Page 367 - SUBSEC October 2017_Neat

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Item 26 refers to the following reaction: 28. Which of the following is NOT a standard
condition for measurement of electrode
CH COOH (aq)  H (aq) + CH COO (aq) potentials?
-
+
3
3
[sodium ethanoate] = 0.5 mol dm and
-3
[ethanoic acid] = 0.3 mol dm -3 (A) Solutions have a concentration of
1 mol dm .
-3
26. Given that K = 1.8 × 10 , what is the pH (B) Gases have a pressure of 1
-5
a
of the buffer solution? atmosphere.
(C) Temperature is 25 C.
o
(A) 3.25 (D) Metal alloys are used as electrodes
(B) 4.97
(C) 6.50
(D) 9.94 29. Which pair of half cell potentials would
produce an E /V of 1.1 when combined to
Θ
form a cell?
27. When solid silver chloride is added to a
solution of potassium iodide, a yellow (A) Cu (aq)/Cu(s) and Fe (aq)/Fe (aq)
2+
2+
3+
precipitate of silver iodide forms because (B) Cu (aq)/Cu(s) and Zn (aq)/Zn(s)
2+
2+
(C) Al (aq)/Al(s) and Zn (aq)/Zn(s)
2+
3+
(A) chlorine is more electronegative (D) Ag /Ag(s) and Cu (aq)/Cu(s)
+
2+
than iodine
(B) potassium chloride and potassium
iodide are both soluble
(C) silver chloride has a lower K
sp
value than silver iodide
(D) silver iodide has a lower K value
sp
than silver chloride
30. The standard electrode potentials for dichromate and iodide are:

1/2 Cr O 7 2- (aq) + 7H (aq) + 3e  Cr (aq) + 7/2 H O + 1.33 V
+
3+
2
2
- 
I (aq) + 2e + 2e 2I (aq) + 0.54 V
-
-
2
From this it can be deduced that

(A) acidified dichromate ions would oxidise iodide ions to iodine
(B) iodine would oxidise chromium (III) ions to dichromate
(C) combining both half cells would produce a cell potential of 1.87
(D) electrons would flow from the dichromate half cell to the iodine half cell if the two are
connected

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02112010/CAPE/SPEC/2017