10/03/08

Report 4. Do not forget to include a sample of each type of the calculation performed in the report. Include a sample calculation for each data table that you have (three total). Show a sample for one trial only. You DO NOT have to show a sample calculation for any of the average or standard deviation calculations.
Remember, you have to have knowledge of the "true value" to be able to evaluate the accuracy of your results. The precision, however, is evaluated in terms of the standard deviation values. The smaller the standard deviation is relative to the average, the more precise is your result.

Post-lab 4, question 3. This is a limiting reactant problem. First, you need to determine the number of moles of both barium hydroxide and hydrochloric acid available before the reaction starts. Then determine which one of the two reactants is going to be leftover after the reaction is complete. Do not forget to take into account the stoichiometry of your reaction.
For example, reactant A reacts with reactant B according to the following chemical equation:
A + 2B ® products. If 1 mole of A is mixed with 1 mole of B, then according to the stoichiometry of the reaction, only 0.5 mole of A will react with 1 mole of B (before you run out of B). That means that after the reaction is complete, you will be left with the excess of reactant A (0.5 moles). If reactant A is your acid (base), then the resulting solution is going to be acidic (basic).

Post-lab 4, question 5. Determine the stoichiometry for the following three reactions: nitric acid plus barium hydroxide, sulfuric acid plus barium hydroxide, and phosphoric acid plus barium hydroxide. Calculate the number of moles of the acid  and the base that went into a titration based on the information given in the problem. Look at the ratio between the number of moles of the acid and the number of moles of the base calculated, and match it up to the stoichiometry of one of the three chemical reactions. Identify the acid.

Pre-lab 5, question 2. In the molecular formula of Fe(NH₄)₂(SO₄)₂·6H₂O, the six waters at the end are the so called "waters of hydration". By definition, waters of hydration are the water molecules that are part of the crystalline structure of the compound and are present in a fixed molar ratio compared to the number of moles of the compound itself. To calculate the molecular weight of the hydrated compound, you need to add the mass of the waters of hydration to the mass of the compound itself.

09/26/08

There is a TYPO in the manual on page 24, Post-lab question 4. It reads: "Write molecular equations for each of the net ionic equations you wrote in problem 2." It should read: "Write molecular equations for each of the net ionic equations you wrote in problem 3."

Report 3. In the Discussion, you have to provide 15 balanced molecular and 15 net ionic equations for the 15 precipitates that you observed in the lab (30 reactions total). That does not include the five "false positives": Na₂S + acids, Na₂S + Ba(NO₃)₂, and NaOH + Ba(NO₃)₂.

Post-lab 3, question 2. You have two beakers containing two different aqueous solutions. One is the solution of sulfuric acid, H₂SO₄, the other - nitric acid, HNO₃. The beakers are not labeled, so you don't know which one is which. Your task is to choose a specific reagent that, when added to both solutions, would selectively form a precipitate in one but not the other, by that allowing you to label the beakers.
For example, if you add a Ba(NO₃)₂ (aq) reagent to both sulfuric and nitric acid solutions, you will see a precipitate formation in the beaker that contains sulfuric acid due to the  formation of BaSO₄ (s). No visible change will be observed in the nitric acid solution. Use Report 3 and Appendix 2 for reference.

09/19/08

Report 2. All of the calculations for this lab can be easily done in Excel, so go ahead and use it. If you do not know how to use Excel, here is a link to a tutorial that can help you get started:
http://www.ischool.utexas.edu/technology/tutorials/office/excel/.
Calculate the averages and the standard deviations for the percentages only, not for the masses (be sure to report them using the correct number of significant figures).
Apply the Q-test (if necessary) to the individual sets of data for:
1) percent of NaCl in the mixture
2) percent of SiO₂ in the mixture
3) percent of CaCO₃ in the mixture
If you do use the Q-test to identify your outliers, you have to show your work (include it into your report under Sample Calculations).
There is no need to apply the Q-test to the total percent recovery data set. If you find an outlier in an individual set of data for either of the components, you can exclude the corresponding total percent recovery from the average and the standard deviation calculations.
Sample Report for Experiment 2 is posted on this website under Sample Report.

09/12/08

Report 1. Sample Report for Experiment 1 is posted on this website under Sample Report. Use it as a reference. Do not forget to include a sample calculation in your report.

Be sure to use the correct number of significant figures when you report your results as Average ± Standard Deviation in your first report.

To be able to evaluate the accuracy of your measurement, you need to compare it to the "true value" (best estimate). In the first approximation, you can use density of water as a point of reference for the density of Diet Coke. A can of Coke states that it has 39 g of sugar as high fructose corn syrup and/or sucrose in 355 ml of soda. That means that Coke is an 11% by weight sugar  solution. The densities of the 11% fructose and sucrose aqueous solutions are 1.0427 g/cm³ and 1.0423 g/cm³ at 20 °C, respectively.

Quiz 1. We are going to have our first quiz next week. This first quiz will be on the material of Experiment 1. To prepare, you need to concentrate on the following:
1) Learn the definitions for all the terms introduced in class (see lecture notes)
2) Understand your experimental procedure (see lecture notes, pre-lab questions, and discussion questions)
3) Understand how to do post-lab type problems (see post-lab).

09/05/08

Make sure to have your Pre-lab 1 done before coming to class. Have it done on a separate page of your notebook, so that you can easily tear out a corresponding copy page and submit it to your TA for grading. Sample Preliminary Write-up for Experiment 1 is posted on this website under Sample Report. You do not have to turn it in next week. Rather you will work with it and it will eventually become your Report 1.

08/21/2008

There will be no classes and no labs on August 27 - September 2 for any of the CH204 sections. We will start on September 3. Please, see your course schedule for respective days and times for your unique number.

In the meantime, you can go ahead and get yourself a Lab Manual (available at the University Co-op), a bound Laboratory Notebook with duplicate numbered pages (sold by the Student Affiliates of the American Chemical Society in the main hallway of the Welch building), and a Combination Lock.