MT2 - questions released

[Zellmer, Robert]

The questions for MT2 have been released.    The following questions seemed to pose
the most problems for most people in all 1220 sections.

A 1.00 L flask is filled with 1.84 mol HF(g) and sealed.
Determine the concentration (in M) of HF(g) when the reaction reaches equilibrium.
H2 (g) + F2 (g) ⇌ 2 HF(g) K = 111
Round your answer to TWO decimal places (i.e. 2.37) and do NOT include any units.

I did a problem like this in lecture, where I had A2 + B2 <=>  2 AB and it was
started with product and went in the reverse direction.  End-of-chapter
exercises 15.56 is like this question.


186.0 mL of 0.012 M Ca(OH)2 is combined with 290.0 mL of 0.035 M HCl. What is the
pH of the combined solution?
Round your answer to TWO decimal places (i.e. 2.37) and do NOT include any units.

The volumes for the base and acid varied.

This is a neutralization problem between a strong base and strong acid.  This is
similar to problems from 1210 and EOCE 16.100.  You could have set up a BCA
table (ICC table as I call them) to figure out which was in excess (the HCl) and the
moles of excess substance (HCl) and then its conc in the final total volume and
the pH.

You could have also treated it as a titration and you would find there’s excess HCl.
Then you could take the volume of excess HCl and it’s original conc. and done a
dilution problem to figure out the conc. of HCl in the final volume and then the pH.

In any case, I think the major mistake most people made was not accounting for
the fact the are 2 OH- in one Ca(OH)2.  The conc. of OH- is twice the conc of the
Ca(OH)2, 0.024 M in OH-.


Sulfuric acid, H2SO4 , is a strong acid for loss of the first proton and a weak acid
(K = 1.2 × 10^-2 ) for loss of the second proton (HSO4^-). In a solution of sulfuric
acid, which of following correctly arranges the concentrations of H2SO4 , HSO4^- ,
SO4^2- , and H3O+ in order from highest concentration to lowest concentration?

When I saw this question I knew it would be challenging, but not impossible.

H2SO4 is a strong acid and thus completely ionizes.

H2SO4   +   H2O   --->    H3O^+   +   HSO4^-

So, there’s essentially no H2SO4 left in solution.

The HSO4^- is a weak acid and thus doesn’t completely ionize.

HSO4^-   +   H2O   <=>    H3O^+   +   SO4^2-

Even though it is a relatively strong weak acid (Ka = 1.2 x 10^-2), it mostly
remains as HSO4^- with a small amount of SO4^2- (less than the equil.
conc of HSO4^-).  It also produces some more H3O^+ which adds to the
H3O^+ from the first ionization step.

Thus, the conc should be in the order,

[H3O+] > [HSO4^-] > [SO4^2-] > [H2SO4]


Phosphoric acid is a triprotic acid with the following acid dissociation constants:
Ka1 = 7.5 × 10^-3 , Ka2 = 6.2 × 10^-8 , Ka3 = 4.2 × 10^-13 .
What is the base dissociation constant for H2PO4^- ?

H2PO4^- is amphoteric (amphiprotic), it can act as an acid or a base.

Acting as an acid,

H2PO4^-   <=>    H+   +   HPO4^2-   Ka2 = 6.2 × 10^-8

Acting as a base,

H2PO4^-   +   H2O   <=>    H3PO4   +   OH-

To get the Kb for H2PO4^- acting as a base you need the Ka for its conjugate
acid, H3PO4,

            Kw            1.0 x 10^-14
Kb = --------   =   -------------------  = 1.3 x 10^-12
           Ka1             7.5 × 10^-3

All the above scored less than 40% across all the 1220 sections.


The following scored around 54%.

What is the pH after 40.0 mL of 0.200 M HCl has been added to 200.00 mL of
a buffer solution consisting of 0.20 M CH3NH2 and 0.15 M of (CH3NH3)Cl?
(Kb of CH3NH2 = 4.4 × 10^-4 ).

I did a problem similar to this for an NH3/NH4Cl buffer system adding NaOH
in lecture.  There was a similar question on Quiz 7 for adding HCl to an
acidic buffer system of HX/X- given the Ka for HX.


Dr. Zellmer



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