Spokane Falls Community College s1

SPOKANE FALLS COMMUNITY COLLEGE CHEMISTRY EXPERIMENT NO. 2

MEASUREMENTS, DENSITY, AND UNIT CONVERSIONS

DISCUSSION In the laboratory, the investigation of matter usually entails five measurement. These five include mass, volume, temperature, pressure, and time. These measurements are taken or derived using metric units which are universally used in all the laboratories of the world. The common measuring units of comparison are: the meter, liter, gram, atmosphere, second, and Celsius degree.

This particular laboratory exercise will help you become familiar with the metric units for distance, volume, and mass, as well as the equipment which is used to make measurements.

In order to measure a piece of matter, there are three things which should be kept in mind.

1. We must understand what is to be measured. 2. We must use some device to measure the piece of matter. 3. Our devices must be compared to some acceptable standard.

The devices we use in the laboratory are the meter stick, a graduated cylinder, a balance, a thermometer, and a clock. The meter stick - or ruler - measures length, the graduated cylinder measures the volume of a liquid, a balance measures mass, and the thermometer measures temperature.

When measuring a piece of matter, one must realize that there are possible sources of error. Because of this, all physical measurements contain a degree of uncertainty. This uncertainty arises from the experimenter misreading the measuring device, and/or the inaccuracy of the measuring instrument itself. This uncertainty leads to the concept of significant figure. Due to uncertainty, any calculation made with measured quantities will also contain uncertainty. The use of the proper number of significant figures in a derived quantity signifies an understanding of the concept of uncertainty and the proper use of data and mathematical expressions.

One of the most important instruments in the chemistry laboratory is the balance. With a balance the chemist is able to weigh materials with great accuracy and precision. There are many types of balances available today. Balances weigh objects to varying degrees of precision. Triple beam balances are capable of a precision of 0.01 grams. Our electronic balances are capable of weighing to 0.001 grams. We even have several balances, known as analytical balances that will weigh an object with a precision of 0.0001 gram.

Always record your measurement with the proper number of decimal places. Use the most precise instrument available to you and record all the information that device provides.

Careful measurements allow us to determine certain physical properties of a substance. Pure compounds and pure elements each have their own distinct set of physical and chemical properties. Physical properties include such characteristics as color, density, melting point, boiling point and solubility in water. One very important physical property, density, is a relationship of an objects weight to its mass. Specifically, the density of an object is defined as its mass per unit volume. Mathematically we can state that as: Mass M Density = , D = Volume V This expression tells us how much a certain volume of a given material will weigh. Water, for example, has a density of approximately 1.0 g/cm3 and mercury (Hg) has a density of 13.6 g/cm3. Mercury is a very dense material, in other words, a very large mass of mercury occupies a small volume. On the other hand, cotton is not a very dense material. It has a very small mass for the large volume it occupies.

In this exercise, you are asked to make observations and measurements about five different pieces of matter. From this data you will be able to calculate such quantities as volume and density. You must always include the proper units on any density value. A unit less number out of context is meaningless.

Materials Needed:

Check out a wood block, metal cylinder, and thermometer from the stockroom. The sugar cubes, ethanol, and waste jars can be found on the supply bench.

Procedure:

A. Density of Wood

1. Check out a block of wood from the stockroom. Measure the height, length, and width in centimeters using the metric ruler. You should be able to read the ruler to the nearest 0.01 cm.

2. Compute the volume of the block by multiplying its length times the width, times the height:

V = l x w x h

3. Write in the data table the volume of the block in cubic centimeters. Report this volume to the proper number of significant figures.

4. Weigh the block of wood and record this in the data table.

5. Compute the density of wood and record this also. Be certain to use the correct number of significant figures and include the proper units.

6. Complete this section of the data table by observing several of the physical properties of the wood block.

B. Density of metal cylinder

1. Weigh the cylinder and record the weight.

2. We shall determine the volume of the iron cylinder by direct measurement. To find the volume of the iron cylinder in this manner, use a metric ruler and measure the height and then the diameter of the cylinder.

3. Compute the volume of the cylinder in centimeters by using the formula:

 d 2 h V = 4 where d is the diameter, h is the height of the cylinder, and π = 3.14.

4. Compute the density of the iron cylinder. Record the density to the proper number of significant figures and with the correct units in the data table.

5. Complete this section of the data table by observing several of the physical properties of the metal cylinder.

C. Distilled Water

1. Place between 10 and 20 mL of distilled water in your graduated cylinder. Record the exact volume to the nearest 0.1 mL.

2. Tare you smallest beaker on the electronic balance. Add the contents of the graduated cylinder (step 1) and record the mass of the water in the data table.

3. Compute the density of the water.

4. Determine the boiling point of the water. Use your smallest beaker and heat the water with a hot plate. You may need to add more water to ensure that the thermometer bulb is completely immersed in water.

5. Complete this section of the data table by observing several of the physical properties of the water.

D. Sugar cube

1. Obtain a sugar cube from the supply bench.

2. Weigh the sugar cube and record it in the data table.

3. Measure the length, width, and height. Record.

4. Calculate the volume of the cube.

5. Calculate the density of the sugar cube.

6. Using the expression: Area = h x w, calculate the area of one side.

7. Using the expression: Total surface area = area side 1+ area side 2+ ...... for six sides, calculate the total surface area of the cube. 8. Place the sugar cube in the labeled waste container when you are finished with this portion of the lab.

E. Ethanol

1. Place between 10 and 20 mL of ethanol in your dry graduated cylinder. Record the exact volume to the nearest Ethanol (Ethyl Alcohol) 0.1 mL. C2H5OH Extremely flammable. 2. Tare you smallest beaker on the electronic balance. Add the contents of the graduated cylinder (step 1) and record the Toxic by ingestion. mass of the ethanol in the data table. LD50 = 7060 mg/kg

3. Compute the density of the ethanol.

4. Determine the boiling point of the ethanol. Use your smallest beaker and heat the ethanol with a hot plate. CAUTION: Flammable - use no flames.

5. Complete this section of the data table by observing several of the physical properties of the ethanol.

6. Pour the used ethanol into the proper waste container. DATA SHEET--EXPERIMENT 11 Name______Class______

A. Wood block

A. Mass B. Length (l)

C. Height (h) D. Width (w)

E. Color F. State

G. Odor

H. Volume of the wood block. V = l x w x h

I. Density of wood block.

B. Metal cylinder

A. mass B. diameter

C. radius D. height

E. color F. state

G. odor

H. Using the formulas below determine:

a. Volume of cylinder (V) where d = diameter h = height π = 3.14  d 2 h V = 4

b. Density of cylinder (D)

Mass D = Vol

C. Distilled water A. Volume of water in graduated cylinder

B. Mass of the water

C. State D. Color

G. Odor

E. Density (Show work)

F. Boiling Point

D. Sugar Cube

A. Mass B. Height

C. Length D. Width

E. Color F. State

G. Odor

H. Volume of the cube.

I. Density of the sugar cube

J. Surface area of one side

K. Total Surface Area

E. Ethyl Alcohol

A. Volume of ethyl alcohol in graduated cylinder B. Mass of only the ethyl alcohol

C. State D. Color

E. Odor

F. Density of Ethanol

G. Boiling Point of Ethanol Some Questions to Ponder and Answer

1. Compare the densities of (1) the metal cylinder, (2) the water, and (3) the wooden block. Why does the wood float in water while the metal sinks?

2. Assuming it didn't dissolve, would the sugar cube float?

3. Knowing the density of an object can be very useful. The following situation demonstrates this usefulness.

A geologist in the desert finds a huge rock in the shape of a rectangle. He would like to know the weight of it. However, since the rock is as big as a three story building, he can't very well weigh it on his triple beam balance. What can he do?

The geologist can solve the problem by finding the density of the rock. If he knows the density, and he can measure the rock to determine its volume, using the formula for density he can compute the weight. Remember that the volume of a rectangle is the length of side one times side two times side three.

V = s3 or V = s x s x s

where s is the length of one side of the rectangle.

The geologist can calculate the density of the rock by chipping off a small piece of the rock. He can weigh these pieces and determine their volume. From this information he can calculate the density. Assuming that the rock is uniform, and recalling that density is a physical property, then the density of the rock chips will be the same as the density of the entire rock. Knowing the rock's density and its volume from above, the geologist can then compute the weight of the rock.

Given the following data, compute the mass of this large rock. Report your answer in kilograms.

mass of rock chips = 50.00 grams volume of rock chips = 25.0 cm3 Length of side one is 20 meters; side two is 30 meters; and side three is 40 meters M Remember that D= V PART B: Unit Conversions

INTRODUCTION:

The process of mathematically changing a measurement taken in one system of units into a comparable value in another system of units is known as a unit conversion. Unit conversions are a fundamental part of all scientific processes, as they enable scientists to conveniently express numerical values in the most convenient form. For example, it may be convenient to measure the volume of a flask in ml, but more practical to refer to the volume of a swimming pool in liters or even gallons. By using simple mathematical operations, each of these units can be inter-converted, and a ready comparison can be made as to the volumes of each object.

Making a conversion is quite easily understood if just a few simple ideas are considered. First, you must know what value you are starting with, and the unit which you will be converting. Second, you must know some relationship between the starting and ending unit, and be able to express this relationship mathematically. Finally, you must be able to add, subtract, multiply, and divide without difficulty.

Although actually the simplest step, many times students have difficulty with the first step - identifying the value to be converted. Always identify the "given" in a problem immediately, so you know where you are starting. Always then identify where you are going; what unit do you wish to end up with? These may seem trivial, but overlooking this simple first step causes many difficulties.

The second major step is to identify the relationship between the starting and ending unit. This is always something that has either been memorized, or you must look up. Twelve inches equals one foot is an example of a relationship between inches and feet that I assume you have all memorized. One inch equals 2.54 centimeters is a similar relationship that many of you probably do not have memorized. These are both just expressions of a relationship, however, any many more will be learned or can be found in reference materials.

Finally, you must use this relationship to write a mathematical equation and perform the appropriate function (multiply, divide, etc.) to complete the unit conversion. For example:

What is the length of a textbook in centimeters that measures 0.26 meters long?

We identify the "given" as a book 0.26 meters long. We wish to convert this to centimeters. 0.26 meters = ? cm

The relationship between centimeters and meters is 100 cm = 1 m. Since all such relationships are equalities, we can express them as fractions equaling 1. For example,

100 cm = 1 , or 1 m = 1 1 m 100 cm

According to the rules of math, any number can be multiplied by one without changing its value. We use the appropriate fraction or ratio above, based on an analysis of the units. We would like to convert meters to cm in this problem, so we use the fraction with cm in the numerator (on the top) and meters in the denominator (on the bottom). This will give us the equation:

0.26 m x 100 cm = 26 cm 1 inch Notice that by using the appropriate fraction (relationship) that an analysis of the units gives us:

meters x cm /meters

This = cm , which was the desired unit.

All unit conversions can be approached in this straightforward manner, and quickly solved. Remember, you must either know the relationship between different units, or look up that relationship in some reference manual before proceeding. An analysis of the units will determine which value in the relationship is placed in the numerator and which is placed in the denominator.

In this portion of the lab, you will take some simple measurements of objects in the lab, and convert these measurements to different units, following the format shown above. Procedure:

Show the complete mathematical set-up for all conversions.

A: 1. Weigh a test tube on the electronic balance.

2. Record the weight in grams.

3. Convert this weight to the following units:

a. Kilograms.

b. Milligrams.

B: 1. Weigh your watch glass on the electronic balance.

2. Record the weight in grams.

3. Convert this weight to the following units:

a. Micrograms.

C: 1. Measure the width of this paper in centimeters, to the

nearest 0.1 cm.

2. Convert this length to the following units:

a. Millimeters.

b. Meters.

D. 1. Fill your smallest graduated cylinder approximately half full of water.

2. Record the volume to the nearest 0.1 mL

3. Convert this volume to the following units:

a. Liters.