Question

PROCEDURE PART I: DILUTING THE VINEGAR SOLUTION The vinegar solution must be diluted by a factor of 5 to be suitable for titration. 1. Obtain - 20 ml of the stock vinegar solution from the fume hood. 2. Using the 10-ml. pipet, pipet" 10 mL of the stock solution to a 50-ml volumetric flask. 3. Fill the volumetric flask to the calibration line with distilled water. Be sure not to go over this line or you will have to start over. 4. Cap the volumetric flask and invert at least 10 times to mix the solution thoroughly. "Review the procedure on how to properly use a pipet in the introduction section of this manual PART II: PREPARING THE VINEGAR SAMPLES 1. Pour some of the diluted vinegar into a clean and dry beaker. Do not put the pipet into the entire amount of sample in case something goes wrong and you contaminate it. 2. Rinse a clean pipet with distilled water. Then rinse the pipet with some of the vinegar solution. Do this by drawing up some vinegar into the pipet...only filling about one-third of the pipet. Rotate the pipet horizontally, much like you do in rinsing a buret, to coat the inside surfaces with the solution. Let this solution drain into the sink. Rinse at least once more. 3. Pipet 10.00 mL samples of the vinegar solution into three clean, dry 125 or 250 ml Erlenmeyer flasks. 4. Add about 40 mL of distilled water to the diluted vinegar sample in each flask. This volume does NOT have to be exact since it is not used in any of the calculations. This distilled water does not affect your results since it is added to the vinegar AFTER the vinegar is measured. The amount of acid present in the flask is determined by how much of the original vinegar solution was placed in the flask, not by how much water is later added to it. 5. Add 3 drops of phenolphthalein to the solution in the flask. PART III: PREPARING THE BURET 1. Clean and fill the buret in the same way that you did for the previous experiment. The buret is to be filled with the NaOH solution whose concentration you determined in the previous experiment. 3. Clamp the buret in place and wipe off the outside of it. Make sure that there isn't a drop of NaOH still clinging to the tip of the buret. EXPERIMENT 11 161

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Answer 1

1) When the amount of vinegar is measured, we can calculate the strength using the data. No matter what amount of water we add , it won't change the amount of base needed to standardise the sample solution taken for titration. The base reacts in one to one molar ratio which means it deals with the no of moles present in the solution. The amount of water added won't change the amount of mole present.

2 ) We use pipette to measure a fixed amount of sample solution to be titrated. If it already contains some amount of water, the the actual amount of sample would be less and the concentration will be lower than usual.

3 ) The concentration of acetic acid in vinegar is too high . If the volumetric flask contains a little amount of water , then it won't change the concentration too diversely. Though, the actual concentration will be lower as the mark of volumetric flask will reach early if it contains some amount of water.

4 ) Let , the concentration of the solution is X moles per liter.

The molar mass of acetic acid is 60 g per mole. Density is 1.0052 g/mL.

So, using the formula ( grams of sample present/ grams of solution) * 100 = % (m/m)

(x * 60*100 )/(1.0052*1000) = 1.55

x = ( 1.55 * 1000 * 1.0052)/ 6000 = 0.25 M

The concentration is 0.25 M