Lab #2: Determining Density

MATERIALS

a)  25-mL graduated cylinder

b)  100-mL graduated cylinder

c)  electronic balance

d)  Chemistry Reference Tables

e)  plastic pipets

f)  Unknown rectangular-shaped materials

g)  Unknown irregularly-shaped materials

h)  water

Procedure:

Station 1: Irregularly-shaped solid #1

1.  Zero (TARE) the balance by pressing the appropriate button.

2.  Place the object on the center of the balance pan, wait for the readout to stabilize, and record the mass in the Data Table with the correct units.

3.  Find the object’s volume using the displacement method described below:

a.  Using a beaker, fill the 100-mL graduated cylinder about ½ way with water. Then read the exact volume of water in the graduated cylinder. Be sure to use the correct number of decimal places (Remember what we learned in class about reading instruments!) Record this value in the Data Table as the initial volume.

b.  Tilt the graduated cylinder at an angle and slide the object into the water so that it is totally submerged. Record the final volume of water in the Data Table.

4.  Empty the graduated cylinder, dry off the object, and continue.

Station 2: Irregularly-shaped solid #2

1. Same as above, only choose a different object

Station 3: Density of water #1

1.  Zero (TARE) the balance by pressing the appropriate button.

2.  Place the 25-mL graduated cylinder on the center of the balance pan, wait for the readout to stabilize, and record the mass of the empty cylinder in the Data Table with the correct units.

3.  Using the pipet, fill the cylinder to 10 mL with water. Then, read the exact volume of water in the graduated cylinder. Be sure to use the correct number of decimal places (Remember what we learned in class about reading instruments!) Record this value in the Data Table as the volume of water.

4.  Find the mass of the cylinder with the water in it and record this value in the Data Table.

5.  Empty the graduated cylinder and clean up the station.

Station 4: Density of water #2

1.  Zero (TARE) the balance by pressing the appropriate button.

2.  Place the 100-mL graduated cylinder on the center of the balance pan, wait for the readout to stabilize, and record the mass of the empty cylinder in the Data Table with the correct units.

3.  Using a beaker, fill the 100-mL graduated cylinder to 50 mL with water. Then read the exact volume of water in the graduated cylinder. Be sure to read the exact volume of water in the graduated cylinder. Be sure to use the correct number of decimal places (Remember what we learned in class about reading instruments!) Record this value in the Data Table as the volume of water.

4.  Find the mass of the cylinder with the water in it and record this value in the Data Table.

5.  Empty the graduated cylinder and clean up the station.

Name: ______Block: ______Date: ______

Lab #2: Determining Density

Density is a physical property that can be used to identify an unknown substance. It is calculated using the following equation.

Mass is usually measured in grams (g) and volume is usually measured in cubic centimeters (cm3), so the units for density are g/cm3 (eg. dgold = 19.31 g/cm3). Since a volume of 1cm3 is equal to a volume 1 mL, densities can also be expressed in g/mL (e.g. dgold = 19.31 g/mL).

The objectives for this activity are

·  To make precise and accurate measurements of mass and volume

·  To determine the density of regularly-shaped solids, irregular solids, and liquids.

·  To identify a material based on its density.

·  To calculate % error and determine sources of error in your experiment.

Data Table

Station 1 / Measurements / Value with unit
Irregularly shaped solid #1 / Mass of solid
Initial Volume in graduated cylinder
Final volume in graduated cylinder
Volume of Irr. Solid

Calculations:

Density ______

Station 2 / Measurements / Value with unit
Irregularly shaped solid #2 / Mass of solid
Initial Volume in graduated cylinder
Final volume in graduated cylinder
Volume of Irr. Solid

Calculations:

Density______

Station 3 / Measurements / Value with unit
Density of Water #1 / Mass of empty grad. cylinder
Mass of grad. cylinder and water
Mass of water ONLY
Volume of water

Calculations:

Density ______

Station 4 / Measurements / Value with unit
Density of Water #2 / Mass of empty grad. cylinder
Mass of grad. cylinder and water
Mass of water ONLY
Volume of water

Calculations:

Density______

Analysis

1.  Find out the identity of the unknown substances and enter their densities (from the Reference Tables) in the table below.

Station / Substance / Density
Solid #1
Solid #2
Water #1 / water
Water #2 / water

2.  Compare the density of water from the Water #1 and the Water #2 experiments. Should the density have been different? Did the amount of water you use significantly affect the density you calculated?

Percent error is a way to indicate how far off your answer is from a generally accepted value. For each of the densities you determined, calculate your percent error using the formula:

The measured value is the value you calculated. The accepted value is the value from the Reference Tables or other source. A positive value for % error means your answer was too high. A negative value for % error means your answer was too low.

3.  Calculate the percent error of your density for Water #1 and Water #2.

4.  Based on your data, do you get a more accurate result when using less water or when using more water. Explain.

5.  List 3 sources of error that might account for the difference between your measured value and the accepted value (exclude human errors, silly mistakes, & calculation errors).

a)  ______

b)  ______

c)  ______