AP Chemistry Unit 1 Lab
Density of a Solid and Organic Liquids
Introduction
All states of matter, gases, solids, and liquids exhibit the property of density. Density is defined as mass per unit volume. Densities of most substances change with an increase or reduction in temperature. Water for example at 4° C is most dense (1.00000 ). The density of water at 25° C is .9970 . The density of ice at 0° C is .9168 . Remember 1 ml = . At varying pressures, the density of liquids and solids is relatively constant. For the case of a gas, the density increases enormously as the pressure increases. Lithium has the low density of .534 while Osmium has a density of 22.5 . Note here the relation as the atomic mass increases then so does the density.
To measure the density of a solid, divide the measured mass by the measured volume. To find the density of a liquid, perform the same calculation as for a solid. The mass can be measured using a balance while the volume can be measured using a pipette or graduated cylinder.
Precision and accuracy play a big role in the outcome of this experiment. Consider the following targets after being shot with a rifle.
The above picture demonstrates the concept of accuracy and precision. See your text for a more elaborate definition of these to terms. To understand the concept of precision, consider the following set of data.
Experiment Measured Density Absolute Deviation
1 0.98 gm/ml 0.01
2 0.96 gm/ml 0.01
3 0.99 gm/ml 0.02
4 0.95 gm/ml 0.02
5 0.97 gm/ml 0.00
So then we can say the density is 0.97 ±0.01. Disregard any negative sign in the calculation of deviation or average deviation.
Table of Known Densities – gm/ml
Pentane 0.626
Hexane 0.659
tert-Butyl Methyl Ether 0.741
Toluene 0.867
Ethyl alcohol 0.785
Ethyl Acetate 0.902
Acetone 0.791
Carbon Tetrachloride 1.594
Dichloromethane 1.325
Materials for this lab
Milligram Balance
1 ml Syringe – graduated
a “TC” or “TD” pipette
2 sample vials and caps
2 cm length of rubber tubing
Graduated Cylinder
Forceps
Safety Considerations
Ware safety goggles
Ware a lab apron
All reagents used in this experiment are combustible. Do not use fire near them.
Pre Lab Questions
1. In the introduction of this experiment, how many significant figures is the density of water given at 4° C and at 25°C?
2. A one-liter flask is placed on a one pan of a very accurate balance. On the other pan brass weights are placed such that both pans balance exactly. This entire apparatus is placed in a complete vacuum. The balance is tipped in favor of the flask. Is seems to weigh more. Explain why.
3. Determine the relative error if the experimental density is measured at: .735 and the actual density is .750 .
4. What does average deviation of a set of data tell us?
Procedure
Solid
1. Measure the mass of the unknown solid and record in the report sheet.
2. Pour a specified volume of water in a Graduated cylinder. Use a cylinder large enough so that the metal object will easily pass into it.
3. Insert the metal object and record the volume change.
Liquids
1. Examine the pipette carefully. Two types are most commonly used. One will be labeled “TC” at the top (to contain) and “TD” at the bottom (to deliver). The “TD” pipette delivers 1.00 ml from the top graduation to the bottom of the pipette whereas the “TC” pipette delivers 1.00 ml from the top graduation to the bottom graduation marked “0”. Either pipette will work. Be sure to familiarize yourself with use of the pipette.
2. Attach a 1 ml syringe to the pipette using a 2 inch section of a gum rubber tube. Determine the masses of the solid metal object supplied by the teacher and 2 labeled and caped vials. Be sure your measure the masses to the nearest .001 gms.
3. Using the syringe as a pump, draw liquid into the pipette. Try to get the liquid near to the top graduation. Record the amount of liquid in the pipette within ±.001 ml. The last figure will be an estimate. Expel a portion of the liquid from the pipette into the vial. Again record the volume reading from the pipette.
4. Determine the masses of the liquids and vials to within ±.001
Report Sheet
Unknown solid
Mass of Solid ______
Final Volume of graduated cylinder ______
Initial volume of the graduated cylinder ______
Volume of the solid ______
Known Liquid
Name of known liquid ______
Mass of empty Graduated Cylinder ______
Mass of Graduated Cylinder and liquid ______
Mass of liquid ______
Volume of liquid ______
Known Liquid
Name of known liquid ______
Mass of empty Graduated Cylinder ______
Mass of Graduated Cylinder and liquid ______
Mass of liquid ______
Volume of liquid ______
Calculations
Calculate the density of the solid
Compare your calculated density to densities of known solids using the CRC handbook. Using the stated density in the CRC handbook perform an error analysis.
Calculate the density of the liquid 1
Using the table provided in this experiment, calculate an error analysis.
Calculate the density of the liquid 2
Using the table provided in this experiment, calculate an error analysis.
Post Lab Questions
1. Explain why there was error in your experiment.
2. Most of your error was probably in measuring the volume used in the calculation of density for the solid. What way could be used to measure the volume of the solid?
3. Can the determination of density be used to identify a material? Why or why not?
4. For the case of the rifle targets shown in the introduction, what would the target look like with high accuracy and high precision? Explain.
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