Chem+1+Unit+6+Lab


 * Formula of an Ionic Compound - Balancing Charges on Ions **

Atoms of different elements combine with one another to form compounds. The formula of an ionic compound indicates the kinds of atoms that are present in the compound as well as the relative number (ratio) of each kind of atom. This lab investigates how the formula of an ionic compound can be determined experimentally.
 * Introduction ** :

Some ions consist of a charged group of covalently bonded atoms. Such ions are called // polyatomic ions //. An example is the nitrate ion (NO3-), which contains one nitrogen atom and three oxygen atoms and has an overall charge of 1-. In calcium nitrate, calcium (Ca2+) ions combine with nitrate ions in a 1:2 ratio in order to balance the positive and negative charges. The formula for calcium nitrate is Ca(NO3)2. Parentheses are used around the nitrate ion to show that the subscript “2” pertains to the nitrate ion as a whole. The overall charge on ionic compounds is always zero.
 * Background ** :

The purpose of this experiment is to determine the formula of an unknown ionic compound. Two solutions containing equal amounts (concentrations) of two reactant ions will be combined in a series of reactions. In each reaction, the total volume of the two solutions will be held constant while the volume ratio of the reactants is varied. The amount of precipitate obtained in each reaction will be measured and plotted against the volume ratio to find the formula of the product.
 * Experiment Overview: **


 * Procedure: **
 * 1) Record the color of the 0.1 M (M is molarity, an expression of concentation) copper (II) chloride solution in the data table.
 * 2) Carefully add the appropriate number of drops of copper (II) chloride solution to each test tube #1-7, as shown in Table- 1. **// Note //** : Exact volumes are very important.
 * 3) Record the color of the 0.1 M sodium phosphate solution in the data table.
 * 4) Carefully add the appropriate number of drops of sodium phosphate solution to each test tube, as shown in Table-1
 * 5) Use a clean stirring rod to stir each reaction mixture in test tubes #1-7. Let the tubes sit undisturbed for 10-15 minutes to allow the precipitate to settle.
 * 6) During this time, determine the volume (drop) ratio of copper (II) chloride and sodium phosphate solutions in each test tube. Write this ratio in the data table. // Example // : In test tube #1, 3 drops of CuCl2and 27 drops of Na3PO4correspond to a 1:9 ratio of CuCl2:Na3PO4.
 * 7) After the precipitates have settled, observe the appearance of the products (both the solid and the solution). Record the observations in the data table in the space provided. Be as detailed as possible.
 * 8) Use a metric ruler to measure the height of the precipitate in millimeters in each test tube. Read from the top of the solid material to the bottom center of the test tube. Record each height in mm in the data table.


 * Table 1: **


 * ** Test Tube ** ||  ** 1 **  ||  ** 2 **  ||  ** 3 **  ||  ** 4 **  ||  ** 5 **  ||  ** 6 **  ||  ** 7 **  ||
 * CuCl2, 0.1 M (drops) || 3 || 6 || 12 || 15 || 18 || 24 || 27 ||
 * Na3PO4, 0.1 M (drops) || 27 || 24 || 18 || 15 || 12 || 6 || 3 ||

Color of copper (II) chloride solution: Color of sodium phosphate solution: Color of precipitate (solid):
 * Data: **


 * Data Table: **

(drops of CuCl2: drops Na3PO4) || 1:9 ||  ||   ||   ||   ||   ||   || // *reduce the volume ratio to the simplest whole-number ratio. //
 * ** Precipitation Reactions ** ||
 * ** Test Tube ** || ** 1 ** || ** 2 ** || ** 3 ** || ** 4 ** || ** 5 ** || ** 6 ** || ** 7 ** ||
 * Volume Ratio*
 * Height of Precipitate (mm) ||  ||   ||   ||   ||   ||   ||   ||

1. The reaction that occurred was: copper (II) chloride + sodium phosphate -> copper (II) phosphate + sodium chloride
 * Questions: **

Write the formulas of the products: copper (II) phosphate:

sodium chloride:

a. Based on common knowledge, which of the products above is likely to be soluble in water? b.The other product must be insoluble in water and therefore precipitate from the solution. Which product formed the solid precipitate? c. What is the ratio of ions in the precipitate above:

2. Complete a bar graph to show the height of the precipitate in each test tube. 3. Which test tube had the greatest amount of precipitate? What is the ratio corresponding to this test tube # from the data table? 4. Are the ratios in 1c and 3a in agreement? 5. Which test tubes showed evidence of unreacted Cu2+ ions in the supernatant when the reaction was complete? a. Explain why unreacted Cu2+ ions were present in these tubes based on the volume ratio of solutions used. b. How could you tell that all of the Cu2+ ions had reacted in a particular test tube? Which test tubes showed such evidence? Explain, based on the volume ratio of solutions used.