Exam 2 - 87 points total

2003

MULTIPLE CHOICE QUESTIONS: (3 pts each)

1.  A "p"-function :

            a) is the negative natural logarithm of another function

            b) equals [- log(the same function)]

            c)  raises a function to some power of ten

            d) is none of the above

2.  The Kb of ammonia is 1.8 x 10-5.  Its conjugate acid

            a) has a pKa  > 5

            b) is NH3+

            c) is a moderately strong acid

            d) all of  above

3. The pH of a solution of sodium acetate will be:

            a) the same as pure water.

            b) > 7

            c) < 7

            d) .... cannot determine without additional information.

4.  Chloride ion has no effect on solution pH because:

           

            a) it is small

            b) it is such a weak acid

            c) it is the conjugate base of a strong acid

            d) it has only a 1- charge

5. Will the equilibrium of the acid/base reaction below go primarily to the right or to the left?  (pKa (HF)  =  3.17;  pKa (HClO) = 7.53) The left side of reaction will be favored. (3 pts)

                     F-  + HClO <==  ClO-  +  HF

6.  Give the chemical formulas for the chemicals named below and circle whether an aqueous solution of the substance is acidic, neutral, or basic.  (3 points each)

            carbon dioxide - CO2 -           acidic     neutral     basic

            sodium nitrate  - NaNO3 -      acidic     neutral     basic

            potassium acetate – KOOC2H3 -        acidic     neutral     basic

            ammonium chloride – NH4Cl -           acidic     neutral     basic

Part A: Discussion QUESTIONS (10 pts each)

1.  Your grandmother loves to drink tea!  But after 60 years, her teapot is completely coated on the inside with a white scum.  You, the excellent chemistry student, can help!   Explain to her that the scum is called scale, what the scale is and how by adding vinegar the scale will be removed from her teapot.

(With gratitude, she will make you a nice chocolate cake to eat with some tea.)

Scale is CaCO3. Vinegar is acetic acid. Dissolving the scale to clean the teapot is essentially the problem of shifting the equilibrium CaCO3 <==> Ca(2+) + CO3 2- to the right. Acetic acid does this by the pH effect, by an acid/base neutralization of carbonate that removes it from the product side and causes more scale to continue dissolving.

 

2. Your mother doesn't believe it: "How could hyper-ventilating* change the pH of my blood?"

You explain it to her; she's your mother.

**hyper-ventilation: excessively rapid and deep breathing, resulting in the decrease of carbon dioxide in the blood.

 The importance of CO2 is that it is the basis of the buffer system in the blood, and this buffer system is required to keep a steady pH. Since hyperventilation decreases the amount of the CO2 products in the blood, H2CO3 and HCO3 -, addition of acid or base into the bloodstream may not be neutralized and teh pH can change. Note also , the H2CO3 formed from CO2 + H2O <==> H2CO3 contributes acidity to the pH and this will be altered with less CO2 dissolved in the blood.

3.     The Ksp for Fe(OH)3 (s) is 10-39.  What is the relationship between this Ksp and the problem of iron bioavailability, the problem where plants and bacteria need to obtain sufficient Fe3+ to grow and live? How does the ever-creative Mother Nature solve the problem?

The problem is that, while there is abundant iron in the earth ( more precisely , in the earth’s crust), the very small value of Ksp means that very little iron(3+) will be soluble, i.e., the equilibrium, Fe(OH)3 <==> Fe(3+) + 3 OH- , lies far to the left.

Mother Nature uses two strategies for increasing the solubilty of iron: 1) the ph effect, where plants produces acidic molecules and increase the acidity ( lower the pH) around their roots and 2) produces large molecules ( called siderophores) to bind strongly ( having large Kf) to iron(3+).

Both of these shift the equilibirium by LeChatelier's principle by reacting and removing one of the "products" of the ksp equilibrium, eithr Fe(3+) oor the OH-.

4. You're helping a classmate finish her lab report and all she has to do is label the titration plot.

But she cannot remember if the titration was sodium phosphate to which was added hydrochloric acid or  phosphoric acid to which was added sodium hydroxide. Good thing she has you for a friend! Help her out then you can both go sit in the sun.

a)     Label each aspect of the titration plot (found at end of exam) that helps to identify which titration reaction was done. 

b) Lindsay has heard of your extensive knowledge of the phosphate system!  

You can help her make a buffer suitable for her experiments on DNA.  DNA needs a buffer that holds pH at 7.0 in order for it to be stable.  Which phosphate species can be combined to make a buffer at pH 7? Give the molecular formulas of the sodium salts that Lindsay should use to make her buffer.

a) The titration was NaOh added to phosphoric acid. You can determine this because the pH begins acidic and because there are two equivalence points, meaning the acid si at least dibasic.

The species in the first buffer region are H3PO4/ H2PO4-.

The species in the second buffer region are H2PO4 - / HPO4 2-.

The species in the third buffer region are HPO4 2-/ PO4 3-.

b)     At pH 7 , the two species are H2PO4 - and HPO4 2-, so the sodium salts are NaH2PO4 and  Na2HPO4.

Part B

PROBLEMS: (10 pts each)

1. The pale green precipitate you may find around the faucets in your bathroom  is Cr(OH)3.  

Can a speck (a speck =1.0 mg or 0.001 g) of Cr(OH)3 be completely dissolved in 100 mL water? 

Ksp for Cr(OH)3 is 6 x 10-31

click here for answer


 2. Remember that wine bottle cork problem?  Those expensive wines having $1.00 corks to stopper the wine bottles? How oxygen from the air reacts with ethanol in wine to form acetic acid?

           

oops! I forgot to replace the cork in the wine bottle three nights ago!  Since I am a chemist, I decide to check on how much acetic acid formed the past three days by measuring the pH of the wine.  If the wine pH is 4.23, what is the concentration of acetic acid in the wine? (You may assume that acetic acid is the only contributor to acidity)

For acetic acid, Ka = 1.8 x 10-5

click here for answer

3. You have just purchased (for $20,000) a marble stature of an angel to grace your garden. Unfortunately, you live in an area of very acid rain.  Rain falling on your garden (and your angel statue) typically has a pH of 4.5.

a) Remembering that marble is calcium carbonate, write the reaction(s) involved as acid rain dissolves marble.

b) If your angel weighs half a ton, how many liters of rain at pH 4.5 will dissolve all of it? (one ton = 907 kg)

click here for answer

4.  4.00 g sodium phosphate, Na3(PO4), is dissolved into 200 mL of water.  To this solution is added 50 mL of a 0.300 M HCl solution.  What it the pH of the final solution?

[For phosphoric acid, pKa1 = 2.12;  pKa2 =  7.21; pKa3  =  12.32]

click here for answer (note: disregard the "15 points" on this linked page. This problem like the rest was 10 points)

5.   Mother Nature is so clever!  A buffer was needed the maintain constant pH for her mammalian life-forms and she made use of one of the most abundant gases that blankets the earth: CO2.

a.  Write the equilibria reaction equations that are involved in converting the gas to a useful buffer for the plasma of mammals.

b. Identify the species that make up the buffer system in mammalian blood.

c. If normal blood pH is 7.43, what is the ratio of buffer species, A- / HA, ?

d. If the blood pH falls to the dangerous (possibly fatal) level of pH 7.0, what is the ratio of buffer species, A- / HA, ?

e.  If there are 5 L of blood in a mammal and at pH 7.43 the concentration of buffer component HA = 3.72 x 10-6 M, how much acid or base was added to the blood to make the pH fall from 7.43 to 7.0?

For HA: Ka1 = 4.2 x 10-7.   Note that Ka2 is not needed in this problem.  

click here for answer (note: disregard the "15 points" on this linked page. This problem like the rest was 10 points)