Introduction to Chemical Analysis SCHA 265 Updated 12/03/06

Experiment 1.

Balance Practice / Unit Conversions and Statistics

In this lab you will practice using the balances and you will apply some simple statistics tools to make inferences about the data that you collect.

The analytes for the course will be coins of the United States and Canada. We are going to ask if these coins are different in some way. Since we will be doing weighing then we are really asking is if they have different weights. We will also be testing the balances. In reality the weighing process is very good and is not often the basis of problems in an analysis. One should be sure of this however and that is the reason for using a check weight.

Balance Procedures (See AnSOP002)

In this course and in any chemistry lab you should always try to use the same balance. We use balances that read to the nearest 0.1 mg (about the mass of one inch of grey hair).

You should always use a weighing dish when weighing. You should never touch items to be weighed with you bare fingers. Use tongs, gloves or a paper towel.

Always clean up the balance area. The little brushes that are around the balances are there for that purpose. If you have a spill, clean it up!!

You should always have a check weight when working in an analytical lab. I would suggest that you use a large paper clip that you will store in a capped scintillation vial. Weigh this item at the start of each days work and record it in you lab book. This gives you a quick check on balance performance.

Part I (Points used for score weighting are listed in the parentheses – i.e. make sure you have made a entry at each place)

Collect a bag of 10 pennies. Weight each coin and record the weight. Also record any observations you make about the coin. I.e. shiny, dull, dirty, year of mintage, mint etc. Prepare a properly formatted Table with your observations. (10), you may wish to prepare the Table to accommodate the next two steps.

Arrange the weights in weight order. Do any values look different? Can you reject any value using the Q-test? (Harris – Chapter 4 page 65) (5) If you can reject a coin, can you reject any others? Show if you can, or can not (5)

Report the weight of a US cent coin as the average of you final set (after removing any values rejected above) plus or minus the standard deviation. Also report any observation you might make about this set. (10)

Find the “correct” weight of the penny on the internet or other reference source. Is your value consistent with this published value at 90% confidence? (Chapter 4, page 69 case 1) Show how you do this (10)

Part II

Find another student that has determined the mass for the penny to be radically different from the value you determined. (Some will find a mass over 3 grams while others will find a mass less than 3 grams). Test to see if your weights are different at 95% confidence. You might need to do a pooled standard deviation. Chapter 4, page 69 case 2) (10)

Part III

Weigh two pennies that are available in the two penny baggies. Which penny matches your value? Explain why there is a difference. (5)

Part IV

Now we want to check to see if two balances give the same readings. We could just weight a number of pennies on each balance and compare the means like in part II. But let us practice with a different statistical tool. Often your analyte will be very limited and you can not do a large number of analyses (in many analyses the sample is destroyed in the process – however, we hope that your coins survive the weighing process). You might split the analyte into two subsets and analyze by the two different methods and then compare the values from these two different methods. You will be given a set of coins and you will want to weigh each one on each balance. Now perform a Case 3 analysis on the data to see the balances give the same values. Report if there is a difference at 99.0 % confidence. Prepare a Table (10), Case 3 test (10) conclusion statement. (5)

Part V

Weigh a piece of Styrofoam on the balance. Report its correct weight after doing a buoyancy correction. The density I found for Styrofoam is 12 kg/m3. The density of the steel weights is 8.0 + 0.5 g/mL and the density of air is 0.0012 + 0.0001 g/mL. Report the corrected weight of a penny for buoyancy. The diameter of a penny is 19.05 mm and its thickness is 1.55 mm (10)

Bonus, determine the density of the Styrofoam that we used in lab. (20). Discuss how you plan to do this with your lab instructor.

Questions

In this lab we have been determining the weight of various things. You reported your results in grams. This is bad science! Tell me why these units might not be appropriate! (5) What is the English system unit equivalent for grams? (5) (This question cuts to a very important but ambiguous distinction between the English and Metric systems.)

You have solid block that has a mass of 10.00 grams on the International Space Station. Convert this to the proper equivalent in the English system. (5)

As an example of the proliferation of different units used in the English system go ahead and determine how many pennyweights are there in a new Lincoln US cent coin. (5)

There is a new movement about to redefine the temperature scale. This group would like to define the new scale when the freezing point of mercury is 0o Hg and the boiling point of mercury is 100o Hg. What is the boiling point of water on this new scale? Provide me with the conversion formula. (20)

Report

For this lab you can just do a linear presentation of your calculations (showing formulas used), conclusions and answers to your questions.

You should use Word to prepare your report. A good portion of this lab is learning how to use Tables and scientific features of Word. You will be using the equation editor too.