Instructions for ChemLab2
Density of a Solid Laboratory Steps
PART I: Density of a Solid by Water Displacement
As mentioned in the introduction, density is determined by a calculation involving at
least two different measurements. The limit of accuracy of each of those
measurements affects the accuracy of the calculated density
When determining the density of a regularly-shaped solid, linear measurements of
length, width, and height can be used to calculate the volume. This volume can then be
compared to its mass to determine its density. Directions for measuring volume using a
graduated cylinder:
Many solids however do not have regular shapes and so their volumes are best
determined using a method called water displacement. This works as long as the
solid does NOT dissolve in water (is insoluble), does NOT react with water, and sinks in
water. Objects that sink in water displace a volume of water equal to their own volume
since they are totally submerged. The graduated cylinder from your lab kit has ten divisions between milliliters. This
means that each smaller mark represents 0.1mL. Because your cylinder has graduation
marks to 0.1mL, the volume can be estimated to 0.01mL. This gives your graduated
cylinder a limit of accuracy of 0.01mL. In this example, the location of the miniscus is
definitely greater than 7.10mL and less that 7.20mL, so an appropriate estimate could
be 7.12mL. If two different people made this measurement, it is likely that they would esitmate slightly different values (ex: 7.12mL vs. 7.14mL), but all values should be
between the known values of 7.10mL and 7.20mL. When measuring the volume of the liquid using this cylinder, the bottom of the miniscus
could be read as being exactly on the seven milliliter mark. Because volumes in this
cylinder must be estimated to the closest 0.01mL, you should record this value as 7.00mL in order to report the correct significant figures per the limit of accuracy of this
cylinder. Directions for measuring mass using a
scale:
In our determination of mass, we will be using the electronic balances provided in your
kit. These balances display the mass digitally in grams. In digital readouts, we assume
that the last digit displayed is an estimated digit and use its place as our limit of
accuracy, unless given further information about the precision of the instrument.
Always be sure to record ALL digits from a digital readout, even the zeros,
because they indicate the degree of accuracy of the instrument. The balance in your kit will measure the mass to one decimal number. In the example
for the clip shown, the mass would be read and recorded as 1.2 grams. In the example of the safety pin, the mass should be read and recorded as 0.7 grams.
The zero in this number is referred to as a leading zero and it must be recorded to
indicate that the safety pin has a mass less than 1.0 grams. The mass of the coin is read and recorded as 5.0 grams. The zero in this number is
referred to as a placeholder zero. The balance has a limit of accuracy of one digit past
the decimal point. The zero in this number must be included to keep the correct amount
of significant figures. Experiment: Record all data on the Report Sheet Steps:
1. From your kit obtain the 5/8 inch long brass screw. If you do not have the brass
screw in your kit you may use ten small paper clips. Place the brass screw or paper
clips on the balance and record the mass. 2. Obtain approximately 7 mL of water in your 10 mL graduated cylinder. Read and
record the volume of the water accurately on your report sheet. For example: 7.12
mL or 6.85 mL. 3. Tilt the graduated cylinder slightly and CAREFULLY place the brass screw massed
earlier into the graduated cylinder so as to slide down into the water WITHOUT
splashing any water out of the cylinder. Gently shake the cylinder if necessary to
remove any air bubbles that may be trapped. Carefully read and record the volume
of the screw and the water on your report sheet. 4. Dump out the water and the brass screw and use a paper towel to dry off the
brass screw and the graduated cylinder both inside and out. Be careful NOT to push
the paper towel down tightly into the cylinder or you may not be able to get it out. 5. Calculate the volume of the brass screw by subtracting the initial volume of
water from the combined volume of the water and screw. Show your work clearly on
the report sheet. 6. Calculate the density of your brass screw using the mass data reported earlier
along with your calculated volume. Again, show your work clearly on the report
sheet, and report the density with units and significant digits. 7. Compare your density to the densities of the following metals1. What
conclusions can you draw about the composition of the brass screw? Metal Density,
g/cm3 Aluminum 2.6989 Titanium 4.54 Zinc 7.133 Copper 8.96 Molybdenum 10.22 PART II: Density of Liquids
Density is an intrinsic or intensive property, which means that it is not affected by
the total amount of a material present. The density of pure copper at 25oC is always
8.96 g/cm3 whether you have one piece of copper wire or a 2 inch thick sheet. Intensive
properties are very useful for identifying substances. In this section you will show that
although you are using different volumes of a liquid for each measurement, the
liquid's mass to volume ratio remains constant.
When scientists want to observe how two variables are related and they may do so by
holding all variables constant except those two. The first one of the variables they
control or change in a systematic way and they measure the second variable to see how
it was affected by changing the first variable. The variable that is changed in
a controlled manner is referred to as the independent variable. The variable that
changes as a result of the change in the independent variable is called the dependent
variable. In measuring density, we have two measurable variables: mass and
volume. All other variables should be held constant, for example the temperature and
pressure. By varying the volume systematically and measuring the corresponding
mass, you will gain a set a data that will allow you to see how the mass and volume of a
given substance are related. Experiment: Record all data on the Report Sheet
using your measurements.
You will be determining the density of three different liquids:
1. Room temperature water 2. 20% salt water 3. Isopropyl alcohol. Room Temperature Water
1. Obtain about 10 mL of room temperature water in your clean 50 mL beaker. 2. Measure and record the mass of your clean, dry 10 mL graduated cylinder. 3. Add approximately 2 mL of the water to the cylinder, record its exact volume.
Example: 1.95 mL or 2.23 mL.Read the volume to the nearest tenth and then
estimate the next digit. 4. Measure and record the combined mass of the cylinder and 2 mL of water.
Example: 141.8 grams. Record the mass as shown on the digital readout. 5. Add additional water to the cylinder to have a total of about 8 mL water in the
cylinder, record its exact volume. 6. Measure and record the combined mass of the cylinder and 8 mL of water. 7. Empty the liquid from the cylinder and dry the cylinder. 20% Salt Water
20% salt water contains 20 g of salt (NaCl) in 100 mL of salt water. Create the 20% salt
water solution by using the mass scale and your 50 ml beaker. Add 10.0g of salt to your
50 ml beaker and add water to the 50.0 mL line while stirring.
8. Add approximately 8 mL salt water to the cylinder, record its exact volume. 9. Measure and record the combined mass of the cylinder and 8 mL of salt water. 10. Empty the liquid from the cylinder and dry the cylinder. Isopropyl alcohol
11. Add approximately 8 mL alcohol to the cylinder, record its exact volume. 12. Measure and record the combined mass of the cylinder and 8 mL of alcohol. 13. Empty the liquid from the cylinder and dry the cylinder. The acceptedvalues for the densities of the liquids you used are given below in the
chart below.
Liquid Substance
Density, g/mL
°
Water at 25 C
0.99705
20% Salt Water
NA
100% Isopropyl Alcohol 0.7848 Procedure:
Now that you have completed your lab report: General Tool Kit - Checklist: Beaker 50ML Low Form Candle Tea Light Cylinder 10ML Student Grade Dish Petri & Lid Forceps Med Pt 4.5/Strt S Glove, Nitrile Large pk/100 Goggle Economy Chem Splash 11X5 Printed Pan Ck Aluminum 12X9X1-1/4 Pencil Glass Marking Blue PhydrionInsta-Chek 1-12 (pH paper) Pipette Barrel Thin Stem Rod (Stirring) Glass 8” Scale, Pocket 600X0.1 Gram Scoop Laboratory 6-1/2" Spot Plate Polystyrene Stopper Rubber Sz 00 Test Tube 13X100MM, 1 Dozen Tongs, Crucible 8" Tums X-tra Strength 750, PK/8 W/P Open-top Non-Sterile 4oz Weighing Dish Aluminum, PK/100 Brass Screw 5/8” Lab Kit - Checklist: 1 12 oz. Styrofoam Cup 1 bag of Epson Salt 1 bag of Baking Soda 1 bag of Plaster of Paris 1 bag of Crushed Chalk 1 bag of Maalox 1 bag of Borax 1 bag of White Powder Labeled "Power A" Magnesium Hydroxide, 100G P/KM Food Coloring 4 colors/Box Chromatography Paper pk/4 Common Household Materials Used in Chem 101 Checklist:
Isopropyl alcohol (rubbing alcohol) 50%, 70%, or 91% 70% works best for the Lesson 12 lab o Clear white vinegar Granulated sugar Salt Bottled or distilled water A Ruler that is at least 6" in length. Must be marked in cm's with at least 10
divisions in between the cm's Measuring Spoon Set Scissors Stapler and Staples Matches 2 Regular Ceramic Mugs 1 Clear Plastic or Glass Container A Heat Resistant Plate Disposable Plastic Teaspoon Cotton Swabs Similar to Q-tips Toothpicks Paper Towels Pen and Pencil

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