Determination of Substance through Density

DExperiment #1

DENSITY OF SUBSTANCES

Prepared by Paul Okweye and Malinda Gilmore

Purpose of the Experiment

To learn about the properties of matter such as density that are used as a method of physical identification. In this experiment the objectives are:

1. To teach the correct use of a balance and graduated cylinders,
2. To determine the densities of solids, pure liquids and solutions,
3. To determine percent errors during experimental analysis, and
4. To teach the use of graphing of experimental data.

Background Information

Density, like boiling point, color, odor, solubility, and melting point, is a physical property of matter. Therefore, density may be used in identifying matter. Density is defined as mass per unit volume and is expressed mathematically as d = m / v (Equation 1: d is density, m is mass, and v is volume). The density of a sample of matter represents the mass contained within a unit volume of space in the sample. The units of density, therefore, are quoted in terms of grams per milliliter (g/ml) or grams per cubic centimeter (g/cm³) for most solid and liquid samples of matter. The density of a sample represents the mass of the specific sample divided by its volume.

density (g/ml) = mass (g) ÷ volume (ml or cm³) Eqn. 1

Often, a density varies with temperature because of the volume of the sample such as gases. Therefore, densities are usually determined and reported at room temperature (about 25^oC; see Table 1). References such as chemical handbooks always specify the temperature at which a density was measured.

As previously stated, density can be used as a method of identification. Various things that density can be useful for are listed below:

Table 1. Densities of various substances at room temperature, 25^oC.

1. Density is often used as a point of identification in the determination of an unknown substance. The density of the unknown might be used to characterize the unknown from a list of known substances. It is very unlikely for two substances to have the same density, and when added with boiling point and melting point it adds even more validity to the identity of the substance.

2. Density can also be used to determine the concentration of solutions in certain instances. When a substance is dissolved in water, the density of the solution will be different from that of the pure water itself. Handbooks list detailed information about the densities of solutions as a function of their composition (typically, in terms of percent substance in the solution). If a sample is known to contain only a single substance, the density of the solution can be measured experimentally, and then the handbook can be consulted to determine what concentration of the substance gives rise to the measured solution density.

Several techniques are used for the determination of density of substances. In general, a density determination involves the determination of the mass of the sample divided by the determination of the volume of the sample. However, the method used for determining mass or volume depends on whether or not the sample is a solid or a liquid.

For solid samples, the volume of the solid can be determined using Archimedes’s principle, which states that an insoluble, nonreactive solid will displace a volume of liquid equal to its own volume. Typically, a solid is added to a liquid in a volumetric container (such as a graduated cylinder) and the change in the liquid level is determined.

For liquids, very precise values of density may be determined by measuring an accurate volume of liquid in a container that can then be weighed and then determining the mass of the liquid that was measured. A convenient container for determining the volume of a liquid is to weigh a particular volume of liquid in a graduated cylinder.

The density of substances is very important especially when talking about buoyancy – the tendency or capacity to remain afloat in a liquid or rise in air or gas. Often one asks the question, “Why does ice float in water?” The answer to that question depends totally on density of the substances involved. When dealing with water, water can be in the form of ice, liquid or solid (Table 2). The density of ice is 0.917 g/cm³ and then density of water in its liquid state at 25^oC (room temperature) is 0.999 g/cm³. Therefore, the density of ice is less than the density of water so that is why ice floats in water.

Table 2. Temperature Dependence of Water Density

Safety Precautions

1. Safety goggles and lab coat / apron are required for this lab
2. The solutions used in this lab are flammable. Use them only as directed

Materials and Chemicals

• Graduated cylinders (25 mL, 50 ml, and 100 mL)
• Balance
• Regular Solid Sample
• Irregular Solid Sample
• Liquid Sample (Isopropyl Alcohol)
• Distilled Water
• Sodium Chloride (5%, 10%, 15%, 20% and 25% solutions)

Procedures

A. Determination of the Density of Solids

1. Obtain a regular shaped solid (cubic metal). On your data sheet, write down the name of the solid and describe its appearance.
2. Using a balance, weigh the regular shaped solid. Weigh it on a balance to the nearest 0.01 g. Record the weight on the data sheet in the section labeled “Weight of the Solid.”
3. Using a 100-mL graduated cylinder, add 75 mL of distilled water. Record the exact volume of water in the graduated cylinder to the precision permitted by the calibration marks on the cylinder. Record this volume on the data sheet in the section labeled “Initial Volume of Water for the Solid”.
4. Gently place the regular solid (cubic metal) into the cylinder (do not drop the metal because it could splash the water in the graduated cylinder). Read the level of the water in the graduated cylinder, again making your determination to the precision permitted by the calibration marks on the cylinder. Record this volume on the data sheet in the section labeled “Final Volume of Water for the Solid.” The change in the water (V_solid = V_f – V_i) level represents the volume of the solid.
5. Calculate the density of the regular solid (cubic metal) using Equation 1. Record the calculated value (experimental value) of the density on the data sheet in the section label “Experimental Value of Density of Solid.”
6. Compare the calculated (experimental value) density of the regular solid (cubic metal) with the actual density value provided in Table 3. Record the actual density on the data sheet in the section labeled “Actual Density of the Solid.”
7. Calculate the percent error of your measurement. Record value on the data sheet in the section labeled “Percent Error of Solid”.

Note:

Percent Error = Experimental Value – Actual Value x 100%

Accepted Value

8. Dry the regular solid (cubic metal) with a paper towel and return the sample to your instructor.

B. Density of Pure Liquids

Pure Water (Distilled Water)

9. Clean and dry a 50 ml graduated cylinder. Accurately weigh the dry graduated cylinder using a balance. Record weight on the data sheet in the section labeled “Initial Weight of the Graduated Cylinder (Water).”
10. Add 45 mL of water to the graduated cylinder. Record the exact volume of the water in the cylinder, to the level of precision permitted by the calibration marks on the barrel of the cylinder on the data sheet in the section labeled “Volume of Water”.

11. Weigh the graduated cylinder and water as accurately as possible. Record weight on the data sheet in the section labeled “Final Weight of the Graduated Cylinder (Water).”

12. Calculate the density of the water using Equation 1. Record the calculated value (experimental value) of the density on the data sheet in the section labeled “Experimental Value of Density of Water.”
13. Determine the temperature of the water in the cylinder. You will use the temperature of the water to determine which density value of water to use from Table 2. Record the temperature on the data sheet in the section labeled “Temperature of Water.”

14. Compare the calculated (experimental value) density of the water with the actual density listed in Table 2. Record the actual density on the data sheet in the section labeled “Actual Density of the Water.”

15. Calculate the percent error. Record value on the data sheet in the section labeled “Percent Error of Water”.

16. Clean and dry the graduated cylinder.

Rubbing Alcohol

17. Obtain a sample of rubbing alcohol (isopropyl alcohol = rubbing alcohol).

18. Clean and dry a 10 ml graduated cylinder. Weigh the dry graduated cylinder as accurately as you can with the balances you have available. Record weight on the data sheet in the section labeled “Initial Weight of the Graduated Cylinder (Rubbing Alcohol).”

19. Add 5 mL of rubbing alcohol to the graduated cylinder. Record the exact volume of the alcohol in the cylinder, to the level of precision permitted by the calibration marks on the barrel of the cylinder on the data sheet in the section labeled “Volume of Rubbing Alcohol.”

20. Weigh the graduated cylinder and rubbing alcohol as accurately as possible. Record weight on the data sheet in the section labeled “Final Weight of the Graduated Cylinder (Rubbing Alcohol).”

21. Calculate the density of the rubbing alcohol using Equation 1. Record the calculated value (experimental value) of the density on the data sheet in the section label “Experimental Value of Density of Rubbing Alcohol.”

22. Compare the calculated (experimental value) density of the rubbing alcohol with the actual density listed in Table 3. Record the actual density on the data sheet in the section labeled “Actual Density of the Rubbing Alcohol.”

23. Calculate the percent error. Record value on the data sheet in the section labeled “Percent Error of Rubbing Alcohol”.

24. Clean and dry the graduated cylinder.

C. Density of Solutions

Chemical solutions are often described in concentrations and most times in terms of the solutions’ percent composition on a weight basis. For example, a 1% sodium chloride (NaCl) solution contains 1 g of NaCl in every 100 mL of solution (which corresponds to 1 g of NaCl for every 99 mL of water (H₂O) present).

25. Obtain 50 mL solutions of NaCl in H₂O consisting of the following percents by weight: 5%, 10%, 15%, 20%, and 25%. Make the weight determinations of NaCl and H₂O accurately as possible.
26. Using the method described earlier for samples of pure liquids, determine the mass, volume and density of each of your NaCl solutions. Record that information on the data sheet under the specified section.
27. Compare the calculated (experimental value) density of the NaCl solutions with the actual density listed in Table 3. Calculate the percent errors for each solution. Record value on the data sheet in the section labeled “Percent Error of NaCl Solutions”.

28. Using Excel, construct a graph of the calculated (experimental value) density of your NaCl solutions (y-axis) versus the percent of NaCl the solution contains (x-axis). Obtain the straight line equation (y = mx + b). Record this equation in the designated area on the data sheet.

Name_______________________________________________________________________________

Lab Partner____________________________Section/Day/Time_______________________________

Experiment #1

DENSITY OF SUBSTANCES

DATA SHEET

A. Determination of the Density of Solids

Sample Name ______________________________

Appearance of Solid ______________________________

Weight (g) of the Solid ______________________________

Initial Volume (mL) of Water for the Solid ______________________________

Final Volume (mL) of Water for the Solid ______________________________

Volume (mL) of the Solid ______________________________

Experimental Value of Density (g/mL) of Solid______________________________

Actual Density (g/mL) of the Solid ______________________________

Percent Error of Solid ______________________________

B. Determination of the Density of Pure Liquids

Pure Water (Distilled Water)

Initial Weight (g) of the Graduated Cylinder (Water) ______________________________ Final Weight (g) of the Graduated Cylinder (Water) ______________________________

Weight (g) of Water Sample______________________________

Volume (mL) of Water ______________________________ Experimental Value of Density (g/mL) of Water ______________________________ Actual Density (g/mL) of the Water ______________________________ Percent Error of Water ______________________________

Name_______________________________________________________________________________

Lab Partner____________________________Section/Day/Time_______________________________

Experiment #1

DENSITY OF SUBSTANCES

DATA SHEET

Rubbing Alcohol

Initial Weight (g) of the Graduated Cylinder (Rubbing Alcohol)______________________________ Final Weight (g) of the Graduated Cylinder (Rubbing Alcohol) ______________________________

Weight (g) of Rubbing Alcohol Sample______________________________

Volume (mL) of Rubbing Alcohol ______________________________ Experimental Value of Density (g/mL) of Rubbing Alcohol ______________________________

Actual Density (g/mL) of the Rubbing Alcohol ______________________________ Percent Error of Rubbing Alcohol ______________________________

C. Determination of the Density of Solutions

Note: Show calculations in your lab report.

Name____________________________________________________________________________

Lab.Partner____________________________Section/Day/Time_____________________________

Experiment #1

DENSITY OF SUBSTANCES

HOMEWORK SHEET

1. Explain density in words.

2. What error would be introduced into the determination of the density of the solid if the solid were hollow? Would the density be too high or too low?

3. An insoluble, nonreactive metal sphere weighing 18.45 g is added to 21.7 ml of water in a graduated cylinder. The water level rises to 26.8 ml. Calculate the density of the metal.

4. An empty graduated cylinder weighs 34.4257 g. A 10-ml pipet sample of an unknown liquid is transferred to the graduated cylinder. The graduated cylinder weighs 40.1825 g when weighed with the liquid in it. Calculate the density of the unknown liquid.

5. Your data for the density of the NaCl (sodium chloride) solutions should have produced a straight line when plotted. How could this plot be used to determine the density of any concentration of sodium chloride solution?

6. Examine your graph and determine the density for each of the following percents of NaCl: 3%, 9%, 15%, 21%, and 45%.