EXPERIMENT 2 Measurements of Mass and Volume Outcomes After completing this experiment, the student should be able to: 1. Perform basic laboratory measurements of mass and volume using appropriate glassware and equipment. 2. Express measurements and calculated results with the proper number of significant figures. 3. Find the mass of a liquid from volume and density at a given temperature. 4. Determine the precision of measurements.

Introduction Quantitative measurements are fundamental to chemistry. One must become familiar with different units of measurement used to quantify base quantities (such as temperature, length, time, mass, etc.). The main units which will be considered in this lab are the base quantities of temperature, length and mass and the derived quantities of volume and density. The quantity of volume is determined through multiple length measurements. Density is derived from the quantities of mass and volume. We will discuss below the base quantities of temperature, length and mass, and the derived quantities of volume and density, individually. Increments: The number of decimal places to which any measurement can be read is dependent on the increments in which the tool used is marked. In Figure 2.1 below, notice that there are 10 subdivisions between the 5 cm and 6 cm marks. If you subtract, the difference between the two labeled marks and then divide by the number of spaces or increments, you can find the value of each increment shown on the tool being used, i.e., 3 cm – 2 cm = 1 cm

(There is 1 cm difference between the two marked lines and there are 10 spaces between 2 cm and 3 cm).

1 cm/10 spaces = 0.1 cm

(Each individual space or increment on this measuring stick represents 0.1 cm or a tenth of a centimeter).

Note: Before you attempt any measurements you must always first determine the increment in the measuring tool you will be using. Making a Measurement: You must always keep in mind that every measurement inherently will have an uncertainty associated with it. This means that a measurement is only as certain as the instrument used to make the measurement. The least significant figure that you can claim in a measurement is the digit which value you estimate. It is in this digit that the uncertainty lies. It is  A metric ruler generally accepted that the uncertainty is estimated as ± 1 in that showing increments. digit, but it is the measurer who actually decides how small is the estimate of uncertainty? We have to assume that measuring devices with electronic displays have uncertainties of ± 1 in the last digit displayed. Examine the ruler in the figure given above. What can we say for certain about the length of the arrow 7

being measured? It is certainly at least 2 cm long. In fact, it is certainly at least 2.3 cm long. However, the object is not exactly 2.3 cm. It is possible to estimate another digit in the hundredths place, which will more accurately describe the length of the object. Try this your-self and record the digit. Your estimation of the object’s length: 2.3__ cm Some of your group members or classmates may estimate a different reading for that last hundredth’s place digit. That is okay because there are no increments on the ruler to indicate hundredths of a cm. There is always some error or uncertainty associated with the reading. When you report such a reading (in your data table), others would recognize that the true length of the object is ± 0.01 cm of the reported value. Rule of Thumb: You will generally estimate one digit beyond the increment digit. Note on the above ruler, the increment is 0.1 cm – your length reading involves estimating to the hundredth place (or 0.01cm). Density Density is defined as the amount of substance present in a unit volume. In other words, density is the mass of an object divided by its volume. It is expressed by the equation:

The density equation can be rearranged to determine either the mass or the volume of a substance as follows:

In calculations of density, mass is usually expressed in grams and the volume in milliliters or cubic 3 centimeters. Thus, units of density are commonly expressed as g/mL for a liquid or g/cm for a solid In this experiment, you will use balances, transfer pipets and micropipets. See experiment 1 for the details.

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Safety Precautions Follow all safety rules described in experiment 1.

Materials and Equipment Balance, transfer pipet (10 mL), micropipette (100 L), beakers, thermometer, Erlenmeyer flask (50 mL), distilled water.

Experimental Procedure Getting started 1. Obtain about 50 mL of distilled water in a beaker. Allow the beaker and water to sit on the laboratory bench while you are learning to use the balance and the pipet. The water should come to the temperature of the laboratory during that time. 2. Plan on using the same balance and pipet throughout the experiment. Using Balance 1. Practice using a balance by measuring the mass of an object (such as a coin) several times. 2. Make sure the Erlenmeyer flask is clean and dry. Bring your balance to the zero position. Measure and record the mass of the flask. 3. Remove the flask from the pan of the balance. Bring your balance to the zero position again. Measure and record the mass of the flask once more. 4. Repeat step 3 until you have measured the mass of the Erlenmeyer flask four times. 5. Calculate the average of the 4 mass determinations. 6. The differences between the measured masses and the average should be very small. Ask your laboratory instructor whether your results are satisfactory before you proceed. Using Transfer Pipet 1. Practice with your pipet using distilled water (do not use the water you have set aside) until you are comfortable with the technique. 2. Using the thermometer, note the temperature of the laboratory and of the distilled water that you have set aside. When the temperatures are identical or very nearly identical, you are ready to begin. Record the temperature to the nearest degree. 3. Measure and record the mass of the flask again. 4. Remove the flask from the balance. Pipet 10.0 mL of the room-temperature water into the flask. 5. Bring your balance to the zero position. Measure and record the mass of the flask now with the 10.0 mL water. 6. Remove the flask from the balance. Pipet another 10.0-mL sample into the flask. Do not pour out the first sample (the volume of water in the flask should now be 20.0 mL). Weigh and record the new mass. 7. Repeat until four samples of water have been delivered to the flask and the final volume is 40.0 mL.

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8. Calculate the mass of water that was delivered each time from your pipet. These masses should be approximately identical. Table l. Density (g/mL) of Water at Various Temperatures (oC) Temp. (oC) 17 18 19 20 21

Density (g/mL) 0.9988 0.9986 0.9984 0.9982 0.9980

Temp. (oC) 22 23 24 25 26

Density (g/mL) 0.9978 0.9976 0.9973 0.9971 0.9968

Temp. (oC) 27 28 29 30 31

Density (g/mL) 0.9965 0.9962 0.9959 0.9956 0.9953

9. You can now calculate the volume of each sample from the mass and density of water. Use the density in Table l that corresponds to your recorded temperature. Using Micropipet Clean and dry a small beaker. Weigh the beaker on the 1 mg top-loading balance. Practice using the micropipet, then deliver 5x 100. μL into a dry beaker and reweigh. Record the mass of the 500. μL added to the beaker.

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Experimental General Chemistry 1 Experiment 2: Measurements of Mass and Volume Laboratory Data Sheet Name: ______________________________________________

Section:

____________

1. Using analytical balance: Addition Number

1

2

3

4

Mass of the flask Average mass (g) Show calculations for the average mass

2. Using transfer pipet (10.0 mL): Temperature (oC) _____________ Density of water (g/mL) (from Table 1) _____________ Addition Number

1

2

3

4

1. Mass before addition (g) 2. Mass after addition (g) 3. Mass of added water (2 – 1) (g) Calculated volume of water: 3/density (calculate for each of the 4 additions) Vaverage (mL) %error = {(10.0 – Vaverage) /10.0}x100%

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3. Using the micropipet: a.

Mass of the beaker before addition

__________________

b. Mass of the beaker after addition of 5x100. μL of water

__________________

c. Mass of added water (b – a)

_________________ (mg)

________________ (g) =

d. Calculated volume of water (use mass and density for the calculations)

_________________ (μL) Show your calculation of the volume:

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