Empirical Formula of Copper Chloride
Empirical Formula: the simplest whole-number ratio in which different kinds of atoms combine to form a compound (atoms combining as single, distinct units). For example,
For ionic compounds: Compound formula is the same as the empirical formula. The compound formula defines the formula unit, the simplest whole-number ratio of positive and negative ions giving an electrically neutral unit.
|name||Empirical Formula||Molecular Formula
Empirical Formulas and mol: The empirical formula is the simplest whole-number ratio of numbers of mols of atoms in one mol of a compound. The mole (mol) is that quantity of matter possessing a mass equal to the formula weight expressed grams. For example:
Empirical formula of Copper Chloride: CuxCly
|Cu (a monatomic element)||H2O||Al2O3
|Atomic Wt.: 63.546||Formula Wt.: 18.016||Formula Wt.: 102.
|Mass of 1 mol = 63.546 g||Mass of 1 mol = 18.016 g||Mass of 1 mol = 102. g
|63.546 g Cu/mol Cu||18.016 g H2O/mol H2O||102. g Al2O3/mol Al2O3
|If the compound is made of ... ||then ... Cl- ions are needed||and the formula would be ...
|Cu1+ and Cl1-||1||CuCl
|Cu2+ and Cl1-||2||CuCl2
|Cu3+ and Cl1-||3||CuCl3
You may work in pairs if you wish.
Producing solid Cu from a Cu(II) solution by reaction with Zn.
|CuxCly (aq) + Zn (s) ---> ||ZnCl2 (aq) + Cu (s)
- Obtain about 25 mL of copper chloride solution in a graduated cylinder. (Be sure to measure the volume to +/- 0.2 mL.) Transfer to a reaction beaker; rinse the graduated cylinder and add the rinse to the beaker.
- Obtain a piece of Zn and obtain its mass to +/- 0.01 g.
- Handling the Zn with tongs, add it to the reaction vessel. Carefully tilt beaker so solution covers much of the Zn.
- Use the rubber policeman attached to your stirring rod to periodically scrape solid Cu from the Zn piece into the solution.
- After the reaction has stopped, add 5-10 drops of HCl solution and stir.
- Using tongs, remove the Zn from the beaker, making sure to leave behind adhering Cu. Dry the Zn with a paper towel and obtain its mass, then dispose of the Zn in a container under the fume hood.
- Carefully decant (pour off) the supernatant liquid from the solid Cu into a waste beaker.
- Wash the solid Cu by adding about 10 mL distilled water to it, stirring vigorously, then decanting the rinse water into your waste beaker. This will remove dissolved ZnCl2.
Removing water from the solid Cu
- Add about 10 mL isopropyl alcohol (isopropanol) to the Cu, stir thoroughly, and decant the alcohol into the waste beaker. Repeat this step twice more.
- Set up a boiling water bath on a hot plate using a 250-mL beaker almost half full of water. Add 1 or 2 boiling stones to the water. You will use this water bath later to dry the Cu.
- Wipe clean an evaporating dish with your cloth towel. Obtain the mass of the dish to +/- 0.01 g.
- Use the rubber policeman to transfer the Cu from the reaction beaker to the evaporating dish. Make sure to transfer every trace of solid Cu. You may use a bit of isopropanol.
- Place the evaporating dish on the water bath and heat the water to boiling.
- When the Cu appears almost dry, stir the Cu and continue heating. When it appears completely dry, use tongs to place the evaporating dish on a wire gauze on the lab bench. Allow the dish and contents to cool to room temperature.
- Discard the contents of your waste beaker into the waste container labeled Discarded Zn(II) Solutions.
- Determine the mass of the dish and its contents to +/- 0.01 g.
- Using tongs, place the evaporating dish back on the water bath and heat for an additional 5 minutes.
- As before, remove the dish, let it cool, and weigh it. Repeat the heating, cooling, and weighing until successive weighings are within +/- 0.05 g.
- Discard the Cu(s) in the waste container labeled Discarded Cu Residue.
- Calculate numbers of mols in original copper chloride sample
g Cl = g copper chloride - g Cu
|1 mol Cu
|Let a = mols Cu = g Cu x||------------
|63.55 g Cu|
|1 mol Cl
|Let b = mols Cl = g Cl x||------------
|35.45 g Cl|
- Write a tentative formula: CuClb/a where a = mols Cu and b = mols Cl. The ratio b/a is the number of moles of Cl needed for every mole of Cu in the compound.
- If necessary, convert these ratios to whole numbers. For example, if you get ...
- ... CuCl0.96, you have approximately CuCl1, that is, CuCl.
- ... CuCl0.48, you have Cu2Cl0.96 (because 0.48 moles Cl for every mole of Cu is the same ratio as 0.96 moles of Cl for every 2 moles of Cu), which is approximately Cu2Cl.
- ... CuCl1.5, you have Cu2Cl3.
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