The purpose of this experiment is to determine the empirical formula of a sample of Copper Chloride

International Baccalaureate Chemistry

Zack Jarrett

CHM 151 Lab

0800-1040

Instructor Husowitz

14-Feb-2007

Lab Report

Determination of an Empirical Formula

I. Purpose of Experiment

The purpose of this experiment is to determine the empirical formula of a sample of Copper Chloride (CuxCly)

II. Chemicals and Equipment

III. Experimental Procedure

NOTE: The following steps were performed for three samples (marked Red, Pink, and Yellow) simultaneously.

Measured out approximately 1.0 g CuxCly sample into a beaker.
Added approximately 25.0 mL 1.0M HCl to the beaker and stirred until CuxCly sample was completely dissolved.
Weighed a piece of Zn(s) metal and added to solution in beaker.
Stirred the solution and periodically scraped the plated Cu(s) from the piece of Zn until the solution above the Zn turned clear and the bubbling from the Zn almost completely ceased.
Scraped all remaining Cu plating off of the Zn, then removed and dried the Zn solid with a kimwipe.
Weighed the Zn solid.
Weighed a piece of filter paper, then placed into a glass funnel over a beaker (see Figure 1) to create a filter system.

Figure 1

Poured solution bearing precipitated Cu into the filter system. Rinsed beaker with a small amount of 1.0M HCl to remove any remaining bits of Cu and poured solution into filter.
Rinsed precipitate in filter with distilled H20.
Rinsed precipitate in filter with 70% isopropyl alcohol in order to remove any remaining H20.
Weighed a watch glass.
Transferred filter paper bearing precipitate onto watch glass and inserted assembly into a 77 °C drying oven for drying.
Removed assembly from drying oven at 5 minute intervals, weighing assembly with each removal.
Repeated ...

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Figure 1

Poured solution bearing precipitated Cu into the filter system. Rinsed beaker with a small amount of 1.0M HCl to remove any remaining bits of Cu and poured solution into filter.
Rinsed precipitate in filter with distilled H20.
Rinsed precipitate in filter with 70% isopropyl alcohol in order to remove any remaining H20.
Weighed a watch glass.
Transferred filter paper bearing precipitate onto watch glass and inserted assembly into a 77 °C drying oven for drying.
Removed assembly from drying oven at 5 minute intervals, weighing assembly with each removal.
Repeated the dry-weigh-dry process until three successive mass measurements within 0.05 g were observed.

IV. Data Tables

V. Data Analysis

The empirical formula for the sample of Copper Chloride can be determined through mathematical analysis of the recorded data, as follows.

The mass of the precipitated Cu can be found by subtracting the mass of the watch glass and original filter paper from the mass of the entire drying assembly after drying.

NOTE: The Pink Sample results are obviously incorrect; we cannot have a negative quantity of Cu precipitated. From this point forward, the Pink Sample is disregarded and left out of all equations and analysis. See the conclusion for further discussion.

The mass of the Cl in CuxCly can be found by subtracting the mass of precipitated Cu from the mass of the original sample of CuxCly.

% Composition by mass of Cu and Cl in CuxCly can be found by dividing the mass of each element by the mass of the sample and multiplying the quotient by 100.

The empirical formula is found by converting the % composition to moles, then converting the results to whole numbers.

VI. Conclusion

The purpose of this experiment was to determine the empirical formula of an unknown copper chloride substance. The experiment procedure required separating the two parts of the substance, then analyzing observed quantities of each.

The results of the analysis of the observed data indicate that the empirical formula for the unknown sample of Copper Chloride is actually Cu1Cl1 or, more simply, CuCl, copper (I) chloride.

The experiment worked as expected except in the case of the Pink Sample (see Data analysis: 1). When performing analysis on the data I came to the unexpected result that my precipitate weighed less than nothing, even though common sense and direct observation prove this as impossible. The cause of the problem I believe to be human error in measuring the mass of the watch glass. I believe this to be the case because the measurements of the mass of the drying assembly are rational and follow a decreasing pattern throughout the drying period, proving precision and accuracy in measurement. There is no such body of evidence to prove that the weighing of the watch glass itself was performed with precision or accuracy. In addition, I was personally involved in weighing the drying assemblies, but did not directly observe the weighing of the watch glasses individually. Another possibility is that of human error on my own part in transcribing values from spoken word.

VII. References