Percent Copper and Molecular Mass of a Copper Compound

[Pages:6]Percent Copper and Molecular Mass of a Copper Compound

Prelab Assignment Read the entire lab. Write an objective and any hazards associated with this lab in your laboratory notebook. Answer the following three questions in your laboratory notebook before coming to lab.

1. List at least 4 changes that indicate a chemical reaction is or has occurred.

2. Calculate the percent by weight of copper in copper (II) sulfate, CuSO4.

3. Calculate the molar mass of an unknown copper compound from the data below. Assume one copper atom per formula unit or n=1. The equation you need for this calculation is in the following lab write up.

Mass of the test tube and unknown sample Mass of the empty test tube Mass of empty filter paper Mass of filter paper and copper

14.653g 12.876g 0.551g 1.098g

Introduction The law of definite proportions states that in a given compound, the constituent

elements are always combined in the same proportions. An empirical formula is based on the smallest ratio of the number of moles of each element in the compound. A molecular formula gives the actual number of moles of each element in a molecule of the compound.

The first step in the determination of the percent copper in a copper-containing compound is to use a type of chemical reaction called a reduction to convert copper (II) ions (Cu2+) in a weighed sample of the compound of interest to metallic copper, by using a reducing agent, in this case magnesium metal. Copper nitrate is an example of a soluble copper salt containing the copper (II) ion. The net ionic equation for the reduction of the copper (II) ion to copper metal by magnesium is

Cu2+(aq) + Mg(s) Cu(s) + Mg2+(aq).

Use of excess metallic magnesium ensures complete reduction of the copper (II) ion and complete removal of the copper (II) ion from solution. After the removal of copper from solution the addition of hydrochloric acid dissolves the excess magnesium producing hydrogen gas.

In the second step of this lab exercise, the metallic copper produced in the first step will be separated from the solution by filtration. The metallic copper collected on the filter will be washed, dried, and weighed. The percent copper in the copper containing compound under study can now be calculated as

% Cu =

Recall that the theoretical percent copper in a compound can be calculated as

Theoretical % Cu =

.

If we substitute the molar mass of Cu, 63.55 g/mole, into the previous equation, we obtain the following mathematical formula which represents the relationship between the theoretical percent copper in a particular copper compound and the compound's molar mass

% Cu =

,

where n is the moles of copper in the compound. We can solve this equation to yield the molar mass of an unknown copper compound:

molar mass of unknown copper compound =

.

Your instructor will tell you if your unknown compound contains more than one mole of copper.

CAUTIONS: - Wear safety goggles during the entire experiment. - Wear gloves when handling the 6 M HCl.

Procedure: Wash two 250 mL beakers and rinse them with distilled water. Dry the beakers.

Label one beaker 1. Weigh the beaker and record its weight. Add approximately 0.75 ? 0.8 g of one of the unknown compounds to beaker 1. Weigh the beaker and unknown compound and record the weight. Label the second beaker 2. Weigh the beaker and record its weight. Add approximately 0.75 ? 0.8 g of the same unknown compound added to beaker 1 to beaker 2. Weigh the beaker and unknown compound and record the weight.

Place a clean stirring rod fitted with a rubber tip (rubber policeman) in each beaker. (Note: Leave each stirring rod in its own beaker for the rest of this lab exercise.) Carefully add about 40 mL of distilled water to each beaker. Pour the distilled water along the stirring rod held low in the beaker to prevent splashing.

Use a graduated cylinder to measure 6 mL of 6 M HCl. Add 6 mL of 6 M HCl to each beaker. Stir until the solid is dissolved. If solid remains after stirring add 6 M HCl in 2 mL increments. Stir at least one minute after each HCl addition until the solid dissolves. Avoid adding any more HCl than necessary. Remember to record your observations in your lab notebook.

Add 0.35 g of magnesium turnings to each beaker and stir well. Stir frequently but gently for at least 30 minutes. There should be solid copper on the bottom of the beakers and the solution should be colorless. If the solution becomes cloudy during this time Mg(OH)2 may have formed. Add 6 M HCl drop by drop with stirring until the Mg(OH)2 is dissolved and the solution again becomes clear.

Add about 2 mL of 6 M HCl to each beaker to dissolve any unreacted Mg. If no hydrogen bubbles are produced, you may not have added sufficient Mg to react with all of the Cu in your sample. Add about 0.1 g Mg turnings (1-2 pieces of Mg) and stir frequently for 5 min. When the rate of hydrogen gas production slows, add about 1 mL more HCl until the production of hydrogen gas has stopped. Your solution should be completely clear (no trace of blue color should be left) at this point.

Label two pieces of filter paper 1 and 2 with a pencil or water insoluble ink. Also label both pieces of filter paper with your initials or some other form of information that will distinguish your samples from those of all the other students in your lab. Weigh each piece of filter paper individually and record their masses in your lab notebook. Fold the weighed filter paper as illustrated by your instructor.

Place the filter and funnel in an Erlenmeyer flask. Seat (or seal) the filter to the funnel surface by filtering about 5 mL of distilled water through the filter. Gently swirl your copper solution to suspend the solid copper. Quickly and carefully pour the copper solution into the filter and funnel. Make sure you pour the solution from beaker 1 into filter 1. Use your stirring rod and rubber policemen to transfer (push) as much of the solid copper as possible into the filter and funnel. Use a wash bottle of distilled water to wash any remaining copper out of your beaker and into the filter and funnel. Wash the solid copper and the surface of your filter with approximately 5 mL of distilled water from a wash bottle and allow the water to completely drain from the filter. Repeat this washing step at least 5 more times.

Carefully remove the pieces of filter paper and spread them on watch glasses. Allow the filters to dry until your next lab. At the beginning of the next lab when the filter is completely dry, weigh each filter and copper and record the masses in your lab notebook.

Data Unknown selected _________ n value (number of moles copper) in your unknown copper compound _________

Mass of empty clean dry beaker: replicate 1 ______g replicate 2 ______g

Mass of beaker and copper compound: replicate 1 ______g replicate 2 ______g

Mass of clean unused filter: replicate 1 ______g replicate 2 ______g

Mass of filter and copper: replicate 1 ______g replicate 2 ______g

Calculations Mass of beaker and copper compound: replicate 1 ______g

- Mass of empty clean dry beaker:

replicate 1 ______g

Mass of unknown copper compound replicate 1 ________g

replicate 2 ________g replicate 2 ________g replicate 2 ________g

Mass of filter and copper:

replicate 1 ______g

- Mass of clean unused filter:

replicate 1 ______g

Mass of metallic copper in unknown: replicate 1 _______g

replicate 2 ________g replicate 2 ________g replicate 2 ________g

% Cu =

% Cu replicate 1__________ average % Cu __________

% Cu replicate 2___________

molar mass of unknown copper compound =

.

molar mass of unknown copper compound = ______________

Questions: 1. List at least 3 observations you made after adding magnesium to your copper solution that indicate a chemical reaction took place.

2. Calculate the range and standard deviation for your duplicate determinations.

3. How would the following occurrences influence your determination of the percent by weight of copper in the unknown compound? Will your value be too large, too small, or not affected? Will your standard deviation be larger or smaller? Explain your answer. (a) You spill some of the copper sample as you are pouring it into the filter funnel.

(b) The solution was pale green when you filtered the metallic copper from the solution.

(c) No hydrogen gas bubbles were produced when acid was added to dissolve the remaining magnesium in your sample.

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