Recording Significant Figures

Transcription

1 Recording Significant Figures Having the correct significant figures in your lab reports begins with recording the correct significant figures from the data you obtain in lab. This packet will demonstrate how to correctly read scales and glassware used in lab and evaluate the accuracy of each piece of equipment. Dimitrios Giarikos, Ph.D.; and Natasha Antonovich, Laboratory Assistant Nova Southeastern University Farquhar College of Arts and Sciences Math, Science, and Technology Division General Chemistry Labs Recording Data from Scales After obtaining the mass of the object on the scale, you will have to note the error of the scale to know how many decimal places are significant. This can be found on the side or the back of the scale. 1

2 Recording Data from Scales In this example, the scale display shows that the object weighs g. Note that the scale is only accurate to g. This means that the ten thousandths place is not accurate on this scale, and therefore, not significant. Thus, the correct way to record this mass would be g Recording Data from Scales What is the correct way to record this mass? 2

3 Recording Data from Scales Notice that the error for this scale is different (0.001g). The correct significant figures for this scale is 50.56g. Recording Data from Glassware The first step in recording data from glassware is to make the initial reading of the volume. This is done by reading the number at the tip of the meniscus. Then the error of the glassware must be noted to understand how accurate the reading is. Note that the meniscus can be either concave (ex: water) or convex (ex: mercury). Either way, the volume is read at the tip of the curve. The volume would be read at this line. 3

4 5mL Graduated Cylinder Error bar Since the graduated cylinder has an error bar that goes to hundredths of a milliliter, that is how many decimals are significant. By reading the meniscus, the volume is 2.29mL. The error bar shows that the measurement has a standard deviation of 0.10mL. Thus, understand that the actual volume could range anywhere from 2.19mL to 2.39mL. 10mL Graduated Cylinder What is the correct way to record this volume? 4

5 10mL Graduated Cylinder Notice that the gradations are different here. They go by two tenths of a milliliter. The error bar is also different (+/- 0.20mL). Thus, the correct measurement is 2.35mL with the understanding that the volume could actually range from 2.15mL to 2.55mL. 25mL Graduated Cylinder Notice that here each gradation is 2.5mL and the error bar is +/- 0.34mL. The meniscus is right on the 10mL mark, thus the correct measurement is 10.00mL, knowing that the actual volume could range between 9.66mL 10.34mL. 5

6 Comparing Graduated Cylinders 50mL graduated cylinder 100mL graduated cylinder *Both cylinders read 20.0mL at the meniscus* The error bar is +/- 0.5mL. Thus the actual volume could range from 19.5mL to 20.5mL. The error bar is +/- 1.0mL. Thus the actual volume could range from 19.0mL to 21.0mL. Comparing Graduated Cylinders Given the previous example, it becomes apparent that seemingly identical measurements can really differ depending on the equipment used. To make your data more accurate, you should use the instrument with the smallest error bar. This is usually the smallest instrument that can still measure the volume desired. 6

7 Thermometer Thermometers are read like graduated cylinders. This thermometer has gradations every 0.1 o C and no error bar. Thus, the correct reading for this temperature is 40.0 o C. Pipettes and Burettes Graduated pipettes and burettes are read the same way graduated cylinders are. The number read, however, means something different with these instruments. When liquid is filled in these instruments to the 0mL mark, the number read represents the volume that has been let out of the instrument not the amount that remains. Giarikos 6/7/2009 7

8 25mL Graduated Pipette error Here, the error bar goes to the hundredths place, thus that is how many significant figures there will be. Notice the gradations are by 0.1mL. So, by reading the meniscus, the volume dispensed was 18.18mL. Given the error bar of +/- 0.10mL, understand that the actual volume dispensed could range between 18.08mL and 18.28mL. 2mL Graduated Pipette Some pipettes do not have error bars. In these cases, the next decimal place after that of the gradations may be estimated only if it can be clearly seen. In this example the pipette has gradations every 0.1mL and no error bar. Thus, the correct way to record the dispensed volume would be 0.82mL. 8

9 10mL Graduated Pipette What is the correct way to record the dispensed volume from this pipette? 10mL Graduated Pipette Since the gradations go by tenths of a milliliter and there is no error bar, the volume can be estimated to the hundredths. The correct dispensed volume would be 4.27mL. 9

10 50mL Burette Burettes are read the same way as pipettes. Burettes do not have error bars, however, notice how close the gradations are. This makes it difficult to estimate a further decimal place. Thus, the volume will only be recorded to the tenths place. By reading the meniscus, the volume dispensed is 36.4mL Volumetric Flasks Volumetric flasks are different from the previous glassware because they are only capable of measuring one specific volume. The liquid is filled to the one and only gradation mark on the flask. Nevertheless, it is important to know that there is still error in the measurement. 10

11 50mL Volumetric Flask The only volume this flask can measure is 50mL. Thus the correct way to record this volume would be 50.00mL knowing that the actual volume could range between 49.92mL and 50.08mL. 11