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Experiment 2 – Density Name __________________Lab Section __________________

Experiment 2 – Density

Introduction

Density (given the symbol d or depending upon which book you read) is an intrinsic property of materials. The term intrinsic means that it is independent upon the amount of the substance. Thus, the density of anything remains the same, no matter the shape and size of the sample. The

density of water at 4°C is 1.000 gmL regardless if the sample is 1 cup or 1 swimming pool.

For regular solids, those that have a simple formula for calculating their volume, calculating the density is straightforward: simply weigh the object and measure its dimensions. The density is calculated by dividing the mass by the volume. Each regular solid has its own formula for calculating its volume. For this lab, the volume equals length times width times height will work for calculating the volume (V=l x w xh¿. (Note: 1 mL = 1cm3)

For irregular solids, those that do not have a formula for calculating their volume, the volume can be determined by measuring the volume of liquid that the solid displaces. To do this, the solid is submerged in a liquid and the volume displaced is measured. This is done by taking an initial reading and a final reading and determining the volume difference. The mass of the object is then divided by the volume, and the density is determined.

Measuring the density of a liquid is very similar. Although the volume cannot be measured with a ruler, it can be determined using volumetric glassware, for instance, a pipet. This measured volume can be weighed in a container, and its mass determined. Typically this is done by taking the difference of the mass of the empty container before and after the liquid has been added.

A technique needed for this lab is called conditioning. This is a technical term for “rinsing out” something. To condition a pipet, you will suck up some of the liquid you are going to measure using a pipet wheel, and allow it to drain out. Be sure that you rinse out the entire working area of the pipet, not just the end, but be sure to not invert the pipet, as this will contaminate the pipet wheel.

In this lab you will determine the densities of the following liquids and solids: a regular solid, an irregular solid, water at room temperature, an unknown liquid. In addition your instructor will demonstrate the effect that density has on different liquids and solids. Also, you will determine the thickness of aluminum foil using its known density.

Equipment Needed:

Erlenmeyer flask100 mL beaker250 mL beaker

Aluminum Foil10 mL pipet and wheelRubber stopper

Unknown SolidUnknown liquid


Examples

Density of a Regular Solid

Consider the following regular solid. The dimensions are measured to be: 3.85 cm long, 1.20 cm wide, and 2.45 cm high. It weighs 99.391 g.

2.45 cm

1.20 cm

3.85 cm

The volume of the solid is calculated by: V=l x w xh:

(3.85 cm ) (1.20 cm ) (2.45 cm )=11.319cm3

The density is then calculated by dividing the mass by the volume:

99.391g11.319 cm3 =8.780899 g

cm3 or in proper significant figures, = 8.78 gcm3

Density of an Irregular Solid – Volume by Difference

The volume of a 71.356 g, unknown, irregular solid is determined by difference. Calculate its density.

46.5 mL

Irregular Solid

71.356 g

60.0 mLThe initial volume of the graduated cylinder is 46.5 mL. Once the object is placed in the graduated cylinder the volume increases to a final volume of 64.0 mL. The difference in the volumes yields the volume of the object placed inside: 60.0 mL – 41.5 mL = 13.5 mL.

The density of the solid can now be calculated. Divide the mass of the solid by the volume it displaced:


71.356g13.5mL

=5.2856 gmL or in proper significant figures, = 5.29

gmL

Density of a liquid – Mass by Difference

The mass of a 25.0 mL sample of an unknown liquid is determined by difference. Calculate its density.

25.0 mL liquid added

145.123 g 183.249 g

Mass of empty beaker is 145.123 g. The mass after 25.0 mL of liquid is added is 183.249 g. Taking the difference of these masses yields the mass of the liquid: 183.249 g – 145.123 g = 38.126 g.

The density of this liquid can now be calculated. Divide the mass of the liquid by the volume of the liquid:

38.126g25.0mL

=1.52504 gmL in proper significant figures = 1.53

gmL

Thickness of a Material

A 10.0 cm by 22.5 cm piece of tin weighs 20.700 g. If the density of tin is 7.310 gcm3 , how thick

is this foil?

The volume of the object can be found by dividing the mass by the density, or multiplying the

mass by the inverse of the density: (recall: ¿mV )

20.700g7.310g1cm3

=2.831737 cm3

or 20.700 g[ 1cm3

7.310 g ]=2.831737 cm3

Remembering that V=l x w xh, or alternatively, V=Area x thickness

Area=(10.0 cm ) (22.5cm )=225 cm2


thickness= volumearea

=2.831737 cm3

225cm2 =0.012585499 cm

In proper significant figures: 0.0126 cm or 126 m


Prelaboratory Questions

Which is denser, water or ice? How do you know?

What does it mean to condition a pipet?

What is meant by “mass by difference”? How do you do it?

What is meant by “volume by difference”? How do you do it?

A rectangular solid measures 2.55 cm by 1.20 cm by 4.15 cm on each side. If it weighs 110.989 g, what is its density?

An unknown, irregular solid weighing 136.092 g is dropped into a graduated cylinder containing 50.0 mL of water. If the water level rose to 65.5 mL, what is the density of the material?

A thin sheet of lead measures 15.5 cm by 23.0 cm and weighs 0.627 g. If the density of lead is

11.34 gcm3 , how thick is the sheet of lead?


Procedure

Relative Densities – Instructor Demonstration

Record each of the components that your instructor uses during this demonstration including: the different liquids and their relative positions, as well as the solids and their final resting places within the liquids

Density of an Unknown Regular Solid

Obtain an unknown solid from your instructor.

Weigh the solid.

Measure its length, width, and height using the centimeter ruler and calculate the volume of the solid.

Calculate the density of the unknown solid.

Repeat these steps using the millimeter ruler.

Density of an Irregular Solid – Volume by Difference

Weigh the irregular solid provided by your instructor.

Fill a graduated cylinder to between 50 and 70 mL with water. Record this initial volume.

Gently place the irregular solid into the graduated cylinder. This is best accomplished by tilting the graduated cylinder and sliding the solid into the water. This will avoid splashing.

Record the new, final volume of the graduated cylinder. Determine the total volume displaced by difference.

Calculate the density of the irregular solid by dividing the mass of the solid by the volume of water it displaced.

Repeat these steps for reproducibility.


Density of Water – Mass by Difference

Weigh the Erlenmeyer flask.

Obtain approximately 100 mL of deionized water in a 250 mL beaker. Use this water to condition your pipet. Transfer 10.0 mL of the water from the beaker into the Erlenmeyer flask.

Reweigh the Erlenmeyer flask containing the 10.0 mL of water.

Determine the mass of the water by difference.

Calculate the density of water for each trial by dividing the mass of the water by the volume of water added

Repeat these steps for reproducibility. Note: it is unnecessary to dry the flask in between measurements as the mass of the added water is obtained by the differences in mass. However, it is necessary to reweigh the Erlenmeyer flask between runs.

Density of the Unknown Liquid – Mass by Difference

Obtain approximately 50 mL of an unknown liquid sample from your instructor in a clean, dry 100 mL beaker. Record the unknown liquid number.

Weigh your Erlenmeyer flask

Condition your pipet with the unknown liquid. Put all waste into a waste beaker for disposal at the completion of the lab.

Once the pipet has been conditioned, transfer 10.0 mL of the liquid into the Erlenmeyer flask and reweigh the flask. Determine the mass of the liquid by difference.

Calculate the density of the unknown liquid by dividing the mass of the liquid by the volume of liquid added.

Repeat these steps for reproducibility.

Thickness of Aluminum Foil

Measure the length and width of the aluminum foil using the millimeter ruler.

Weigh the piece of aluminum foil. To minimize errors due to wind currents, fold the foil in fourths before weighing.

Using the density of aluminum, ¿2.70 gcm3 , calculate the thickness of the aluminum foil.


Data Table

Relative Densities

Observations


Density of Unknown Solid

Initial mass of Unknown Solid

Volume of solid

Length of solid

Width of solid

Height of solid

Volume of solid – show calculation:

Trial 1(centimeter ruler)

___________ g

___________ cm

___________ cm

___________ cm

___________ cm3

Trial 2(millimeter ruler)

___________ g

___________ cm

___________ cm

___________ cm

___________ cm3

Density of Unknown Solid – show calculation:

_________ gcm3 _________

gcm3


Density of Irregular Solid

Mass of irregular solid

Initial volume

Final volume

Total volume displaced

Trial 1

___________ g

___________ mL

___________ mL

___________ mL

Trial 2

___________ g

___________ mL

___________ mL

___________ mL

Density of Irregular Solid – show calculation:

Trial 1 ________ gmL Trial 2 ________

gmL Average density of solid _______

gmL


Density of Water

Initial mass of Erlenmeyer flask

Final mass of Erlenmeyer flask

Volume of water

Trial 1

___________ g

___________ g

___________ mL

Trial 2

___________ g

___________ g

___________ mL

Density of water – show calculation:

Trial 1 ________ gmL Trial 2 ________

gmL Average density of water ________

gmL

Density of Unknown Liquid

Initial mass of Erlenmeyer flask

Final mass of Erlenmeyer flask

Volume of Unknown Liquid

Trial 1

___________ g

___________ g

___________ mL

Trial 2

___________ g

___________ g

___________ mL

Density of Unknown Liquid – show calculation:

Trial 1 ________ gmL Trial 2 ________

gmL Average density of liquid ________

gmL

Thickness of Aluminum Foil


Mass of aluminum foil

Length of aluminum foil

Width of aluminum foil

Volume of aluminum foil – show calculation:

Thickness of aluminum foil – show calculation:

__________ g

__________ cm

__________ cm

__________ cm3

__________ cm


Postlaboratory Questions:

1) Why is it unnecessary to dry the Erlenmeyer flask in between measurements for the water and the unknown liquid?

2) A 400 troy ounce gold bar is the standard in gold trading. The U.S. Mint reports that a

standard gold bar weighs 27.428 pounds, or 12.441 kg. It measures 7 inches by 3 58 inches by 1

34 inches, which is 17.78 cm by 9.21 cm by 4.45 cm. What is the density of gold in

gcm3?

Knowing that the density of gold is 19.3 gcm3 , is this a pure gold bar?

2) An 18.715 g, uncut diamond was dropped inside of a graduated cylinder containing 45.5 mL of water. If the water level rose to 51.0 mL, what was the density of the diamond?

3) A 150 mL beaker weighing 125.326 g had a 50.0 mL sample of an unknown liquid put inside of it. The beaker was then reweighed and found to be 164.776 g. What was the density of the liquid?


4) A block of iron was measured to be 2.00 cm by 3.15 cm by 5.25 cm. If the density of iron is

7.87 gcm3 , how heavy was the block of iron?

5) A thin sheet of nickel measured 5.00 cm by 7.00 cm was found to weigh 0.350 g. If the

density of nickel is 8.90 gcm3 , what is the thickness of this sheet?

6) Cats love to play with balls of yarn. If a 3.75 inch diameter ball of yarn weighs 0.110 lbs,

what is its density in gcm3 ? [volume of a sphere is

43r3]

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