Question

based on instructions please help me fill out chart

Use appropriate safety precautions for working with concentrated acids and bases. This includes splash-proof goggles, long pants, and closed-toe shoes. No eating, drinking, or chewing at any time in the laboratory. Part I: Comparing the Behavior of the Buffer with Distilled Water 1. Obtain a pH meter or probe. Make sure beakers are clean and dry. 2. Place 25 mL of distilled, deionized water in a clean dry 100-ml beaker. 3. Measure the pH of the DI water and record the pH. 4. Using a disposable pipet add one (1) drop (-0.05 mL) of 0.50 M HCl to the beaker. Record the change in pH. 5. Discard the solution down the sink with lots of water and place a fresh 25 mL of distilled, deionized water in a clean dry 100-ml beaker. Buffers 151

Answer 1

This is a experiment where you measure the buffer capacity by adding drops of a strong acid (HCl) and strong base (NaOH).

To fill the part I, you have to measure the pH with a pH-meter of the distilled water, this should be around 7 (neutral) but could be displaced by impurities in the water. Then you have to add one drop of 0.50 M HCl and measure the pH again, you'll see the pH decreases due to the acid. The opposite occurs when you add NaOH, after that you'll see the pH increases due to the base.

To fill the part II, you need your assigned pH to calculate the mass of NaOAc·3H₂O and the volume of HOAc you have to use to prepare the buffer.

With the Henderson-Hasselbalch equation to do this calculation:

Base pH = pK, +log [Acid

The acid of an acetate buffer is HOAc and its conjugated base is NaOAc:

NaO AC pH = pK+log [H04c

The data here is:

pKa = 4.76 (acetate buffer)

pH -> is your assigned pH (must be between pKa $\pm$ 1 since this is the range in which buffer works, so must be 3.76 - 5.76)

The unknowns in this equation are: [NaOAc] and [HOAc]. Hence, you need another information. Could be the concentration of one of them or the total concentration of the buffer. For the last one you have this equation:

this way you can obtain one of the concentration and substitute in the Henderson-Hasselbalch equation to find the other that you missing.

Once you have both concentration you can calculate the mass with the molar mass (given on your sheet) of NaOAc and the volume of HOAc you need to dilute to obtain your required concentration.

I hope this explanation can help you.