142 Quiz 5 Sp12 Key

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142 Quiz 5 Sp12 Key Powered By Docstoc
					Grossmont College                                  Name: _____________________________
Chemistry 142
Spring 2011                                        Lab Section: ________Date: ____________

                                               Quiz 5

Directions: Answer each of the following questions. Be sure to use complete sentences where
appropriate. For full credit be sure to show all of your work. Where appropriate answers should be
boxed for clarity, written to the correct number of significant figures, and include the proper units.

      1. What indicator is being used in this week’s experiment (1 point)? Starch

      2. The common ion is (1 point)                                                 Ca2+

      3. A certain weak acid, HA, with a Ka = 5.61 x 10-6, is titrated with NaOH (10
         points).
            a. A solution is made by mixing 9.00 mmol of HA with 3.00 mmol of the
                 strong base. What is the resulting pH?

Stoichiometry First
  HA (aq)                       + OH- (aq)                   A- (aq)           + H2O (l)
I 9.00 mmol                       3.00 mmol                  0M                  n/a
C -3.00 mmol                      -3.00 mmol                 +3.00 mmol          n/a
E 6.00 mmol                       0.00 mol                   3.00 mmol           n/a

Equilibrium Second
  HA (aq)                       + H2O (l)          A- (aq)                     + H3O+ (aq)
I 6.00 mmol                       n/a              3.00 mmol                     ~10-7 M
C -x                                               +x                            +x
E




or




               b. More strong base is added until the equivalence point is reached. What is
                  the pH of this solution at the equivalence point if the total volume is 46.0
                  mL?
     A- (aq)                    + H2O (l)          HA (aq)                     + OH- (aq)
I                                                  0M                            10-7 M
C -x                                               +x                              +x


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Grossmont College                                   Name: _____________________________
Chemistry 142
Spring 2011                                         Lab Section: ________Date: ____________

E 0.196 M – x =                                     x = 5.91 x 10-5 M            x = 5.91 x 10-5 M
  0.196 M – 5.91 x 10-5 M
  ≈0.196 M




    4. Lead(II) thiocyanate, Pb(SCN)2, has a Ksp value of 2.00 x 10-5 (8 points).
          a. Calculate the molar solubility of lead(II) thiocyanate in pure water.

    Pb(SCN)2 (s)            Pb2+ (aq)           +    2 SCN- (aq)

I                           0M                       0M
C                           +s                       +2s
E                           s = 0.0171 M             2s = 2(0.0171 M) = 0.0342 M




            b. Calculate the molar solubility of lead(II) thiocyanate in 0.800 M
               potassium thiocyanate.

    Pb(SCN)2 (s)            Pb2+ (aq)                +   2 SCN- (aq)

I                           0M                           0.800 M
C                           +s                           +2s
E                           s = 3.13 x 10-5 M            0.800 M + 2(3.13 x 10-5 M)
                                                          ≈ 0.800 M




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