1230 Chemisty - Week 2
Experiments 1C and 2 Isotopes and Mass Spectrometry
Department of Chemistry CHM 1230
Lab notes for Expt. 1C (pp. 33-42) and for Expt. 2 (pp. 43-57).
Experiment 1C: Some Measurements of Mass and Volume
This Presentation AUTHORED BY PROF. David M. Coleman. FOR PERSONAL STUDY USE ONLY by students enrolled in CHM 1230. All rights reserved. MAY NOT BE COPIED OR DISTRIBUTED FOR ANY OTHER PURPOSE. SOME PORTIONS ARE Copyright 2005 by Houghton Mifflin Company. Text: Ebbing, 8th Ed. Appreciation also expressed to Drs. Maryfran Barber and Martin McClain for valuable contributions.
Background
In this experiment you will work with a balance and a pipet. It is an opportunity to learn how to properly use these instruments. It is also an opportunity to introduce you to some common statistical calculations. The procedure is 1. You will deliver a certain volume of water (10 cc) using a pipet, and 2. You will measure the mass of the water delivered using a balance The ratio mass / volume is the density : d = m / V. Therefore V = m / d.
Take the density from Table 1C.1, p. 35, which shows how the density of water varies with temperature The pipette is more difficult to use accurately than the balance, so you will weigh four deliveries of 10 cc, and then find the amounts more accurately by weighing the water and dividing by its known density. This will give you a feeling for the accuracy of volumetric transfers, as done by a person who is just starting to learn pipette technique. The small differences in the weight of the water delivered are "random errors", and you will also learn to report the mean and standard deviation of data with random error in it.
Appendix A. Mistakes, Errors, Accuracy and Precision Random Error random error is as likely to make a value too large as it is to make a value too small random error affects precision; the larger the random error, the lower the precision and the larger the range of values when a large number of measurements is made, you can construct a "distribution curve" for the random error. The wider the distribution curve the lower the precision; the more narrow the distribution the greater the precision.
Systematic Error systematic error always impacts a measurement in the same direction, making it always either too large or too small. Example: A poorly calibrated pipette. systematic error affects accuracy; the larger the systematic error, the lower the accuracy
Page <#>
Precision and Standard Deviation
standard deviation is a a single number that measures the width of the distribution: the larger the standard deviation, the less precise the measurement
Precision and Significant Figures the standard deviation should be rounded to 1 significant figure; the measurement should then be reported so that its least significant digit corresponds to the standard deviation digit. For example, suppose the mean is 1.24366 and the standard deviation is 0. 00372. Then you should report 1.244 0.004 . Significant Figures in Calculations See Wentworth (lab manual) pages 469470 or Ebbing (CHM 1220/1225 text book) pages 1721.
Good Accuracy & Good Precision
Accuracy vs. Precision
Good Accuracy & Poor Precision
Poor Accuracy & Good Precision
Page <#>
Poor Accuracy & Poor Precision
Performing the Experiment Since density varies with the temperature of water, fill a beaker of water as soon as you come to lab and put a thermometer in it. Let its temperature stabilize before you measure volume. Be sure to record the stable temperature of the water. All data must be recorded in ink. You are required to obtain your TA's initials on your data before you leave the lab. After you have performed the measurements required, you will calculate the standard deviation in two ways: by hand and using a spreadsheet program. Directions for each follow.
Pipette Technique How to set meniscus on a pipette
Bad Technique
Analytical Balance
Page <#>
Safety
In addition to the standard precautions Never use your mouth to pipet use a bulb.
Conclusions and Report
Your conclusion consists of identifying the precision you had in your volume measurements in two ways (by hand and a using spreadsheet program). Your report must include the data sheets and answers to all Questions (1a, 1b, 1c, 1d, 2a, 2b, 3, and 4<-NOT optional), as well as those at the end of the experiment, pages 40-42.
Background
Department of Chemistry CHM 1230
Read Ebbing and Gammon, Section 2.4 . In this experiment you will use the mass spectra given in your lab manual to determine the isotopes present and the molecular weight of an element. For compounds, you will identify each of the various fragments present and determine the molecular weight of the compound.
In performing this experiment the most critical aspect is reading the correct number of decimal places from your ruler. The ruler is marked to 0.1 cm. As a result your measurement is exact to 0.1 cm; you must estimate one more digit. Record your measurements to 0.01 cm. All data must be recorded in ink. You are required to obtain your TA's initials on your data before you leave the lab.
Experiment 2: Isotopes and Mass Spectrometry
Atomic Theory of Matter (Historical Perspective)
Nuclear Model of the Atom E. Rutherford (1911) Nuclei ca. 10-15 M (10-5 !) Atomic dia. ca 10-10 M (1 A)
Atomic Theory of Matter (Term Notation)
Atomic Number (Z): # of protons in nucleus of an atom. Element: substance whose atoms all have the same atomic number (Z) Neutron: nuclear particle w/ mass ! mass of proton but charge = 0. Mass Number (A): # protons + # neutrons e.g. Write nuclide for nucleus w/ 19 protons and 20 neutrons:
Not to scale!!
3 miles
golf ball
39
19
K
Isotopes: Atoms with same # of protons but different number of neutrons. (e.g. C12 vs. C13)
Page <#>
Representation of 2 Isotopes of Carbon
Simple Mass Spectrometer
02_12
Simple Mass Spectrometer
Mass Spectrometry
Fractional abundance
20.1719
10Ne
18
19
20
21
22
Atomic mass (amu)
Mass Spectrometry
Mass Spectrometry
Page <#>
Determine the Atomic Weight from Isotopic Masses and Fractional Abundances
An element has 4 naturally occuring isotopes. The mass and percentage abundance of each isotope is: % Abun Mass Fraction amu 1.48 203.973 x 0.0148 3.018 23.6 205.9745 x 0.236 48.609 22.6 206.9759 x 0.226 46.777 52.3 207.9766 x 0.523 108.772 207.176
Determine the Atomic Weight from Isotopic Masses and Fractional Abundances
An element has 4 naturally occurring isotopes. The mass and percentage abundance of each isotope is: % Abun Mass Fraction amu 1.48 203.973 x 0.0148 3.018 23.6 205.9745 x 0.236 48.609 22.6 206.9759 x 0.226 46.777 52.3 207.9766 x 0.523 108.772 207.176 What is atomic weight and name of element?? Atomic Weight = 207 Element = lead (Pb)
Determine the Atomic Weight from Isotopic Masses and Fractional Abundances
An element has 4 naturally occuring isotopes. The mass and percentage abundance of each isotope is: % Abun Mass Fraction amu 1.48 203.973 x 0.0148 3.018 23.6 205.9745 x 0.236 48.609 22.6 206.9759 x 0.226 46.777 52.3 207.9766 x 0.523 108.772 207.176 What is atomic weight and name of element?? Atomic Weight = 207 Element = lead (Pb)
Percent abundance
Look at Fig. 2.3, a mass spectrum of elemental mercury. You see six isotope lines. Measure all of them as accurately as possible. Add the heights and divide each individual height by the sum of the heights. These numbers, all between 0 and 1, are the fractional abundances of the isotopes. Now multiply each fractional abundance by 100. There are the percent abundances.
Conclusions and Report
Your conclusions consist of your answers to the questions (including the Optional Questions) at the end of the experiment. Your report includes the data sheets and answers to all Questions (1, 2, and 3) at the end of the experiment, page 55-56.
Fragmentation
All the mass lines seen are the masses of positive ions. The heaviest one is often the parent molecule that has simply lost one electron. But very frequently the ionization process also knocks off atoms, so you see mass lines that correspond to molecular fragments that have been made into positive ions. The HCl spectrum shows this very clearly.
Page <#>
Safety In addition to the standard precautions This is a "dry" lab. Don't fall off your lab stool.
Page <#>