CHEM1612 2013-N-3 November 2013 Marks Calculate the molar solubility of silver sulfide, Ag S, given that K is 8 × 10–51 • 2 sp 3 at 25 °C.
The dissolution reaction and solubility product are:
+ 2- + 2 2- Ag2S(s) 2Ag (aq) + S (aq) Ksp = [Ag (aq)] [S (aq)]
+ 2- If x mol of Ag2S dissolve in one litre, then [Ag (aq)] = 2x M and [S (aq)] = x M. Hence:
2 3 –51 –17 Ksp = (2x) (x) = 4x = 8 × 10 so x = 1 × 10
Answer: 1 × 10–17
CHEM1612 2009-N-8 November 2009 Marks • Will AgCl precipitate if solutions of 25.0 mL of 2.0 × 10 –5 M KCl and 75.0 mL –5 2 of 1 × 10 M AgNO 3 are added to one another? Show your reasoning. –10 Ksp for AgCl = 1.8 × 10 at 25 °C.
After mixing the solution has a volume of (25.0 + 75.0) mL = 100.0 mL. Using + - c1V1 = c2V2, this leads to Ag and Cl concentrations of: [Ag +(aq)] = (75.0 / 100.0) × 1 × 10 -5 M = 7.5 × 10 -6 M [Cl -(aq)] = (25.0 / 100.0) × 2.0 × 10 -5 M = 5 × 10 -5 M + - AgCl(s) dissolves to give Ag (aq) + Cl (aq) with the ionic product, Qsp : + - -6 -5 -11 Qsp = [Ag (aq)][Cl (aq)] = (7.5 × 10 ) × (5 × 10 ) = 4 × 10
As Qsp << Ksp , there will be no precipitate.
Answer: No precipitate forms
CHEM1612 2009-N-11 November 2009
• A saturated solution of lithium carbonate in pure water at 20 °C contains 1.33 g of 4 solute per 100.0 mL of solution. Calculate the aqueous solubility product of lithium carbonate at this temperature.
-1 The molar mass of Li2CO3 is (2 × 6.941 (Li) + 12.01 (C) + 3 × 16.00 (O)) g mol = 73.892 g mol-1. A mass of 1.33 g therefore corresponds to: ���� �.�� � number of moles = = = 0.0180 mol ����� ���� ��.��� � ���!�
The reaction table for the dissolution of Li2CO3 is:
+ 2- Li2CO3 2Li (aq) CO3 (aq) Initial 0.0180 0 0 Change -x +2x +x Equilibrium - 0.0360 0.0180
+ 2- These number of moles of Li (aq) and CO3 (aq) in 100.0 mL. In a litre, the + 2- concentrations are therefore [Li (aq)] = 0.360 M and [CO3 (aq)] = 0.180 M. The solubility product is therefore: + 2 2- 2 Ksp = [Li (aq)] [CO3 (aq)] = (0.360) (0.180) = 0.0233
Ksp = 0.0233
When the temperature of the same solution is raised to 40 °C, the solubility is reduced to 1.17 g per 100.0 mL of solution. What conclusions can be drawn about the sign of the standard enthalpy of dissolution of lithium carbonate?
Increasing the temperature leads to less dissolution: the equilibrium has shifted towards reactants (to the left). According to Le Chatelier’s principle, this is consistent with an exothermic reaction: ΔH < 0.
CHEM1612 2008-N-10 November 2008
• A standard test for the presence of chloride ion in water involves the appearance of a 3 precipitate of AgCl upon addition of 0.05 mL of AgNO 3 (0.03 M) to 100 mL of sample. What is the minimum concentration of Cl – detectable by this method? –10 The Ksp of AgCl = 1.8 × 10 .
+ The number of moles of Ag (aq) in 0.05 mL of a 0.03 M solution of AgNO 3(aq) is:
number of moles = concentration × volume = (0.03 mol L -1) × (0.05 × 10 -3 L) = 1.5 × 10 -6 mol
When this is added to 100. mL of the sample:
[Ag +(aq)] = number of moles / volume = (`1.5 × 10 -6 mol) / (0.100 L) = 1.5 × 10 -5 M
+ - For AgCl(s), Ksp = [Ag (aq)][Cl (aq)] and so:
- + -10 -5 -5 [Cl (aq)] = Ksp / [Ag (aq)] = 1.8 × 10 / 1.5 × 10 = 1.2 × 10
Answer: 1.2 × 10 -5
CHEM1612 2007-N-8 November 2007
• –21 3 Marks What is the molar solubility of Cu(OH) 2 at 25 °C given its Ksp = 4.5 × 10 M ? 2
The solubility expression and product for Cu(OH) 2(s) are:
2+ - 2+ - 2 Cu(OH) 2 Cu (aq) + 2OH (aq) K sp = [Cu (aq)][OH (aq)]
If the molar solubility is S, [Cu 2+ (aq)] = S and [OH -(aq)] = 2S. Hence,
2 3 -21 Ksp = (S) × (2S) = 4S = 4.5 × 10
S = 1.0 × 10 -7 M
Answer: 1.0 × 10 -7 M
CHEM1612 2006-N-9 November 2006
• –3 ° Marks The molar solubility of lead(II) fluoride, PbF 2, is found to be 2.6 × 10 M at 25 C. 2 Calculate the value of Ksp for this compound at this temperature.
The solubility equilibrium and constant for PbF 2(s) are,
2+ - 2+ - 2 PbF 2(s) Pb (aq) + 2F (aq) Ksp = [Pb (aq)][F (aq)]
As one moles of Pb 2+ (aq) and two moles of F -(aq) are produced for every mole of 2+ -3 - -3 PbF 2(s) which dissolves, [Pb (aq)] = 2.6 × 10 M and [F (aq)] = (2 × 2.6 × 10 ) = 5.2 × 10 -3 M. Hence,
-3 -3 2 -8 Ksp = (2.6 × 10 ) × (5.2 × 10 ) = 7.0 × 10
-8 Ksp = 7.0 × 10
CHEM1612 2005-N-6 November 2005 Marks The active ingredient in aspirin is the monoprotic acid, acetylsalicylic acid (HC9H7O4) –4 3 that has a Ka of 3.3 10 M at 25 C. What is the pH of a solution obtained when a tablet containing 200 mg of acetylsalicylic acid is dissolved in 125 mL of water?
The molar mass is (8×1.008 (H)) + (9×12.01) + (4×16.00 (O)) g mol-1 = 180.154 g mol1 so 200 mg contains:
3 mass (200 10 g) 3 n(HC9H7O4) = 1.11 10 mol molarmass (180.154gmol1 )
When this amount is dissolved in 125 mL, the concentration is:
3 numberof moles (1.11 10 mol) 3 n[HC9H7O4] = 8.88 10 M volume (125 1000L)
The reaction table is:
+ – HC9H7O4 H2O H3O C9H7O4 initial 8.88×10-3 large 0 0 change -x negligible +x +x final 8.88×10-3– x large x x
The equilibrium constant Ka is given by:
2 [H3 O (aq)][C 9 H 7 O 4 (aq)] x -4 Ka 3 = 3.3 × 10 [HC9 H 7 O 4 (aq)] (8.88 10x )
Ka is not sufficiently small in comparison to the initial concentration of acid that any approximation to this equation can be made. Hence, the quadratic expression must be solved:
x2 = (3.3×10-4 × 8.88×10-3) –(3.3×10-4)x
x2 + (3.3×10-4)x –(2.9×10-6)
With a = 1, b = +3.3×10-4 and c = -2.9×10-6, the roots are:
-b ± b2 - 4 ac -(3.3 10-4 ) ± (3.3 10 -4 ) 2 - (4 1 2.9 10 5 ) x = 2a (2 1)
Only the positive root has physical significance so x = 1.55 × 10-3
+ -3 + -3 [H3O (aq)] = x = 1.55×10 M, pH = -log10([H3O (aq)]) = -log10(1.55 × 10 ) = 2.81
Answer: pH = 2.81
ANSWER CONTIUNES ON THE NEXT PAGE
CHEM1612 2005-N-6 November 2005 2 A standard test for the presence of chloride ion in water involves the appearance of a precipitate of AgCl upon addition of 1 mL of AgNO3 (0.03 M) to 100 mL of the water sample. What is the minimum concentration of Cl– detectable by this method? –10 2 Ksp (AgCl) = 1.8 10 M .
+ The number of moles of Ag (aq) in 1 mL of 0.03 M AgNO3 is:
n(Ag+(aq)) = concentration × volume = (0.03 M) × (1 × 10-3 L) = 3 × 10-5 mol
In the test, this amount is present in (100 + 1) = 101 mL so its concentration is:
numberof moles (3 105 mol) [Ag+(aq)] = 3 104 M volume (101 103 L)
The solubility equilibrium and product for AgCl are given by:
AgCl(s) Ag+(aq) + Cl-(aq)
+ - -10 Ksp = [Ag (aq)][Cl (aq)] = 1.8 × 10
Hence,
K (1.8 1010 ) [Cl-(aq)] = sp 6 107 M [Ag (aq)] (3 104 )
If [Cl-(aq)] is less than this value, AgCl will not precipitate.
Answer: [Cl-(aq)] = 6 107 M
CHEM1612 2004-N-6 November 2004
–1 Marks • Uric acid, C5H5N4O3, is a weak diprotic acid with a low solubility of 70 mg L . The 7 extremely painful inflammation known as gout occurs when crystals of uric acid are deposited in the joints. Given that the pH of a saturated solution of uric acid is 4.58, calculate the pKa1 of uric acid at 25 °C?
+ + 4.58 -5 As pH = -log10([H3O (aq)]) 4.58, [H3O (aq)] = 10 = 2.63 × 10 M.
The molar mass of uric acid is:
((5×12.01 (C)) + (5×1.008 (H)) + (4×14.01 (N)) + (3×16.00 (O)) g mol-1 = 169.13 g mol-1
A one litre solution contains 70 mg corresponding to
(70× 10−3 g) number of moles = =4.1× 10−4 mol . (169.13gmol−1 ) For this weak acid, the reaction table is:
+ – C5H5N4O3 H2O H3O C5H4N4O3 initial 4.1×10-4 large 0 0 final (4.1×10-4) – (2.63 × 10-5) large 2.63 × 10-5 2.63 × 10-5
The equilibrium constant Ka1 is given by: [H O+ (aq)][C H N O− (aq)] (2.63××× 10−−55 ) (2.63 10 ) K 35443 = 1.8 × 10-6 a1 ==−4 [C5543 H N O (aq)] (3.9× 10 )
-6 Hence, pKa1 = -log10Ka1 = -log10(1.8 × 10 ) = 5.7
Answer: pK = 5.7 a1
The monosodium salt of uric acid is slightly more soluble, 8 × 10–4 g mL–1. Calculate the solubility product constant, Ksp, of sodium urate at 25 °C. Assume no hydrolysis of the urate ion occurs.
The formula mass of the monosodium salt, NaC5H4N4O3 is 22.99 (Na) + (5×12.01 (C)) + (4×1.008 (H)) + (4×14.01 (N)) + (3×16.00 (O)) = 191.112. The molar solubility is:
solubility (8× 10−−41 gmL ) molar solubility = = formulamass (191.112g mol−1 ) = =410molmL×−−−613= 410M×
+ – -3 -3 -5 Hence, Ksp = [Na (aq)][C5H4N4O3 (aq)] = (4 × 10 ) × (4 × 10 ) = 2 × 10
-5 Answer: Ksp = 2 × 10
ANSWER CONTIUNES ON THE NEXT PAGE CHEM1612 2004-N-6 November 2004
Suggest a possible reason why the pH of blood plasma remains near 7.4 even when saturated with uric acid.
2– – Blood is buffered by a CO3 / HCO3 buffering system which resists changes in pH.