To identify an unknown metal based on density measurements. To determine the density of diluted rubbing alcohol.
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/cm3 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 top-loading 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= a3 = (2 cm)3 = 8 cm3.
Determining the volume of irregularly shaped solids (e.g. metal samples in this experiment), requires more ingenuity. Archimedesí 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.
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 5-6 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
2-5 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.
|Metal sample form||Amount of metal used||Size of the graduated cylinder used||Amount of water used in the cylinder|
|uniform silver-color pellets||10-15 pieces||if the pieces will fit:
if do not fit: 50 mL
|approx. 5 mL|
|irregular silver-color 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|
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.
Metal Density, g/cm3 Metal Density, g/cm3
Aluminium 2.70 Zinc 7.13
Copper 8.96 Osmium 22.57
Lead 11.36 Gold 19.31
I. Rubbing Alcohol
|Alcohol solution concentration, % weight||
Mass of empty
|Mass of cylinder with
|Mass of alcohol
|Volume of alcohol
Density of the unknown sample =
The unknown rubbing alcohol
solution #_______ was determined to have the
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 and
| Mass of metal,
|Volume of water in the cylinder,
|Volume of water after sample
Mass of metal =
Volume of metal =
Density of metal =
The uknown metal sample #
_____ was identified as _______________ beacuse its experimentally
determined density ________g/cm3 closely matched the density
of that metal sample listed as ________g/cm3.
CLICK ON IT!
1. Calculate the volume (in m3 ) of a 25,000 tonne iceberg. ( 1tonne = 1,000 kg)