UW Department of Chemistry Lab Lectures Online

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1 Lab : Chemical Composition of Compounds Part I. Identification Based on Percent Metal Compositions Part II. Determination of Percent KClO 3 in a KCl/KClO 3 Mixture Introduction Often in research and industry, it is necessary to know the elemental composition of a sample. There are many techniques to determine composition. In this experiment, you will synthesize either magnesium oxide or magnesium nitride and, from the starting mass of the magnesium, you will determine which reaction occurs by calculating the % magnesium in your product. You will also decompose copper(ii) carbonate hydroxide to determine the % copper and, from the % copper, identify the compound as either Cu (CO 3 )(OH) or Cu 3 (CO 3 ) (OH). Lastly, you will use information obtained from the decomposition reaction for KClO 3 to determine how much KClO 3 was present in your sample mixture. Part I. A. Magnesium The composition of air is 78% nitrogen, 1% oxygen, and 1% other gases. Most metals, when heated in air, react with either the oxygen in the air to form a metal oxide or with nitrogen to form a metal nitride. The temperature at which these reactions occur varies with the type of metal. The most reactive metals are those of groups I and II of the Periodic Table. For some metals, such as aluminum, all of the metal will not react before the surface of the metal is coated with the metal oxide, preventing further oxidation. In this experiment you will attempt to oxidize a magnesium metal sample by heating it in air: Mg(s) + O(g) MgO(s) Magnesium could instead react with the nitrogen in the air to form magnesium nitride: 3 Mg(s) + N (g) Mg3N (s) By determining the % Mg in your product, you can identify the product as being either MgO or Mg 3 N. From the mass of magnesium before heating and the mass of the product, you can calculate the percent Mg in the product: Mass Mg % Mg 100% Mass Product You will then compare your experimental value for % Mg to the theoretical % Mg values for both MgO and Mg 3 N, as calculated from the Periodic Table. By collecting data from the other groups in your section, you will also be able to calculate an average and standard deviation for the data and evaluate the class s data compared to the theoretical values for % Mg in the two possible products. 1 of 5

2 B. Copper in Copper(II) Carbonate Hydroxide Copper(II) carbonate exists as a solid in combination with copper hydroxide and can be in two possible forms: Cu (CO 3 )(OH) or Cu 3 (CO 3 ) (OH). When heated above 00 C, Cu (CO 3 )(OH) decomposes to form copper(ii) oxide (s), carbon dioxide (g), and steam: Cu CO (OH) (s) CuO(s) + CO (g) + H O(g) 3 Heating of Cu 3 (CO 3 ) (OH) yields the same products, but in different ratios: Cu (CO ) (OH) (s) 3CuO(s) + CO (g) + H O(g) 3 3 You will be given an unknown sample of copper(ii) carbonate hydroxide and attempt to determine, using information from these reactions, which of the two forms your unknown is. The mass of Cu in the CuO (s) product will be determined by measuring the mass of CuO formed and multiplying that mass times the percent Cu in CuO (see Helpful Information section below). Since there is no other source of Cu besides the unknown sample, all of the Cu in the product must have come from the original sample. From the mass of the original sample and the mass of the Cu found in the product, the % Cu in the unknown sample can be determined: Cu in Sample Product (CuO) x % Cu in CuO Cu in Sample % Cu in Sample 100% Reactant The experimentally determined % Cu can then be compared to the theoretical % Cu calculated for each of the two possible compounds and the unknown sample can be identified. Part II. Percent KClO 3 in a KCl/KClO 3 Mixture KClO 3 can be decomposed according to the following equation: KClO (s) KCl(s) + 3O (g) 3 This reaction is one in which the absorption of heat, designated by the triangle above the reaction arrow, results in the breaking of bonds. If you heat a mixture of KCl and KClO 3, only the KClO 3 will decompose. The KCl from the mixture and the KCl formed from the decomposition KClO 3 will remain in the crucible, but the mass of your product will be less than of 5

3 the mass of your starting sample. If you carefully heat a known mass of KCl/KClO 3 in a crucible, any decrease in mass can be attributed to the oxygen (O ) that escapes from the crucible as a gas. The change in mass, along with the percent oxygen in KClO 3 you calculate from the Periodic Table, will allow you to determine the amount of KClO 3 in the original sample mixture. The prelab assignment on WebAssign focuses on the following: Stepwise calculation of the percent KClO 3 in a KCl/KClO 3 mixture Calculating the % oxygen in KClO 3, based on the atomic mass of each the elements listed on the Periodic Table Calculating the grams of oxygen in 100 grams of KClO 3 (once you know the % O in KClO 3 from the previous question) Figuring out what mass of KClO 3 contains a given mass of oxygen (using the number of grams of oxygen in 100 grams of KClO 3 from the previous question) Calculating the % KClO 3 in the mixture if you know the mass of oxygen lost when a specific mass of KCl/KClO 3 mixture is heated in a crucible Calculating the % Cu in a sample of Cu (CO 3 )(OH) or Cu 3 (CO 3 ) (OH) if the mass of the sample and the mass of the CuO product remaining in the crucible are known. Helpful information Unit conversions and dimensional analysis are covered in Appendix Two (section A.) in Zumdahl. This section may be helpful when figuring out how to perform some of the required calculations for this lab. A systematic approach, in which you analyze the units as well as the numbers, will verify that you have done the calculation correctly. As you saw in Lab 1, to convert from mass to volume, you need to divide the mass by the density. When you do this, the units of grams cancel out and you are left with milliliters. If you were to accidentally multiply by the density, the resulting units would be g /ml, which is obviously incorrect if you are solving for volume. 3 of 5

4 Calculating % composition is covered in Zumdahl Chapter 3, Section 4. This consists of comparing the mass of each element present in 1 mole of the compound (from the Periodic Table) with the total mass of 1 mole of the compound. Let s look at water (H O) as an example: H O ( mol) ( 1mol) g mol g mol.016 g g 1mol H O g Mass % of mass of H in 1mol H O H in H O mass of 1mol H O.016 g x % g x 100 Another way to present % composition is to give the fraction of hydrogen in H O. The 11.19% calculated above is the same as saying that there are g of H in 100 g of H O. This is helpful if you know the mass of water and want to determine the mass of hydrogen present g H 100 g H O ( 5.0 g H O). 80 g H In Lab 1, we learned about averages and standard deviation in physical measurements. With any experiment, there is always the chance that there is error associated with your results. Doing multiple trials can help you determine what the true answer probably is, but you will not have enough time in any of the lab sessions to do more than a couple of trials, at most. For Part I of this lab, you are asked to compare your measured experimental values with known, or theoretical, percent composition values in order to determine the identity of the compounds involved. In Part I A. you will determine if your product is MgO or Mg 3 N and in Part I B. you will figure out which copper-containing compound you started with. In order to justify your decision about which compounds you have, you will collect data from all of the groups in your lab and find the average and standard deviation for each measurement. By combining an average and standard deviation, you can calculate a range of values to use for comparison with a set of known values: average: 50%, standard deviation: ±5% range: % 4 of 5

5 If the theoretical value for % Mg in MgO, which you calculate from the Periodic Table, falls within the range of experimentally determined values for the Mg product, then most of the class produced MgO during the reaction. If it is the % Mg in Mg 3 N that falls within the range, then the class produced Mg 3 N. This will be a fairly straightforward decision for Part I A. For Part I B., the two copper-containing compounds have % Cu values very close together. It may be that the range determined from the class data for Part I B. may include both, or neither, of the theoretical % Cu values, which may make it difficult or impossible to tell for sure which compound you started with. In the post lab report, you will be asked to evaluate the results from the class, as well as your own individual values, compared to the theoretical values and comment on the accuracy of the data collected. Safety Considerations Be sure to follow the instructions for the disposal of the solid waste from this lab. The biggest safety concerns for this lab are the open flames and hot crucibles. Keep your lab bench neat and any long hair tied back so that nothing is at risk of catching on fire. Remember that hot crucibles and room temperature crucibles look exactly the same. Once you have washed your crucibles and covers at the beginning of lab, only use the tongs to handle them from that point on. The crucibles are porcelain and will break if dropped. Practice using the tongs to transport the crucibles and their lids before they are hot or are full of the product that you still need to weigh. 5 of 5