Exp. 1: Laboratory Measurements

Although there are popular myths to the contrary, the secrets of nature do not simply pop into the heads of gifted people. Those secrets are uncovered laboriously by people whose minds are prepared to receive them, and only after careful and critical examination of their own observations of nature or of the properly recorded observations of others. So there you have it--good science begins with a careful examination of nature by trained, discriminating observers. Sloppy observations are about as likely to help us to understand the truths of nature as wild shots in the air are to hit a desired target. Even though observations may be made with utmost care, our memory of the details is very short; hence, observations must be recorded carefully as they are made and as soon as possible.
Since this is not an introductory course in chemistry, it is unlikely that you are totally lacking in observational experience. What is emphasized in this exercise, and the sequence of experiments you will encounter in this course, is the need for careful observations, and the immediate recording of those observations in a legible fashion.
The physical sciences like chemistry and physics are fortunate in that the properties or events that we wish to observe are measurable; that is, a numerical value may be obtained through the use of an instrument. Instruments vary in the ability to measure the property in question and therefore, the quality of the numerical value obtained varies considerable. It is hoped that you will develop a sense of the quality of the measured values obtained from the various instruments you use. In treating the data you collect the two principal indicators of the quality of measurements will be emphasized--precision and accuracy.
Accuracy is nothing more than the comparison of an experimental value with the true or accepted value, and is reported as percent error. The repetition of measurement of a single property in the same way for at least three trials gives us a way of judging how close to each other our repeated values are. This is what is meant by precision.

Procedure:

Part A. Accuracy: Measurement of the accuracy of calibrations of selected laboratory glassware.

1. Accuracy of a 10 mL and 100 mL graduated cylinder and a 50 mL beaker compared to a buret.
Wash, rinse and fill a 50 mL buret with tap water, recording the liquid level to the nearest 0.01 mL.
While keeping your eye on the liquid level in the cylinder, deliver water from the buret into the 10 mL graduate until the level reaches the 10.0 mL mark. Record the liquid level in the buret.
Repeat this procedure using the 100 mL graduate and then the 50 mL beaker, filling to the 10.0 mL mark each time. Record the liquid levels in the buret after each time.
Helpful tips for using a buret
2. Accuracy of a buret compared to the balance.
Fill a 250 mL beaker about 3/4 full with tap water. Measure and record the temperature of the water. This same water will be used in the rest of Part A. Obtain a dry 125 mL Erlenmeyer flask. Wipe the flask clean and then mass on the analytical balance to the nearest 0.0001 g. Fill the buret with water from the beaker then read the liquid level to the nearest 0.01 mL. Deliver water from the buret into the massed flask until approximately 15 mL have been delivered. Record the actual liquid level in the buret to the nearest 0.01 mL.
Re-mass the flask and water to the nearest 0.0001 g.
3. Accuracy of a 25 mL pipet compared to the balance.
Retake the temperature of the water recording the value only if different from before. Fill the pipet to the mark with water from the beaker and then deliver the water into the partially filled Erlenmeyer flask. Let the pipet tip touch the side of the flask and drain naturally until it stops. DO NOT blow or shake out the last drop.
Re-mass the flask and water to the nearest 0.0001 g.
4. Accuracy of a 100 mL graduated cylinder compared to the balance.
Check the temperature of the water again, recording only if changed. Fill a clean 100 mL graduate to the 20.0 mL mark with water from the beaker, and then deliver the water into the partially filled Erlenmeyer flask.
Re-mass the flask and water to the nearest 0.0001 g.

B. Precision: the precision of measurements of volume.

Empty and dry the 50 mL beaker. Then refill the buret, reading the liquid level to the nearest 0.01 mL. Deliver water from the buret into the beaker until the liquid level in the beaker is at the 20 mL line. Record the liquid level in the buret. Empty and dry the beaker, and repeat the procedure until five separate trials have been made.
NOTE: It is essential that you keep your eye on the liquid level in the beaker as it is being filled and NOT on the liquid level in the buret.

Results and Discussion:

Part A
In Part A of the procedure you measured the volume of the water required to fill various vessels to the 10 mL mark, and the mass of 15, 25, and 20 mL of water as dispensed by various volumetric equipment. To determine the accuracy of these measurements, we have to assume that the buret reading in A(1) and balance readings in A(2), (3), and (4) are correct. In other words we are going to say that there are no errors in these readings. They will constitute the accepted values.
Therefore Calculate:

the % error in each of the 10 mL volumes measured in A(1)

the % error in the 15, 25 and 20 mL volumes measured in A(2), (3) and (4)

(the true volumes here must be calculated from the masses of water using the density of water at the temperature recorded for the water during each trial. See a reference book for the density of water at the given temperatures.)
Briefly discuss the choice we made for the "true" values in these calculations. You might want to answer the following questions in your discussion. What factors affect the appropriateness of the "true" values we chose? How could better "true" values for any measurement be obtained? Is there a relationship between accuracy of the measurement and the diameter of the measuring instrument?
Part B
In Part B you carried out the measurement of a single property; a 20 mL volume in a 50 mL beaker, and repeated it four times. The determination of the precision of these five volumes is obtained by first calculating the average value for the volumes, and then finding out how much each value differs or deviates from the set.
Calculate:
The average, the average deviation, and the standard deviation, s, for the five volumes actually measured out in the 50 mL beaker.
Since it is much easier to calculate average deviation than standard deviation, what is the reason standard deviation is used in so much research? Why might chemists be interested merely in the average deviation?