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GROUP-1-EXPERIMENT-1-LAB-REPORT

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Experiment 1
Weighing Techniques, Errors, and Statistics
Kristele Angeline G. Bilan
Joe Vincent E. Cacay
Alyssa Lakhsmi M. Maranan
Group 1
Bachelor of Science in Chemical Engineering
Submitted to the
Department of Chemistry
University of the Philippines Visayas
In partial fulfillment of the course
Chemistry 23.1
Under the supervision of
Professor Cathrina F. Bagarinao
September, 2019
ABSTRACT
Using the top loading and analytical balance, samples NaCl, NaOH pellets, ethyl alcohol,
and sulfur were weighed separately. Using different weighing techniques, each sample was subject
to either weighing by addition, subtraction, error, and statistics. Weights were successfully
measured by addition for NaCl(0.58g) and subtraction for NaOH pellets(0.45g), ethyl
alcohol(0.18g), and sulfur(0.6g) using their initial and final weights. Weighing errors were
measured using factors like temperture, moisture gain or evaporation, and electrostatistically
charging before and after the application. Separate weighing was done to evaluate data using
statistics. 10 new coins was weighed and the data was used to calculate the mean(5.97199),
standard deviation(0.045946), relative standard deviation(0.77%), and confidence interval at the
90% confidence level(5.97199 ± 0.0239) and also underwent the Dixon’s Q-test beforehand to
test any outliers. After all data were recorded and compared for final conclusions, it was
understood that in weighing samples, one should take into consideration the different factors that
affects its weight and the different techniques applicable to the samples. One should also be able
to use the different balances in weighing to avoid personal errors and correctly analyze data using
statistical instruments.
INTRODUCTION
To weigh different samples means to
have adequate skills in using weighing
instruments and in identifying the different
balance. Many weighings are rough and
require the top-loading balance. However,
semi-micro quantities must be weighed using
an analytical balance.
factors that affect the final weight. Different
All measurements contain some
procedures should be individually taken into
variability so no conclusion can be drawn
consideration for any possible errors and the
with certainty. Statistics gives us tools to
use of different weighing techniques and
accept
statistics should be evident in acquiring
probability of being correct and to reject
results. A thorough knowledge and care of
conclusions that do not. The mean, is the sum
the balances used in the laboratory is
of the measured values divided by the
essential. The two most common types of
number of measurements, n, while the
weighing balance used in the laboratory are
standard deviation, s, measures how closely
the top-loading balance and the analytical
the data are clustered about the mean.
conclusions
that
have
a
high
Dixon’s Q test detects a single outlier from a
techniques in obtaining the weights of
small set of replicate observations (about 3-
different
10) in a normal distribution.
statistical tests to determine the precision of a
This experiment aims to allow the
materials
and
apply
simple
measurement.
students to learn the proper weighing
MATERIALS AND METHODS
The
analytical
top-loading
balance
were
balance
and
cup and the weight was recorded. To obtain
used
this
the weight of the sample, the weight of the
in
experiment. The materials used in this
weighing
cupthat
containedNaCl
was
experiment were ten (10) pieces one-peso
subtracted by the weight of the empty
coins minted in the same year, watch glass,
weighing cup.
100-mL beaker, tweezers, weighing cups, test
Weighing by difference. The balance used
tube, and woollen cloth. EtOH (Ethanol),
was a top-loading balance. Three (3) reagents
NaCl(Sodium chloride), NaOH (Sodium
were used in this test: sodium hydroxide
hydroxide) pellets, and Sulfur were the
(NaOH) pellets, ethyl alcohol (EtOH), and
reagents used in this experiment.
Sulfur. The initial weight of the containers
Weighing Techniques
containing the samples were recorded. Three
Weighing by addition. The top-loading
(3) NaOH pellets were transferred into a
balance was used. An empty, dry, clean
clean, dry, pre-weighed watch glass and
weighing cup was weighed and the weight
weighed immediately upon transfer. Ten
was recorded. Three (3) spatulas of sodium
drops of EtOH and three (3) spatulas of sulfur
chloride (NaCl) were added into the weighing
were transferred into two (2) separate clean,
dry, pre-weighed weighing cups and weighed
EtOH, and sulfur were weighed. The weights
immediately. The containers of the samples
were calculated using weighing by addition.
were weighed again. The weight of the
The weights of the samples were then
sample
by
compared to the weight when the samples
subtracting the initial weight of the container
were first transferred to the watch glass and
containing the sample by the weight of the
weighing cups.
container
transferred
after
was
the
calculated
sample
was
Effect of electrostatically charged weighing
transferred.The watch glass and weighing
materials. A clean, dry, plastic weighing cup
cups which contained the samples were set
was initially weighed and recorded. The
aside for some time.
weighing cup was then rubbed using a
Weighing Errors
woollen cloth and weighed again. The
Effect of temperature. A test tube holder was
weights obtained were then compared to
used to hold the test tube. A clean, dry, test
know the effect of electrostatically charged
tube was weighed. The same test tube was
weighing materials.
then placed inside a freezer for one (1) minute
Weighing Statistics
and weighed afterwards. The test tube was
The analytical balance was used in
then placed inside the oven for approximately
this experiment. Ten (10) new Philippine
ten (10) seconds and the weight was
peso coins were each weighed and recorded.
recorded.
The values were then arranged from the
Effect of moisture gain or evaporation. After
lowest to highest and Dixon’s Q test was used
a few minutes, the weight of the watch glass
to reject any questionable result. The mean,
and the weighing cups containing NaOH,
standard
deviation,
relative
standard
deviation, and the confidence interval of the
mean at 99% confidence level was then
calculated.
RESULTS
Table 1. Weighing by addition
weight in (g)
Empty weighing bottle
Weighing bottle + NaCl
1.67g
Weight of NaCl
0.58g
Table 2. Weighing by difference
NaOH
EtOH
Samples
Initial weight of
bottle containing 122.82 98.47
the sample
Final weight of
bottle containing
the sample
Weight of
sample that was
dispensed
122.37
0.45
98.29
0.18
Table 4. Effect of moisture gain or
evaporation
Sample
S
NaOH
EtOH
Sulfur
Net weight
immediately
after transfer
(g)
0.47g
0.16g
0.57g
Net weight
after some
time (g)
0.54g
0.0117g
1.00g
194.52
Table 5. Effect of electrostatically charged
sample
193.92
0.6
Weight of
empty watch
glass
35.05
1.62
1.64
Weight of watch
glass + sample
35.52
1.78
2.21
Weight of
sample in the
watch glass
0.47
0.16
0.57
Table 3. Effect of temperature
Temp. condition
Weight of test tube (g)
Cold
11.2203g
Room temp. (24oC)
11.2131g
Hot
11.2130g
Weight (g)
Initial weight of
weighing bottle
(g)
1.6822
Weight
of
weighing bottle
after rubbing
with woollen
cloth (g)
1.6872
Table 6. Weighing statistics
Sample coin
number
1
2
3
4
5
Weight (g)
5.9861
5.9740
5.9870
5.8910
6.0620
6
5.9948
7
8
9
10
5.9177
5.9710
5.9825
5.9538
clean weighing cup was initially weighed
before NaClwas added and weighed again,
comparing its initial weight and final weight
when it already contained the NaClsample.
Another technique used was the weighing by
Table 7. Dixon’s Q test for outliers
Data
Group
Suspect
Values
H: 6.0620 g
L: 5.8910 g
Qexp
Qcrit
0.156
0.466
difference which is an alternative way of
computing the mass of a sample reagent. In
this method, the difference of the initial
For a sample size of 10 at 95%
weight of the NaCl container and its final
confidence level, the Qexp 0.156 is not greater
weight after transferring the sample is the
than the Qvalue 0.466, so this point is not at an
weight of NaCl.[1] The weight obtained from
outlier at an alpha level of 5%.
weighing the container before transferring
Table 8. Summary of Statistical Results
some NaOH increased after it was weighed
Statistical parameters
Value
from the watch glass. Because different
Mean
Standard deviation, s
Relative standard deviation,
RSD
Confidence interval at the
90% confidence level
5.97199
0.04595
materials were used to hold the NaOH in
0.78%
5.91799 ±
0.0239
5.91799 ±
0.0285
weighed and there was transferring involved,
external factors may have affected the weight
like moisture gain.[2] Comparing both
techniques, weighing by difference proves to
DISCUSSION
be more accurate because it only involves the
In this experiment, we used two
difference between the initial weight of the
weighing techniques in determining the
container before transferring the sample and
weight of the samples. The first one is
the final weight. It has less to no external
weighing by addition in which an empty, dry,
factors that can affect the weight like how
immediately upon transfer and were set aside
moisture
gain
during
the
for a few minutes before obtaining their
increases
net
weight
of
transferring
sample
weight again. The initial and the final weight
reagent.[3]Weighing by difference is used if
recorded were different from each other. The
one wants to find out the accurate computed
weight of the NaOH pellets increased while
mass of a sample. While weighing by
there was a decrease in the weight of the
addition on the other hand can be used to
EtOH and sulfur. This is due to the effect of
measure
after
moisture gain and evaporation. [6]As a certain
transferring the sample which may be
material gains moisture, its weight record
affected by external factors and to measure an
would have an increase compared to its initial
estimate mass of a great amount of sample
weight before getting exposed to any external
which may be less prone to external factors
factors. Evaporation, on the other hand,
and in instances that does not need that much
reduces weight of a material as any water
accuracy.
an
[4]
amount
the
acquired
Since the test tube was exposed
content will be lost as gas and will not be
to different temperatures and its weight at
weighed together with it.[7] Electrostatically
different temperatures was recorded, we
charged samples or containers can be difficult
could see the effect of temperature on the
to weigh. Static charges can cause unstable
weight of materials. As it was shown in Table
balance readings and measurement drift,
3, as the temperature increases, the weight of
leading to slow and inaccurate weighing
the test tube decreases. It could be concluded
results.A negative charge on the material and
that the temperature is inversely proportional
a positive charge on the balance housing
to the weight of the materials.
[5]
NaOH,
EtOH, and sulfur were first weighed
cause a force to be exerted between the
balance
and
the
vessel.
The
vertical
component of this force adds to the weight of
by the observer such as using the right
the flask and influences the weighing
weighing technique
result.[8]
accuracy of weighing and by avoiding or
that
increases
the
controlling the factors that could affect the
SUMMARY AND CONCLUSION
measurement.
Furthermore,
statistical
The conduct of the experiment
analysis gives us a better evaluation of the
showed different techniques that could be
data and basis to accept conclusions that have
used in the laboratory to weigh different
a high probability of being correct and to
samples in certain situations accurately
reject conclusions that do not.
depending on the desired results. It showed
the factors considered as common sources of
REFERENCES
errors which affect the weight of the samples
“Electrostatic Charges During Weighing.”
Mettler Toledo. Retrieved from
https://www.mt.com/ph/en/home/libr
ary/white-papers/laboratoryweighing/excellence/electrostaticcharges-during-weighing.html
and cause the recorded data to vary such as
effect of temperature, moisture gain, and
evaporation as well as the material used being
electrostatically charged.
It is important to note that variations
in measurements is normal and no conclusion
can be drawn with certainty. In addition, the
errors committed by the observer and other
systematic errors like environmental and
instrumental errors can never be avoided.
There are certain methods however to
minimize the mistakes and errors committed
“Laboratory
Weighing.”
Chemistry.
Retrieved
from
https://www.cerritos.edu/chemistry/_
includes/docs/Chem_111/Lab/Exp%
203%20Laboratory%20Weighing%2
0Fall%2008.pdf on September 07,
2019.
“What Precautions Should Be Taken for
Correct Weighing of Laboratory
Samples?” Lab-Training.com, LabTraining.com, 22 Jan. 2016, labtraining.com/2014/06/24/correctweig
hing-samples/.