Lab Data Practice Workbook
Problem 1
Consider a calorimetry experiment that gave the following results. Calculate the energy given off by a particular chemical
reaction in kJ/mol. (The “true” value is 40.5 kJ/mol)
Absolute
Percent
uncertainty
uncertainty
Initial temperature/ºC (± 0.1 ºC)
21.6
Final temperature/ ºC (± 0.1 ºC)
24.2
ΔT=
Mass of water heated/g (± 0.5 g)
200.0
Amount of limiting reagent chemical/ mol (± 0.0005 mol)
0.0500
ΔT =
Given: Specific Heat capacity of water = 4.184 ± 0.005 J/g º C
Problem 2
The following data has been collected in the lab. A group of students used a dry 100 ml graduated cylinder to record the mass
and the volume of various samples of pure ethanol. The goal of this “dry” lab activity is to determine the density of the ethanol
and propagate error due to the uncertainties in the instruments through the calculations.
The table below lists sample data collected by a pair of chemistry students using standard lab equipment. The graduated
cylinder was marked in 1 mL intervals and the electronic balance had a precision of 0.01 g. Thus, all volumes were recorded to
the nearest 0.1 mL and all masses are recorded to 0.01 g.
The students found the mass and volume of 5 different samples of ethanol using a range of different volumes.
Data Table 1: The Mass and Volume Measurements of Samples of Pure Ethanol at Room Temperature
Measurement
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Mass of grad. Cylinder only /g (± 0.01 g)
43.43
43.43
43.43
43.43
43.43
Mass of grad. Cylinder and ethanol / g (± 0.01 g)
47.78
55.22
65.96
73.27
124.29
Volume of ethanol / mL (± 0.1 mL)
5.0
15.0
27.0
37.3
100.0
Trial 6
Qualitative Data: The ethanol was a clear, colorless, transparent liquid. The odor was that of alcohol,
pungent and strong.
Results Table 1: Calculated Results for the Mass and Density of Pure Ethanol
Measurement
Trial 1
Trial 2
Trial 3
Mass of ethanol /g (±
Trial 4
Trial 5
Trial 6
g)
Density of the ethanol /g mL-1 (±
g mL-1)
Note: as in all recording of data, calculations, and results, pay particular attention to the proper use of significant figures. Show
sample work.
1.
2.
3.
4.
5.
6.
7.
Calculate the density of ethanol in each trial. Fill in the densities in Results Table 1.
At 20ºC, the density of ethanol is 0.789 g/ml. Which of the five trials is the most accurate? Defend your choice.
Determine the average density of the five trials.
Calculate the percentage error.
Propagate the error to determine the overall percentage uncertainty and the overall absolute uncertainty in the new
average density.
Answer to the problem: The Density of Ethanol = ______________ ±% or
±
What type of errors would account for a percentage error larger than the range of uncertainty? Give 2 specific
examples of limitations in the procedure that might account for this.
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Problem 3
Classify each of the following laboratory errors.
Type of error
(systematic or random)
Description of Experimental Error in a lab
a.
You mass the same aluminum bar 4 times and get masses of
10.34, 10.33g, 10.33 g, and 10.34g. You wonder why the mass is not
consistent.
b.
The thermometer you are using does not record a temperature of
100.0ºC ± 0.5ºC in boiling water. Instead your thermometer displays a
temperature of 97.5ºC in boiling water. Now you use the same
thermometer to read the room temperature as 21.0ºC.
c.
Tim carefully masses out 10.5634 grams of a salt to dissolve in
water and make a particular concentration of a solution. However, he
misses the opening of the bottle as he pours the salt and ends up spilling
some of the salt on the lab counter.
d.
Vicki is reading a graduated cylinder of water by looking down at
the cylinder which is placed on the lab counter at waist height.
e.
The temperature of water in a can being heated with a Bunsen
burner is measured with the thermometer touching the inside bottom of
the can instead of being suspended in the water.
f.
5 different students measure the length of a strip of magnesium
with a centimeter ruler and the lengths range from 35.10cm to 35.20cm
±0.05cm. The average length is 35.15cm.
g.
Bill uses a ruler to measure the length of a block of metal but
notices that the end of the ruler is worn down. Several millimeters are
missing from the end of the ruler.
h.
i.
Sue misreads a 6 for a 9 on the balance with a digital readout.
The thermostat to regulate your furnace is set to turn on at 18ºC.
However, the thermostat is located in the same room as the wood
burning fireplace.
j.
Becky uses a beaker to measure 25 mL of water for a lab
experiment in which a precise quantity of water is required.
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