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.

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