In this lab you are asked to make a series of volume and temperature measurements and record the number of significant figures in each measurement.

Transcription

1 CHEM Experiment 2 Measurement Objectives 1. To understand the accuracy of various types of graduated glassware. 2. To measure volumes of liquids using a graduated cylinder. 3. To measure the volume of liquids dispensed from a burette. 4. To determine the temperature of liquids using a thermometer. 5. To record measurements with correct number of significant figures. Introduction In this lab you are asked to make a series of volume and temperature measurements and record the number of significant figures in each measurement. Three stations will be set up in the laboratory. Each station is set up for you to determine the two volume measurements and a temperature measurement. The measuring instruments at the three stations are: 1. a graduated cylinder for volume measurement 2. a burette for volume measurement 3. a thermometer for temperature measurement Volume measurements can be made with various types of glassware. The glassware that is used determines the accuracy of the measurement.

2 Accuracy of the Various Types of Graduated Glassware 2 Type of Glassware Graduated Erlenmeyer flask Target Volume Accuracy Accuracy Range 125 ml +/- 5% 125 ml +/ ml Graduation interval=25 ml roughly accurate Graduated beaker 100 ml +/- 5% 100 ml +/- 5 ml Graduation interval=10 ml roughly accurate Graduated cylinder 10 ml +/- 0.5% 10 ml +/ ml Graduation interval=0.1 ml accurate Graduated cylinder 100 ml +/- 0.4% 100 ml +/ ml Graduation interval=1 ml accurate Volumetric flask 100 ml +/- 0.08% 100 ml +/ ml No graduation interval very accurate Volumetric pipet 25 ml +/- 0.12% 25.0 ml +/ ml No graduation interval very accurate Burette 50 ml +/- 0.1% 50 ml +/ ml Graduation interval=0.10 ml very accurate In all the measurement determination, it is important that you pay attention to: 1. The determination of the value of the measurement, 2. The unit that is associated with the measurement, 3. The proper number of significant figures that is associated with the measurement.

3 Volume Measurements - Graduated Cylinder 3 In any volume measurement we need to approach the measuring instrument at proper eye position, at eye level. When we are up close to the liquid level at eye level, we will observe that the surface of the liquid forms a "lens-shaped". This is called the meniscus. In taking the measurement, we always read the bottom of the meniscus. Below is an example of a reading taken of a liquid in a graduated cylinder. The diagram shows an expanded view of the portion of the cylinder near the surface of the liquid. Let's analyze this measurement: 1. Upon inspection of the graduated cylinder, we observe that the finest division is a 1 ml division. 2. We can always make an estimate of a reading to within the finest division. Therefore, to the best of my judgement, I conclude that this particular graduated cylinder is able to provide a reading accuracy to +/- 0.2 ml. (Note: A plus or minus 0.2 ml reading accuracy means that a volume measurement using this graduated cylinder will contain uncertainty in the first decimal place.) 3. The bottom of the meniscus lines up with the 4th finest division above the 10 ml mark. 4. The volume reading is determined to be 14.0 ml. This volume reading has 3 significant figures. (Note: the '0' in this measurement is an uncertain figure. This measurement is accurate to /- 0.2 ml. It means that using this graduated cylinder, the accuracy of the volume of the liquid is between 13.8 ml to 14.2 ml.)

4 Volume Measurements - Burette 4 In any volume measurement we need to approach the measuring instrument at proper eye position, at eye level. When we are up close to the liquid level at eye level, we will observe that the surface of the liquid forms a "lens-shaped". This is called the meniscus. In taking the measurement, we always read the bottom of the meniscus. Below is an example of a reading taken of a liquid in a burette. The diagram shows an expanded view of the portion of the burette near the surface of the liquid. Let's analyze this measurement: 1. Upon inspection of the burette, we observe that the finest division is a 0.1 ml division. 2. We can always make an estimate of a reading to within the finest division. Therefore, to the best of my judgement, I conclude that this particular burette is able to provide a reading accuracy to +/ ml. (Note: A plus or minus 0.02 ml reading accuracy means that a volume measurement using this burette will contain uncertainty in the second decimal place.) 3. The bottom of the meniscus lines up with the 6th finest division below the 34 ml mark. 4. The volume reading is determined to be ml. This volume reading has 4 significant figures. (Note: the '0' in this measurement is an uncertain figure. This measurement is accurate to / ml. It means that using this buret, the accuracy of the volume of the liquid which has been dispensed is between ml to ml.)

5 5 Temperature Measurements - Thermometer In any temperature measurement we need to approach the measuring instrument at proper eye position, at eye level. With a mercury thermometer, when we are up close to the mercury level at eye level, we will observe that the surface of the mercury forms a "lens-shaped" that bulges upward. This is called the meniscus. Unlike the meniscus that we observe with a liquid like water, the mercury forms a positive meniscus. In taking the measurement, we always read the top of the positive meniscus. Below is an example of a temperature reading. The diagram shows an expanded view of the portion of the thermometer near the surface of the mercury. Mercury Thermometer Let's analyze this measurement: 1. Upon inspection of the thermometer, we observe that the finest division is a 1 o C division. 2. We can always make an estimate of a reading to within the finest division. Therefore, to the best of my judgement, I conclude that this particular thermometer is able to provide a reading accuracy to +/- 0.2 o C. (Note: A plus or minus 0.2 o C reading accuracy means that a temperature measurement using this thermometer will contain uncertainty in the first decimal place.) 3. The top of the positive meniscus almost lines up with the 30 o C mark. 4. The temperature reading is determined to be 29.8 o C. This temperature reading has 3 significant figures. (Note: the '8' in this measurement is an uncertain figure. This measurement is accurate to /- 0.2 o C. It means that using this thermometer, the accuracy of the temperature is between the range 29.6 o C to 30.0 o C.)

6 6 Alcohol Thermometer The Alcohol thermometer is an alternative to the mercury thermometer. The range of usefulness of the thermometer is set by the boiling point of the liquid used. In the case of the ethanol-filled thermometer the upper limit for measurement in 78 C. Thus, alcohol thermometers cannot be used to measure high temperatures. The ethanol version is the most widely used due to the low cost and relatively low hazard posed by the liquid in case of breakage. When reading the alcohol thermometer, make sure the eye position is level with the surface of the alcohol level. You will not observe a meniscus, as the alcohol level will be flat. Determine where the alcohol level lines up with the nearest mark on the scale and follow the above procedure to report the temperature to within the finest division.

7 Procedure: 7 Part A: Volume measurements Three stations are set up in the laboratory. At each station: 1. Read the volume of the burette 2. Read the volume of the graduated cylinder 3. Record the volume measurements and the units of the measurements on the data sheet and determine the number of significant figures of the recorded measurements. At the end of Part A, you should have recorded 6 volume measurements: 3 burette readings 3 volume measurements of the graduated cylinders Part B: Temperature measurements Three stations are set up in the laboratory. At each station: 1. Read the temperature of the thermometer 2. Record the temperature measurements and the units of the measurements on the data sheet and determine the number of significant figures of the recorded measurements. At the end of Part B, you should have recorded 3 temperature measurements.

8 8 Name Set Student # Datasheet: Part A: Volume measurements Burette Reading Number of Significant Figures of the Volume dispensed Final Reading: ml Station A Initial Reading: 3.55 ml Volume dispensed: ml Final Reading: ml Station B Initial Reading: 0.80 ml Volume dispensed: ml Final Reading: ml Station C Initial Reading: 1.00 ml Volume dispensed: ml Graduated Cylinder Measurement Volume Number of Significant Figures Recorded in the Measurement Station A Station B Station C

9 Part B: Temperature measurements 9 Thermometer Reading Temperature Number of Significant Figures Recorded in the Measurement Station A Station B Station C Question Draw a picture showing the markings (graduations) on glassware that would allow you to make each of the following volume measurements of water and explain your answers. Label the graduations with the volume increments. (a) 28.7 ml (b) 18 ml