Chem assignment 4

Experiment 4: Density

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Purpose: You are to learn the proper way to report the mass and volume of various samples, and make use of the information to determine the density. Introduction: Density is one of the physical properties of matter that helps us identify it. The density of a sample is defined as the mass per unit volume of the sample. It is the ratio of its mass to its volume and can be calculated by the equation

volume theis V and mass theis M density, theis D where V M

D =

Consider a large rock and a small pebble. The large rock is most likely to be heavier than the small pebble, but it is not necessarily denser. In order to compare their densities the two objects would have to be the same size, or you could compare their ratios of mass to volume. In determining the density of a liquid, you will need the volume and mass of a sample of the liquid. It would not matter how much of the sample you use as long as it does not exceed the capacity of the apparatus you are using, and that the volume and mass are obtained for the same source. Density is said to be an intensive property because it is independent of the sample size. In contrast, mass and volume are said to be extensive properties because they are dependent of the sample size. In this experiment you will determine the density of water, using 8 mL and 10 mL. The densities of the two samples are to be determined to see whether the difference in volume would affect the density. Some of you may recall from past experience that the density for water it is 1.00 g/mL. In actuality, that value is only an approximation, as it will vary depending on the temperature. Your instructor will look up the “correct” density from a handbook. The volume of a liquid is measured using a graduated cylinder. Water has a curved surface called a meniscus. The proper way to measure the volume is to read the bottom of the meniscus at eye-level as shown in Figure 4.3. This is easier said than done with water being colorless. For this reason you will prepare a Reflection Card to make the bottom of the meniscus more visible.

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Figure 4.3

The mass is measured with an electronic balance. Obviously you cannot pour the liquid directly on the balance pan. It is to be obtained by the method of “weighing by difference”. The following example illustrates how density is calculated using this technique:

A student weighed a 50-mL beaker. He then pipetted 10.00 mL of a liquid substance into the beaker and weighed it again. The difference in masses was the mass of the liquid that was pipetted.

Mass beaker + liquid 42.3220 g

Mass beaker (-)30.0830 g Mass liquid 12.2390 g Volume liquid pipetted 10.00 mL

The density was calculated:

density = = 1.224 g/mL

Procedure (Using a pen or pencil, record by hand all of your data and results on the Data Collection and Results Pages.)

Part I: Density of Water

1. Your instructor will either announce on the board or show you how to determine the room temperature and known density of water at that temperature. Record these in your lab notebook.

2. Record the mass of a clean and dry 10-mL graduated cylinder and use this mass in your calculations for both trials. You should use the same balance throughout this part of the experiment.

Always remember to read the meniscus at eye-level.

Reflection card

EXPERIMENT 4: DENSITY 3

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3. Add approximately 8 mL of deionized water to the grad cylinder. Wipe the outside of the grad cylinder totally dry and record the total mass of the grad cylinder and the water.

4. Calculate the mass of water in the 10-mL grad cylinder. 5. Read and record the volume of the water to the correct significant figures with the

use of the Reflection Card. (Hold the card behind the grad cylinder with the black line just slightly below the bottom of the meniscus. The black line will reflect off the bottom surface and make it more visible.) It is essential that you take time to figure out how many decimal places you should be recording.

6. Empty the grad cylinder. (You need not dry it. Just shake out the excess water.) 7. Repeat Steps 4 through 6 using approximately 10 mL instead of 8 mL of deionized

water. 8. Complete the calculations specified on the Data Collection and Results Pages.

Part II: Density of Marbles

In this section you will learn to determine the volume of solids by the application of Archimedes’ Principle, which states that an object is buoyed up with a force equal to the weight of the liquid it displaces. This principle is often applied to the measurement of the volume of a solid. Simply put, the volume of an object is equal to the volume of water it displaces if it is denser than water (and therefore does not float on the surface of the water). Procedure: Determination of the Mass (by the method of weighing by difference) 1. Obtain 3 marbles from your instructor. 2. Weigh a 50-mL beaker and record the mass in your lab notebook. 3. Gently place the 3 marbles into the beaker and record the mass again. (Remember to

use the same balance.) 4. Take the beaker and marbles to your bench. Hold on to your 3 marbles. You

MUST use the same 3 marbles in the next section. 5. Calculate the total mass of the 3 marbles. Determination of the Volume of the Marbles (using Archimedes’ Principle) 6. Place exactly 30.0 mL of water in your 50-mL graduated cylinder and record the

volume to the correct sig. fig. 7. Carefully slide your 3 marbles into the graduated cylinder, taking care that no water

splashes out of it and record the volume again. 8. The difference between the two volume readings you recorded is the volume of

water displaced by the marbles. According to Archimedes’ Principle, this is the volume of the marbles. Calculate and record the total volume of the 3 marbles.

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Determination of the Density 9. Remembering the relationship between density, mass, and volume, calculate the

density of your 3 marbles and record it on the Data Collection and Results Page. Watch your sig. fig. at each step of your calculations.

10. Obtain the density from 3 other students, record their full names and the densities they obtained. Compare the values you and your fellow students obtained. Are there any that look totally unreasonable?

11. Calculate the average density of the marbles that you and other students obtained and record it on the Data Collection and Results Pages.

12. Obtain the correct density of the marbles from your instructor and calculate the error and percent error of the average density of the marbles. Watch your sig. fig. and sign (whether positive or negative).

Units of Volume and Density of a Solid The volume of liquids is generally measured in mL; whereas, that of solids is generally recorded in cm3. Since 1 mL is exactly 1 cm3, the numerical value is unchanged. Record the volume of the marbles as cm3, and the density as g/cm3.

Experiment 4: DENSITY

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Data Collection and Results Pages Name: __________________________ (Show work for all calculations.) Date: ___________________

I. Density of Water Room Temp: ___________ Correct Density of Water at this Temp: ________________ Reference: Calculate the density of water for each trial: Trial 1: Trial 2: Calculate the Average Density of Water: Note: The average value cannot have more precision than the individual values. As a rule of thumb, the average should have the same decimal places as the individual values. Calculate the Error in the Average Density: (Watch your sign and sig. fig.!) Calculate the % Error in the Average Density: (Watch your sign and sig. fig.!)

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II. Density of Marbles

Mass of the 3 marbles = _______ Volume of the 3 marbles = _______ Density of the 3 marbles* = _______

*Show calculation for the density of the 3 marbles: Calculate the Average Density from the class data: Calculate the Error in the Average Density: Calculate for % Error in the Average Density:

8 mL Trial 10 mL Trial Mass of Water in Cylinder Volume of Water in Cylinder Calculated Density of water Average Density of Water Correct Density of Water Error in the Average Density % Error in the Average Density

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CLASS DATA ON DENSITY OF MARBLES

Name of Student DENSITY OF MARBLES reported Yours

1.

2.

3.

Average Density

Correct Density (from instructor)

Error in Average Density

% Error in Average Density