Upon completion of this experiment, students will:
(CLO4). Interpret and utilize mathematical formulas while solving problems
(MLO3-4) Calculate molar mass and convert between mass and moles
(MLO3-5) Calculate the percent composition of an element or % mass of water of a compound or determine the empirical and molecular formula of a compound from percent composition or from combustion analysis data.
Experiment 6 covers the calculations of molar mass, gram to mole conversion and percent composition.
The following assignments must be completed and submitted in Canvas:
Laboratory Report of the experiment and the CLASS DATA
Post Laboratory Problems and Conclusion paragraph using the RERUNS method
When water combines to forms these types of compounds they are called hydrates or hydrated salts. Hydrated salts are written as the ionic compound dot number of waters, for example, Na2SO4●10H2O. These compounds can easily decompose, losing the water and forming an anhydrous salt upon heating. The percent mass due to the water chelated to the ionic compound will be determined by determine the mass that is lost upon heating. The mass percent of water can be calculated by
By comparing the calculated moles of the dehydrated copper sulfate, we can compare this to the moles of water calculated from the loss in mass when the hydrated salt was heated to dryness, the moles of water and therefore the coefficient of water in the hydrated salt can be determined.
CAYERCHEM - Quick video of heating copper sulfate pentahydrate to an anhydrous powder
Another possible video (in Spanish?) https://youtu.be/Ey1V08MCHj0
In the second part of this experiment a copper (II) compound will be analyzed using the gravimetric technique. The hydrated copper compound, CuXn ● mH2O (solid) upon adding water will form Cu2+ (aq) and Xn+(aq) where X can be NO31- , SO42- , Cl1- , Br1- or C2H3O21-
Upon addition of an excess amount of hydroxide ions (OH-) to a solution containing the copper(II) ions (Cu2+), a blue precipitate of copper(II) hydroxide [Cu(OH)2] is formed.
CuSO4 + 2 NaOH → Cu(OH)2 (s) + Na2SO4
Cu2+ (aq) + 2 OH1- (aq) → Cu(OH)2 (s) (blue)
The Na+ ion reacts with the anion, X, of the soluble copper (II) salt, producing a soluble sodium salt of the anion as the second product in the precipitation reaction. Subsequently heating of the Cu(OH)2 precipitate results in the decomposition to copper(II) oxide (CuO) and water. The sodium salt is not affected by the heat.
Cu(OH)2 (s) → CuO (s) Black + H2O (g)
The CuO solid can be separated from the sodium sulfate solution by filtration to isolate the precipitate, then washed to remove the soluble sodium salt, dried to remove the water and finally weighed.
The net reaction of any copper (II) salt reacting with sodium hydroxide results in the same copper (II) hydroxide and upon heating, copper (II) oxide. The mass percent of an unknown copper (II) compound will be determined. The percent composition of an element in any compound can be calculated if the mass of the element can be determined. Since the copper (II) oxide has a known formula, the amount of copper present in the unknown copper (II) salt is calculated from the moles of CuO produced. The moles of copper present in compound and the mass of the Copper present in the unknown is determined by converting the mass of CuO to moles of CuO which equals the moles of Copper in the unknown and then converting the moles of Copper to mass of copper.
The mass of the CuO precipitate is used to determine the mass of copper present in the unknown copper (II) salt. From the calculated % mass of copper, the identity of the unknown is determined.
Possible unknowns may include: CuSO4·5H2O, Cu(NO3)2·3H2O, CuBr2, CuCl2·2H2O, Cu(C2H3O2)2·H2O
The Mole and Atomic Mass
Percent composition, empirical formulas and molecular formulas
The student should complete two assignments before coming to the laboratory discussion. The first is a pre laboratory writing assignment that will be required for most of the laboratory assignments. The pre writing assignment may consist of up to three parts: a purpose statement of what the objectives of the assignment are, a theory paragraph containing background and an understanding of why we are completing this assignment and finally, a summary paragraph of the procedure explaining how the assignment will be completed.
The student should complete a pre laboratory writing assignment for Experiment 6 that contains a purpose statement and a summary paragraph of the procedure explaining how the assignment will be completed and a list of the data that must be collected while performing this experiment.
A pre writing assignment is included in the example of a formal laboratory report at https://docs.google.com/document/d/1BGBPJS3VqNME4cKDrR50HqqrIYZt0sZ9UCg6IrS3MDk
This assignment will only be accepted before the laboratory session begins. No late assignments will be accepted.
The Pre laboratory problems are questions about the laboratory assignment, background, definitions or procedure and calculations. These problems should be completed BEFORE coming to the class where the assignment will be discussed.
The Laboratory Report of the assignment contains questions, data collection, calculation and reporting of results.
We will use the CLASS DATA to share our collected Data for the Laboratory report.
The percent mass of water in an unknown hydrated ionic compound will be determined by heating the compound to dryness. From the collected data, the identity of the unknown hydrated salt will be determined.
Copper in an unknown compound can be converted to a copper compound of known composition. We will convert a copper unknown to CuO and then collect this to determine the amount and mass percent of copper in the unknown compound.
The Post Laboratory Problems and Conclusion paragraph assignment contain questions that determine your understanding of the laboratory assignment.
While the conclusion paragraph is a summary of what you learned in the assignment. The conclusion paragraph should not just be "this is what I learned" statement but a summary of the assignment, analysis of the data and results of the assignment and critical thinking about what was learned and why it is important. The RERUN method is used to prepare a conclusion paragraph