Objective
To identify an unknown metal based on density
measurements. To determine the density of diluted rubbing alcohol.
Introduction
The density is one of the physical properties
of matter. The density of a substance is defined as the ratio of
its mass to its volume:
d = m / V
Eq. 1
The usual units for density in the SI (International
System of Units) are g/mL for liquids, g/cm^{3} for solids,
and g/L for gases. The density of a pure substance in its solid state is
usually greater than its density in the liquid state. Water is an important
exception ice floats on water. Gases are much less dense than solids
or liquids. The density of a substance is usually reported at a specific
temperature, since its value is temperature dependent.
The density is determined indirectly,
based on measurements of the mass and volume of the substance.
The mass of a substance is measured using
a balance. Analytical balances are very accurate instruments that measure
mass to the nearest 0.0001 g. The toploading balances that you will
use in this lab will determine the mass to the nearest 0.01 g, which is
adequate for this experiment.
The volume of liquids is determined
using volumetric glassware such as volumetric pipets, graduated cylinders
or pycnometers. Measurements made with graduated cylinders are not
as precise as those performed with volumetric pipets.
The volume of regularly shaped objects (spheres, cubes, cylinders, etc.) can be determined by measuring the dimensions of the object and calculating the volume based on the appropriate mathematical equation. For example, if the object was a cube with the side length a= 2 cm, the volume would be calculated as V= a^{3} = (2 cm)^{3} = 8 cm^{3}.
Determining the volume of irregularly shaped solids (e.g. metal samples in this experiment), requires more ingenuity. Archimedes’ principle can be used to measure their volume. This principle states that an object, when placed in a liquid, will displace a volume of liquid equal to its own volume. However, to be able to measure the volume of an object by displacement method, the object has to be denser than the liquid used and must be completely immersed in the liquid. The most commonly used liquid is water.
Procedure
I. Determining the Concentration of the
Unknown Isopropyl (Rubbing) Alcohol Solution.
In this part of the experiment,
you will determine the concentration of an unknown solution
of rubbing (isopropyl) alcohol in water. The density of a rubbing alcohol/water
mixture depends on the concentration of alcohol. To find the relationship
between density of the solution and the alcohol concentration in
this solution, you need to determine the density of several
solutions with a known concentration of alcohol first.
1. Obtain three isopropyl alcohol solutions in water with the following concentrations of alcohol (% weight): 20%, 50% and 70%.
2. Weigh a 10 mL graduated cylinder. The cylinder has to be CLEAN AND DRY. Use tongs or test tube holder to handle the cylinder. Record the tare (empty cylinder) mass on the data sheet.
3. Pour 56 mL of the first isopropyl alcohol solution of known concentration into the tared cylinder, taking care not to spill any on the outside of the cylinder. Read and record the volume of the alcohol in the cylinder to the nearest 0.1 mL (or 0.2 ml, depending on the cylinder).
Note: it is convenient to perform the measurements starting with the lowest or highest concentration and continue in increasing or decreasing order.
4. Weigh the cylinder with alcohol and record the weight.
5. Discard the alcohol into the waste container. Clean and dry the cylinder.
6. Repeat steps 2 through 5 with the remaining 2 known alcohol solutions.
7. Obtain an unknown sample and repeat steps
25 using this sample. Record the weights and volume.
II. Identification of an Unknown Metal
Measure the volume of an irregular metal
sample by displacement method. When you are done, pour
the water into the sink. Pour the metal sample on a paper towel. DRY the
metal sample and return to the cart. Identify the metal by comparing
the experimentally obtained value with the values listed in Table
1.
Table 1.
Metal sample form  Amount of metal used  Size of the graduated cylinder used  Amount of water used in the cylinder 
uniform silvercolor pellets  1015 pieces  if the pieces will fit:
10 mL
if do not fit: 50 mL 
approx. 5 mL 
irregular silvercolor chunks  8 smaller pieces or 5 larger  50 mL  approx. 30 mL 
orange/reddish granules/spheres  approx. 15 grams  10 mL  approx. 5 mL 
tiny dark gray spheres  approx. 30 grams  10 mL  approx. 5 mL 
Calculations
I. Rubbing Alcohol
1. Calculate the mass of each alcohol solution
used by subtracting the mass of an empty cylinder from the mass of
the cylinder with alcohol.
2. Calculate the density of each known
solution and the unknown using the formula in Eq. 1.
3. Determine the approximate concentration
of the unknown solution by fitting the value of density obtained
for the unknown into the set of densities obtained for the three
known solutions.
II. Unknown metal
1. Calculate the volume of your unknown
metal sample.
2. Calculate the density of the unknown
metal.
3. Identify the unknown metal by comparing
the obtained density with the values in Table 2.
Table 2.
Metal
Density, g/cm^{3
}Metal
Density, g/cm^{3}
Aluminium
2.70
Zinc
7.13
Copper
8.96
Osmium
22.57
Lead
11.36
Gold
19.31
Magnesium
1.74
Data Sheet
I. Rubbing Alcohol
Uknown #____________
Alcohol solution concentration, % weight 
Mass of empty
cylinder grams 
Mass of cylinder with
alcohol, grams 
Mass of alcohol
solution, grams 
Volume of alcohol
solution, mL 
Density
g/mL 
20  
50  
70  
Unknown = 
Density of the unknown sample =
The unknown rubbing alcohol
solution #_______ was determined to have the
concentration_______________
beacuse______________________________________________________________________________
II. Uknown Metal
Unknown # __________
Description of the metal sample__________________________________________________
Check Table 1 to find out how much metal to use and what size cylinder.
Mass of
weighing dish, grams 
Mass of weighing dish and
sample, grams 
Mass of metal,
grams 
Volume of water in the cylinder,
V_{1}
mL 
Volume of water after sample
was
added, V_{2} mL 
Metal volume,
mL 
Metal density,
g/cm^{3} 







Calculations:
Mass of metal =
Volume of metal =
Density of metal =
The uknown metal sample #
_____ was identified as _______________ beacuse its experimentally
determined density ________g/cm^{3} closely matched the density
of that metal sample listed as ________g/cm^{3}.
PRELAB
CLICK ON IT!
POSTLAB
1. Calculate the volume (in m^{3}
) of a 25,000 tonne iceberg. ( 1tonne = 1,000 kg)