Question

Questions asked after completing quantitative volumetric analysis of pickling vinegar & NaOH (regular vinegar consists 5% acetic acid, CH3COOH. Piclking vinegar consists 7% of the acid).

1. Is there a trend in the values obtained in class? (there was a trend) Explain.

2. If regular vinegar costs $1.00/L & pickling vinegar costs $1.25/L, which is the best by? (i.e use a logical argument to determine which gives you the most acetic acid for your dollar)

3. Why do we only include the trials that agree withing 1% of each other? What is the difference between accuracy & precision for a series of measurements?

4. Identify 4 important sources of systematic (not human) error. Discuss why they are important.

5. If 0.25M potassium hydroxide solution was used as the titrant, instead of 0.1 M NaOH (aq), would your titration have use dmore or less itrant to reach the end point? Use a logical argument to determine how much more or less titrant would be required.

Answer 1

2. 5% CH3COOH means 5g in 100 g water, so 1L or 1000g of water will contain regular Vinegar = 50 g (Acetic Acid).

Where as 1L or 1000g of water, 7% pickling vinegar = 70 g acetic acid

So, per g price =$ 1/50 for 5% = $0.02 and $1.25/70 for 7% = $0.018

Thus 7% acetic acid or pickling vinegar is cheaper.

3. When we use significant figures then an implicit uncertainty is considered, the last digit is considered uncertain. For example, a result reported as 1.15 implies a minimum uncertainty of ±0.01 or 1% and the value lies in between 1.14 to 1.16.

The accuracy of a set of observations is the variance between the average of the measured values and the true value of the observed quantity. The precision of a set of measurements is a measure of the range of values found, that is, of the reproducibility of the measurements.

Accuracy and precision are related and this is illustrated by an example of firing a rifle at a target where the blue dots below represent hits on the target:

4.

• Incomplete definition (may be systematic or random) - One reason that it is impossible to make exact measurements is that the measurement is not always clearly defined.
• Environmental factors (systematic or random) - Be aware of errors introduced by your immediate working environment. You may need to take account for or protect your experiment from vibrations, drafts, changes in temperature, electronic noise or other effects from nearby apparatus.
• Instrument resolution (random) - All instruments have finite precision that limits the ability to resolve small measurement differences. For instance, a meter stick cannot distinguish distances to a precision much better than about half of its smallest scale division (0.5 mm in this case).
• Improper calibration – it is important to have calibrated system as per the recommendation of the manufacturer and proper calibration can give accurate results.

5. From acidimetry and alkalimetry and law of equivalence, we know that

V1S1 = V2S2

In this case V2S2 is fixed and let us consider S1=0.1M NaOH,

Then considering, V1, volume of NaOH = V2S2/0.1 mL

If we consider S3 = 0.25 KOH and its volume = V3

Then V3 = V2S2/0.25

Thus the ratio V3/V1=0.1/0.25 =0.4 or V3 = 0.4xV1

The volume of the titrant of 0.25 M KOH will 0.4 times of the voulme of 0.1M NaOH solution.

1. Information not availabale.