Pre-Lab Discussion:
Gases have no definite volume. They spread out, or diffuse, and occupy all of the space available to them. This spreading of gases is called diffusion. It has been observed that different gases diffuse at different rates. The rate at which a gas diffuses may be related to it's molecular mass.
In this investigation, two gases-HCl and NH3 - will be introduced simultaneously into opposite ends of a glass tube. At the point where the two gases meet inside the tube, a chemical reaction will occur that produces white powder. Using the point where the powder forms as a reference point, the distance traveled by each gas can be measured. By comparing the ratio of these distance traveled by each gas can be measured. By comparing the ratio of these distance to the ratio of the molecular masses, a relationship between molecular mass and rate of diffusion may be found.
Purpose: To determine the relationship between the molecular mass of a gas to its rate of diffusion.
Materials:
glass tubing, (10mm x 60 cm) wax marking pencil Q-tips
wax marking pencil metric ruler safety glasses
vials containing HCl and NH3·H20 with dropping pipets
acetone
Procedure
1. Obtain the glass tubing. Make sure it is clean and completely dry. If the tube is not clean, clean and dry it as instructed by the teacher. Lay the tubing on your lab bench.
2. Place one Q-tip in each end of the tubing to a distance of 1-2 cm. Using a marking pencil, mark the glass to indicate the position of the end of each cotton tip.
3. Remove the Q-tips. Mark the stem of one of the Q-tips with the marker for purposes of identification. Using dropper pipets, place about 3-5 drops of the HCl solution on the unmarked Q-tip and 3-5 drops the concentrated NH3·H20 on the marked one. CAUTION: HANDLE THESE CHEMICALS WITH CARE! They can cause painful burns is they come in contact with the skin, and could cause permanent damage to the eyes.
4. Immediately and simultaneously insert the moistened ends of the Q-tips into opposite ends of the tube to the lines previously marked.
5. After several minutes, a white ring will form where the gases HCl and NH3 meet inside the tube to form the white compound NH4Cl (ammonium chloride). Mark the point on the tube where the white ring is first formed.
6. Remove the Q-tips, rinse them with water, and dispose of them as instructed by the teacher.
7. Measure the distance traveled by each gas and record your results in a table like the one below.
Distance traveled by HCl Distance traveled by NH3
Trial 1
Trial 2
8. If time permits, clean and dry the tubing as instructed by the teacher and repeat the procedures.
Calculations:
1. Calculate the following ratio: Distance NH3/Distance HCl
2. Calculate the following ratio: Molecular mass HCl/Molecular mass NH3
3. Calculate the square root of the ratio obtained in calculation #2.
Conclusions/Questions:
1. How does your answer to calculation #1 compare to the answer for calculation #3. Is there any apparent relationship between the molecular mass of a gas and it's rate of diffusion?
2. Assume that within limits of experimental error, your answer to calculation #1 was the same as #3, state Graham's Law of gas diffusion.
3. How could what you learned in this investigation be used to separate a mixture of gases.
4. How could this investigation be used to find the molecular mass of an unknown gas.?
Pre-Lab Discussion
The normal electron configuration of atoms or ions of an element is known as the ground state. In this most stable energy state, all electrons are in the lowest energy levels available. When atoms or ions in the ground state are heated to high temperatures, some electrons may absorb enough energy to allow them to jump to higher energy levels. The element is then said to be in the excited state. This excited configuration is unstable, and the electrons fall back to their normal positions of lower energy. As the electrons return to their normal levels, the energy that was absorbed is emitted in the form of electromagnetic energy. Some of this energy may be in the form of visible light. The color of this light can be used as a means of identifying the elements involved. Such analyses are known as flame tests.
Only metals, with their loosely held electrons, are excited in the flame of a laboratory burner. Thus, flame tests are useful in the identification of metallic ions. Many metallic ions exhibit characteristic colors when vaporized in the burner flame. In this experiment, characteristics colors of several different metallic ions will be observed, and an unidentified ion will be identified by means of its flame test.
Purpose: Observe the characteristic colors produced by certain metallic ions when vaporized in a flame. Identify an unknown metallic ion by means of its flame test.
Equipment
10 mL graduated cylinder nichrome wire loop
laboratory burner safety glasses 2 - 50 mL beakers
Materials
Concentrated hydrochloric acid (HCl)
Water
Salts: sodium chloride (NaCl), potassium chloride (KCl), lithium chloride (LiCl), calcium chloride (CaCl2), strontium chloride (SrCl2), barium chloride (BaCl2), copper (II) chloride (CuCl2)
Unidentified salts
Procedure
1. Pour about 10 mL of concentrated hydrochloric acid into a 50 mL beaker. CAUTION: Use extreme care in handling this acid. To clean the wire loop, dip the loop in the acid and then heat the loop in the outer edge of the burner flame. Continue to clean the loop in this manner until no color is observed in the flame.
2. Put about 25 mL of water in a 50 mL beaker.
3. Dip the CLEAN wire loop into the water, then dip the loop into your vial of the salt. Place the loop in the outer edge of the burner flame and move the loop up and down. Note the color of the flame. Record your observations in the data table provided.
4. Clean the wire loop as described in step 1. Repeat step 3 using a different salt.
5. Test each salt in the same manner, cleaning the loop thoroughly between tests. Record all your observations in the data table.
6. Obtain a sample of an unknown salt. Perform a flame test and identify the metallic ion present by the color of the flame..
Observations and Data
Salt Metal Ion Formed Color in Flame Observed
sodium chloride ______________ ____________________
potassium chloride ______________ ____________________
lithium chloride ______________ ____________________
calcium chloride ______________ ____________________
strontium chloride ______________ ____________________
copper (II) chloride ______________ ____________________
barium chloride ______________ ____________________
Unknown_________ ______________ ____________________
Conclusions and Questions
1. What inaccuracies may be involved in using flame tests for identification purposes?
2. Which pairs of ions produce similar colors in the flame tests?
3. Explain how the colors observed in the flame tests are produced.
4. Define these terms: (a) quanta (b) ground state (c) excited state
5. What is a spectroscope? What is observed if the flame tests are viewed through a spectroscope?
6. What is the identity of your unknown salt? How do you know you are correct?
PRE-LAB DISCUSSION
The nuclei of most atoms are extremely stable. Even when atoms are involded in chemical reactions, their nuclei do not change. However, a few nuclei are unstable and undergo change. These changes are called nuclear reactions.
Nuclei that undergo changes are said to be radioactive. Their nuclei change in such a way that they disintegrate into other elements. Such changes are called transmutations. These transmutations are accompanied by radioactivity - the emission of particles or rays from the nucleus of the atom. There are 3 types of radiation produced in a transmutation: Alpha particles (mass 4, charge +2), Beta particles (mass nearly 0, charge -1), and Gamma rays (no mass nor charge).
In this investigation you will become familiar with the effects of distance and various types of shielding materials with the intensity of radiation.
PURPOSE:
To determine the effect of distance from a radioactive source on the intensity of radiation. To determine the abilities of certian materials to shield from radiation.
MATERIALS:
Geiger counter aluminum sheets(12)
meter stick lead sheets (12)
cardboard sheets (12) beta emitter
ring stand various radioactive samples
utility clamp
PROCEDURES:
Part I - Background Radiation
1. Turn on the Geiger counter and follow the manufacturers directions to get a background count. Record this value.
2. Set up the Geiger tube a certain distance (about 10-20 cm) above the table top using a ring stand and a utility clamp.
3. Make a table with the following headings:
Sample Counts per Minute
4. Place a sample on the lab table under the Geiger counter.
5. Obtain the number of counts per minute and record it in the table. Repeat for all the samples.
Part II - Shielding Effects of Different Materials
6. Make a table with the following headings:
Number Counts per Minute
of Sheets cardboard aluminum lead
7. Place the beta sample on the lab table.
8. Adjust the height of the Geiger counter so that it as 10.0 cm above the sample.
9. Place a sheet of cardboard between the tube opening and the radioactive source.
10. Read the Counter and record the data.
11. Place another sheet of cardboard between the tube opening and the radioactive source, read the Counter and record the data with the two sheets of cardboard in place.
12. Repeat step 11 until all the sheets of cardboard have been used.
13. Repeat steps 9-12 using sheets of aluminum instead of cardboard.
14. Repeat steps 9-12 using sheets of lead instead of cardboard.
Part III - Relationship between Distance and Radiation
15. Make a table with the following headings:
Distance (cm) Counts per Minute
16. Place the Geiger counter on the table next to a meter stick.
17. Place the beta sample 2cm from the Geiger tube. Note and record the distance and the reading on the Geiger counter in the table.
18. Move the radioactive sample 4 cm farther from the tube (6 cm total distance). Note and record the distance and the Geiger counter reading.
19. Continue to move the sample away from the tube at 4 cm intervals until it is about 30 cm from the tube. Note and record the distance and Geiger counter reading after each move.
ANALYSIS
1. Plot the data from Part II on a sheet of graph paper (or the computer) with the independent variable on the x-axis. Plot all three curves on the same grid using different colors (or symbols if on the computer) for each material.
2. Plot the data for Part III on a sheet of graph paper (or the computer) with the independent variable on the x-axis.
3. Square each value of each distance and then calculate the multiplicative inverse of this value. Be sure to include these calculations on your calculation page.Plot the counts per minute vs. the inverse of the square of the distance on a sheet of graph paper (or the computer) with the counts per minute on the y-axis.
CONCLUSIONS AND QUESTIONS
1. Why is it necessary to take the background count before doing this lab?
2. In your Discussion be sure to describe any relationships that are indicated by the three graphs.