Question

This is for the lab practical I take on Tuesday night. If you could answer each one and show your work/ explain that'd be amazing so I can do well on the actual practical on Tuesday! Thank you!

Chemistry 211   Lab Final Study Guide   Spring 2015

Useful Information:
1 ampere =coulomb/sec
Faraday’s constant = 96,500 coulombs/mole of electrons
Molar mass of Au = 197.0 g/mole, Molar mass of Cu = 63.55 g/mole
m = moles ,   '   ’

c   kg

ΔTf

= Tf

− Tf

where Tf is for the pure solvent and Tf

is for the mixture.

ΔT = (K

)(m ) ,

m = (

(mass of

solute, g )
)(   ).

f   f   c

c   molar

mass, g

mol −1

mass of

solvent, kg

7.   Next question

(a)   If you weigh 6.355 grams of copper, add concentrated nitric acid to dissolve it, and then dilute the solution to a total volume of 50 mL, how many moles of copper are in this solution?

(b)   For the equilibrium reaction 3H2(g) + N2(g) → 2NH3(g), what is the equilibrium constant expression, Kc, for this reaction? (5pts)

8.   Suppose at some temperature, the equilibrium concentration of H2 is 0.45M, the equilibrium concentration of N2 is 0.38M, and the equilibrium concentration of NH3 is 0.42M. What is the value of the equilibrium constant for the reaction 3H2(g) + N2(g) → 2NH3(g) at this temperature? (Show your work. No credit will be given if your work is not shown.)

9.   Next question
(a)   If you need 0.0534 moles of NaOH to reach equivalent point in the titration with acetic acid, how many mL of 0.09876M NaOH will you need?

(b)   The Ksp of Cu(OH)2 is 2.2x10-20, what is the solubility of Cu(OH)2?

10.   Next question
(a)   In non-sulfide qualitative analysis of representative cations, the Group A cations comprising of Pb2+,

Hg2+, and Ag+ give a precipitate with   solution.
(b)   From a solution containing mixture of Pb2+, Fe3+, and Al3+, Pb2+ is removed by adding HCl solution. To the remaining cation mixture, on making the solution basic with NH3 (pH=9 – 10), both the cations form precipitate. Further analysis of the cation mixture containing Fe3+ and Al3+ is carried out by adding NaOH solution.

Aluminum hydroxide    (dissolves/remains undissolved) in NaOH solution while Ferric hydroxide        ( dissolves/remains undissolved).
(c)   Fe(OH)3 precipitate is dissolved in minimal amount of aqueous HCl which gives a

yellow solution. To this solution addition of    solution (at pH = 4 – 5) yields a dark blue solid known as Prussian Blue.

(d)   Describe the method for checking for completeness of precipitate in qualitative analysis of ions.

Answer 1

7 a) Molarity = (mass of Cu /molar mass of Cu) x (1/ volume in Litres)

= (6.355 g/ 63.5 g/mol ) x (1/0.05 L)

= 2 moles/ Litre

Therefore, 2 moles of Copper present per litre of solution.

b) N₂ (g) + 3H₂ (g) ------------> 2NH₃ (g)

Equilibrium constant, Kc = concentration of products/ concentration of reactants

Equilibrium constant, Kc = [NH3]² / [N₂] [H₂]³

8) Given that N₂ (g) + 3H₂ (g) ------------> 2NH₃ (g)

Equilibrium concentration of H2 is 0.45 M,

Equilibrium concentration of N2 is 0.38 M,

Equilibrium concentration of NH3 is 0.42 M

Hence,

Equilibrium constant, Kc = concentration of products/ concentration of reactants

Kc = [NH3]² / [N₂] [H₂]³

   =  [0.42]² / [0.38] [0.45]³

= 5.09

Therefore, Equilibrium constant, Kc = 5.09

9) b) The Ksp of Cu(OH)2 is 2.2x10-20, what is the solubility of Cu(OH)2?

  Cu(OH)2 -------> Cu2+ + 2OH-

s 2s s = solubility of Cu(OH)2

Ksp = [Cu2+] [OH-]2

2.2x10^-20 = s.(2s)²

2.2x10^-20 = 4s³

Hence,

s = 1.76 x 10^-7 M

Therefore,

solubility of Cu(OH)2 = 1.76 x 10^-7 M