Hydrolysis
and Buffers, take 2
Hydrolysis section
In this
part of the experiment you will test several different salt solutions
(in a spot plate) with
pH paper determine their acidity or basicity and then determine which
solution
is which.
The 8
solutions are 0.1 M solutions of copper (II) sulfate, sodium
sulfide,
calcium hydroxide, ammonium carbonate, magnesium bromide, sodium
chloride,
potassium
phosphate, and copper (II) acetate. You will measure the pH of each
solution using pH paper and then knowing what you know about
hydroylsis, you are going to make educated guesses as to which solution
is which. Recall that pure water will have a pH slightly lower than 7
because of carbon dioxide from the air dissolving in the water to form
carbonic acid. Re-read section in
your text about hydrolysis and your lecture notes on hydrolysis to prepare you for the
experiment.
Buffers
A mixture of relatively large amounts of weak acid and
its salt or a weak base and its salt is known as a buffer.
Buffers have the property of maintaining a relatively
constant pH even when considerable acid or base has been
added.
Re-read section of your text on buffers to prepare for this
experiment.
Buffer
'procedure':
1. Make a sodium acetate/ acetic acid
solution in a 100 mL volumetric flask. Use ~3 g of sodium acetate and
~6 mL of 6M acetic acid, dulite to 100 mL (with DI water of course). You will need to know the exact amount of
each for step 2.
2. Calculate the pH of what your buffer
should be (Henderson Hasselbach?) Measure the pH of 50 mL of this
solution. Do they match? Explain the difference if there is one.
3.
To your 50 mL of buffer, add exactly 20 drops
of 6M NaOH, remeasure the pH.
4. Measure the pH of a 50 mL
sample of distilled water. With the pH probe in the DI water, add 20 drops of 6M
NaOH. Is the change in pH bigger than when you added the NaOH to the buffer?
Why or why not (be complete!)
Lab report: (only 15 points total) Very
simple. For the hydroylsis section, give the results of your pH paper
testing, and what you thik everyone is and exaplin why for each. You
would do well by having 8 sentences that start with "I believe solution
XX to be YY because ..."
For
each step number in the buffer section, give nice organized tables of
data (volumes, pH measurements, whatever) and words to tie them
together (it should be clear what you are doing) and answer all the
questions posed.
Oh
and that pesky commentary section, don't forget that. Remember, getting
better results or the lab getting done faster are not valid 'how to
make this a better learning experience' answers.Prelab
Questions: (worth 5 points)
1. Calculate the pH of a 0.87 M NaCN solution. Solve it exactly (no 'assume x=0.1')
HINT:
NaCN
dissociates into Na+ (an unexciting acid) and CN-, the conjugate base
of HCN. You can look up the Ka for HCN, and therefore calculate the Kb
for the BASE, CN-. It is this equation (Kb) that is your equilibrium
expression for this problem.
2. What is initial concentration of a sodium acetate if the pH
(at equilibrium) is measured to be 9.30? HINT: Same sort of
thing
as question 1, sodium acetate dissociates into Na+ and acetate ion (a
base)......
~MEO 22 Mar 06