used, what would be the effect on the quality of the chromium layer
plated onto the bumper?
3.
(7 marks)
Phenolphthalein (C20H14O4) is an acid-base indicator, it can be prepared from the reaction between phthalic
anhydride (C8H4O3) with phenol (C6H6O). The equation of the reaction is:
HO
OH
O
O
OH
+
+
2
H 2O
O
O
O
phthalic anhydride
(a)
phenol
phenolphthalein
2.0 kg of phthalic anhydride reacts with 2.0 kg of phenol to prepare phenolphthalein.
State, with calculation, which compound is the limiting reactant in the reaction.
(b)
If 900 g of phenolphthalein is obtained in the preparation, calculate the percentage yield of
phenolphthalein.
(c)
Phenolphthalein exhibits colourless in a solution of pH 6. If more concentrated hydrochloric acid is added,
the solution turns to orange. Write a possible structure of phenolphthalein which exhibits orange in colour.
(6 marks)
4.
Tartaric acid can be used as a flavoring agent to make foods taste sour. Tartaric acid contains carbon, hydrogen
and oxygen only. 3.72 g of tartaric acid burns in excess oxygen completely to give 4.36 g of carbon dioxide and
1.34 g of water.
(a)
Calculate the empirical formula of tartaric acid.
(b)
Tartaric acid is a dibasic acid. 0.372 g of tartaric acid can be completely neutralised with 19.84 cm 3 of
0.250 M KOH(aq). Deduce the molecular formula of tartaric acid.
(c)
Tartaric acid contains two hydroxyl groups. It is a straight chain and chiral compound. Deduce the
structure of tartaric acid.
5.
(8 marks)
A pain killer contains aspirin as active ingredient and other inert substances. 5.432 g of the pain killer was heated
under reflux with 25.00 cm3 of 2.056 M NaOH(aq) for about two hours. The reaction can be represented by the
following equation:
_
_
3 OH
O
OCOCH3
+
++
+
CO2 H
_
CH3 CO2
+
2 H 2O
_
CO2
heat
(a)
Suggest a compound that can be used to standardise NaOH(aq).
(b)
Draw a labelled diagram to show the assembly of the set-up of the above reaction.
(c)
After the reaction had been completed, the mixture was cooled down and made up to 250.00 cm3. Then
25.00 cm3 of the resulting solution was titrated against 0.1234 M HCl(aq) using phenolphthalein as
indicator, and the average titre of the HCl(aq) was 28.30 cm 3.
6.
(i)
Suggest a compound that can be used to standardise HCl(aq).
(ii)
Suggest the colour change of the solution at the end point of the titration.
(iii)
Calculate the percentage by mass of aspirin in the pain killer.
(9 marks)
Caffeine is an addictive drug used in soft drinks as a stimulant. The structure of caffeine is shown below:
O
CH3
N
HN
N
O
N
CH3
(a)
Calculate the percentage by mass of nitrogen in caffeine.
(b)
If a sample of soft drink contains caffeine as the only nitrogen-containing compound. It was found that the
soft drink sample contained 12.8 % by mass of nitrogen. Calculate the percentage by mass of caffeine in
the soft drink sample.
(c)
Outline a method for determining the percentage by mass of caffeine in the soft drink sample.
(8 marks)
End
of
Paper
Chemistry test I
1.
2.
S6
29/10/2010
101 x 80.0 x 103 = (0.160 M) x 8.31 x 359
1
M = 59.08 g mol1
1
(a)
D.C. source, ammeter, rheostat, car bumper as the cathode, chromium as anode,
CrO3(s) and H2SO4(aq) as chromium-plating bath

2
(b)
Cr2O72 + 14 H+ + 12 e
(c)
3.0 x 103 x 3.0 x 104 x 7.2  52.0 = 0.125 mol of Cr(s)
2 Cr + 7 H2O
1
1
0.125 x (1/6) x 96500 = 35 t
1
t = 57.4 s
3.
1
(c)
The rate of plating is too fast. Chromium layer is attached very loosely on the surface of bumper.
1
(a)
no. of moles of phthalic anhydride = 2000 148 = 13.5
½
no. of moles of phenol = 2000  94 = 21.3
½
since
(b)
( 21.3 / 13.5 ) < 2
phenol is the limiting reactant
(900  318)  (21.3 x ½) = 26.5 %
1+1
(c)
4.
(a)
½+½
2
no. of mole of carbon = 4.36  44.0 = 0.0990
no. of mole of hydrogen = 2 x (1.34  18.0) = 0.149
no. of mole of oxygen = (3.72  0.0990 x 12.0  0.149 x 1.0)  16.0 = 0.149
mole ratio of C : H : O = 0.0990 : 0.149 : 0.149 = 2 : 3 : 3
Empirical formula of tartaric acid is C 2H3O3
(b)
1
1
1
0.372  M = 0.250 x 0.01984 x ½
M = 150
1
(12.0 x 2 + 1.0 x 3 + 16.0 x 3) n = 150
n=2
5.
1
Molecular formula of tartaric acid is C 4H6O6
1
(c)
HO2CCHOHCHOHCO2H
2
(a)
hydrolysis / saponification
1
(b)
oxalic acid / benzoic acid / potassium hydrogen phthalate
1
(c)
(d)
2
(i)
anhydrous Na2CO3 / Na2B4O7 10 H2O
1
(ii)
pink / red to colourless
1
(iii) 0.1234 x 0.02830 x (250  25) = 0.03492
(2.056 x 0.02500  0.03492) x ½ = 8.239 x 10
1
3
1
Mass of aspirin in the pain killer = 8.239 x 10 3 x 180 = 1.483 g
1
% aspirin in the pain killer = (1.483  5.432) x 100 % = 27.3 %
1
6.
(a)
Caffeine is C7H8N4O2
1
14.0 x 4  108 = 51.9 %
1
(b)
12.8 %  51.9 % = 24.7 %
(c)
The sample is heated with concentrated sulphuric acid.
1
Excess NaOH(aq) is added and the mixture is heated.
1
NH3(g) is distilled into known volume of standard HCl(aq).
1
Excess HCl(aq) is titrated against standard NaOH(aq) with phenolphthalein as indicator.
1
Mole of nitrogen = mole of HCl(aq)  mole of NaOH(aq)
14.0 x mole of nitrogen  mass of sample = % of nitrogen
1+1