CH 339K Problem Set 1

 

1)      Dihydroxyacetone phosphate (DHAP)and  Glyceraldehyde-3-Phosphate (G3P) are both intermediates in the metabolic breakdown of glucose.  They can be interconverted by the enzyme Triose Phosphate Isomerase.

 

DHAP

 

G3P

 

For the conversion of DHAP to G3P, DGo = 7.5 kJ/mol

 

            a)         Calculate Keq and the equilibrium fraction of G3P at 37o C.

 

           

 

b)         What is DG if [G3P] = 1/100 [DHAP]?

 

                       

 

 

2)      A Cl- ion is separated from a Na+ ion by a distance of 0.5 nm.  What is the interaction energy (in Joules/mole) in

a.      Water (ε = 78.54)

 

b.      n-Pentane (ε = 1.84)

 

The Point: electrostatic interactions are stronger in environments with lower dielectric constants (like, say, organic solvents or the hydrophobic interiors of macromolecules.)

 

 

3)      What is the pH of each of the following unbuffered solutions?

a.      0.35 M HCl

 

HCl is a strong acid, so it dissociates completely to (formally speaking) H+ and Cl-.

pH = -log[H+] = -log(0.35) = 0.456

 

b.      0.35 M Acetic Acid

 

HAc is a weak acid: we can solve using the

quadratic equation:

or the approximation:

 

c.       0.035 M Acetic Acid

 

To heck with the quadratic equation:


 

4)      What is the pH of the following buffer mixtures?

d.      1 M Acetic Acid and 0.5 M Sodium Acetate?

 

Conjugate acid is HAc; conjugate base is Ac-; Na+ is spectator.

 

 

e.       0.3 M H3PO4 and 0.8 M KH2PO4?

 

Conjugate acid is H3PO4; conjugate base is H2PO4-; K+ is spectator.

 

 

5)      You want a buffer of pH 7, made using KH2PO4 and Na2HPO4:

f.       What is the conjugate acid in this system?  What is theconjugate base?

 

Conjugate acid is H2PO4-; conjugate base is HPO4-2

 

g.      If the buffer is 0.1 M KH2PO4, what must the concentration of Na2HPO4 be?

                 

 

h.      You want the same buffer, but with a total phosphate concentration of 0.3 M.  What concentrations of each species do you need?

 

 

6)      You have a beaker containing 500 ml of 0.10 M potassium formate buffer pH 3.75.  What is the pH if you add 5 ml of 1.0 M KOH?  (Hint – the change in volume can be ignored)

 

The conjugate acid is formic (HCOOH, pKa = 3.75); the conjugate base is formate ion (HCOO-).  Adding the strong base, KOH, will convert formic acid to formate, changing the base/acid ratio and changing the pH.

 

First, how much of each species do we have in our starting solution?

 

Now, when we add 5 ml of 1M KOH, we are adding strong base.  This will convert 0.01M HCOOH to 0.01M HCOO-.  The new pH then becomes:

 

 

 

7)      On my benchtop, I have a jar of the amino acid glycine, a solid powder with MW = 75.07 g/mol.  I also have a bottle of 1 M NaOH, all the distilled, deionized water I can use, and a 2 liter graduated cylinder.  Tell me how to make 2.0 liters of 0.1 M glycine buffer, pH 9.

 

The conjugate acid is glycine (H3N+CH2COO-), the conjugate base is gly- (H2NCH2COO-).

How much glycine do I need?

Now, how much strong base do I need to add to convert the correct amount of glycine to gly- to give the correct conjugate base/conjugate acid ratio to generate a pH of 9:

In order to convert 0.02M of the free acid (glycine) to the conjugate base (gly-), I have to add 0.02M strong base (KOH)

 

So I need to mix 15.14g glycine, 40 ml of 1M KOH, and add water to 2 liters total volume.

 

Useful Physical Constants

 

Constant

Value

Avagadro’s Number

6.02 * 1023

Boltzmann’s Constant

1.3807 * 10-23 JK-1

Charge on electron

-1.602 x 10-19 coulomb

Gas Constant (R)

8.314 JK-1mol-1

Faraday’s Constant

96,485 JV-1mol-1

k = 1/(4pe)

8.99 x 109 Nm2coulomb-2

ln(x)

2.303 log (x)

Dielectric constant of water

78.54