Lab report rubric – Formula for a hydrated salt
This lab must be typed and submitted individually. Additionally, submit this rubric sheet with your lab report.
Your name: ______
Names of people with whom you worked while collecting data:
Names of people with whom you consulted while preparing your lab report:
Sources you consulted while preparing your lab report:
Only the following sections are required for this lab report.
Raw data table with clear description of what was measured (“table headings”), including units and uncertainty for every measurement. Uncertainty must be exactly one significant figure, and the final digit of the data value must match the place value of the uncertainty.
(4 points)
You MUST show multiple “after heating” measurements in your raw data.
(points included in the above 4 points)
Qualitative:
Provide a description of “drying to constant mass” with reference to your data. (1) Also provide a descriptive, evaluative judgement of your success in dehydrating your crystal without “overcooking” the salt. (1) Remark on color change, formation of condensation along the sides of the test tube, evolution of a gas, etc. Stating, “It was not possible to observe a color change since the hydrated and anhydrous species are both white,” shows scientifically meaningful thinking. (1)
(3 points)
{FYI: “Processing raw data” section of Lab writing hints… document on course webpage is being skipped for this lab but should be considered on your IA.}
Sample Calculations:
One typed calculation for mass, with a brief description. Calculated value must have uncertainty (explicit statement, e.g., ± 0.02 g, not just following significant figure rules).
(2 points)
One typed calculation for moles, with a brief description; again, calculated value must have explicit uncertainty.
(2 points)
Typed or neatly handwritten calculation for mole ratio, showing that you divided moles of each substance by the smaller number of moles and what the actual value of the second number came out to be (“first number” being presumed to be the moles of salt that came out to 1.0000). Use correct significant figure conventions here.
(2 points)
Conclusion/Claim:
Type the chemical formula of the original hydrated salt sample, with the n in the formula replaced with an integer (1). Also, state the name of the hydrated salt (1). Finally, provide a brief, typed justification of your empirical formula conclusion/claim (1).
(3 points)
Error Analysis:
Read the attached and consider a few possibilities for error analysis.
At a minimum:
· Provide a percent error comparison with the accepted value (based on the label for the salt). This may require a bit of creative thinking; one way to do this is to calculate your experimental percent composition of water (mass of water removed from your sample/total initial mass of sample x 100%) versus the “accepted percent composition of water” from the formula on the label.
· Comment on what part of your method may have contributed to the error (e.g., amount of substance, choice of heating vessel, observations while heating—which must have been documented in the qualitative section above for consistency, etc.).
· Comment on the type of error that was identified (systematic or random); justify your choice.
· Consider non-method-based sources of error. Describe one or more material-based potential error. Explicitly state the error and its effect on the results (e.g., “If the sample had dried slightly while in storage, it is reasonable to expect a lower percent composition of water in the experimental results than the chemical label suggests…”)
· Suggest a realistic improvement you might make if you were to repeat this experiment, and describe/predict how that improvement would affect future experimental errors compared to the error found here.
(5 points)
Error analysis in chemistry
A large part of work in studying Chemistry is based on scientific evidence, accumulated through laboratory work. Inherent in all such work are certain assumptions and errors. An essential part of interpreting scientific data is therefore an ability to consider the extent to which a certain result may be compromised by the specific errors present. Broadly the types of error which arise in chemistry experiments are:
Systematic errors (determinate)
· These errors are due to identifiable causes.
· They are likely to give results which are consistently too high or consistently too low
· Sources of systematic errors can usually be identified
e.g. solubility of a gas when collected over water
· Systematic errors can in principle be eliminated or at least ameliorated by modifications to the experiment
Random errors (indeterminate)
· These errors generally arise from the limit of accuracy of the apparatus.
· They arise from fluctuations that cause about half the measurements to be too high and about half to be too low.
· Sources of random errors cannot always be identified. Possible sources:
a) observational e.g. reading burette, judging a colour change
b) environmental e.g. convection currents
· Random errors can generally not be ameliorated
· Random errors can be quantified.
The random error is equivalent to the uncertainty in measurement. This is usually given by the manufacturer of the equipment and expressed as + / - a certain value. If this information is not available, a good guideline is:
a) for analogue equipment the uncertainty is approximately +/- half the smallest scale division (often, the equipment will be marked with +/- a specified value)
b) for digital equipment the uncertainty = +/- the smallest measure (the least count)
Note when the uncertainty is recorded, it should be to the same number of significant figures as the measured value. For example a balance reading to 53.457g +/- 0.001
Propagation of uncertainties
The overall uncertainty arising in an experiment is determined by the manner in which the data values and their associated uncertainties are processed. This is known as propagation of uncertainties through the calculation.
The principle is that the overall uncertainty is the sum of the absolute uncertainties.
When values are being added or subtracted, the uncertainties associated with them must be added together:
e.g. initial temperature = 20.1 +/- 0.1 C
final temperature = 27.9 +/- 0.1C
Þ temperature change = 27.9 – 20.1 +/- 0.2C
In experiments where values are being multiplied or divided, and / or when there are several measurements made - each with its own uncertainty, the absolute uncertainties must be expressed as percentage uncertainties. These can then be added together, and finally converted back into absolute uncertainties.
e.g. mass reading 5.456g +/- 0.001
% uncertainty = 0.001/ 5.456 x 100 = 0.0183%
temperature reading = 27.8C +/- 0.2C
% uncertainty = 0.2 / 27.8 x 100 = 0.7%
Þ total uncertainty = sum of % uncertainties = 0.0183 + 0.7 = 0.72%
So, if the answer is 55.8 J, then the total uncertainty = 0.72 / 100 x 55.8 = 0.40
Therefore final answer = 55.8 J +/- 0.4
Experimental error
The difference between the experimental and theoretical results.
% error = experimental result – theoretical result x 100
theoretical result
When the final uncertainty arising from random errors is calculated, this can then be compared with the experimental error as described above. If the experimental error is larger than the total uncertainty, then random error alone cannot explain the discrepancy and systematic errors must be involved.