# Calculations and results of quantitive analysis

Results & Calculations

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Pool your density values with another student who has the same unknown so that you have six trials to analyze. Calculate the average density and absolute standard deviation for the pooled trials of solution.

Do a Grubbs’ test on the most reasonable outlier.

Calculate the relative standard deviation for the pooled densities. Depending on your Grubbs test, this may require recalculating the average and absolute standard deviation of a new data set.

Obtain the “true” value for the solution’s density from the instructor and calculate % error.

Several standard curves are posted in the lab room.  Use the appropriate standard curve to determine the concentration of your unknown solution.  Notice that the concentration units in the standard curve are in “M”, which stands for “molarity” and means “moles of solute per liter of solution”. You will learn more about molarity in later Skill Builders, but for now you just need to know that the higher the molarity, the greater the concentration.

A table of results is often used to summarize information from a given experiment, and is particularly useful in a multipart experiment.  Generate a table of results to summarize the results of Experiments 1 and 2. Remember, the overall goals were to determine the identity and density of the unknown solution.

Write a one-sentence conclusion for Experiments 1 and 2. Put it in its own section titled “Conclusion”.

Chem 151L Skill Builders
Chem 151L Skill Builders
SKILL BUILDER #5: STANDARD CURVES
Learning Objectives
1) Generate a standard curve on a graphing calculator
2) Draw a standard curve that contains an appropriate title and properly labeled axes.
3) Evaluate the quality of the graph by assessing the correlation coefficient (r value).
Special Things Worth Noting
A standard curve that is drawn or printed should always contain these features:
A
Clear labd of each axis to include units
A title that provides enough information about the standard curve that someone could look
at it and know it could be used without having to refer to any other sources (chemical name,
special parameters used, etc.) – i.e. it can stand on its own. In Chem ISIL, the most
common parameters to report are wavelength setting and path length for a standard curve
in a spectroscopy experiment
Axis ranges established so that the data points take up the space of the graph (not crunched
into a small portion of the graph space.)
The equation of the curve-fit line and ther’ value.
Example: A calibration curve for protein analysis was by measuring the absorbance of six
standard protein samples at 350m nm and generating a standard curve with best fit line.
Background Information
A standard curve is often used in chemistry in order to determine the concentration of an unknown
solution. “Unknown” in this case refers to the fact that the concentration is unknown: we know the
identity of the solute, but we do not how much of it is present. A set of standards (solutions of
known concentration) is analyzed and the unknown solution is compared to those standards. Here
is how it is done:
0.45
04
0.35
03
Concentration of Protein vs Abs at 350 nm
y00154x +0.0111
R0.9938
ug/ml. Absorbance
protein
0.0 0.000
5.0 0,086
100 0.183
150 0.246
200 0.326
250 0.384
If we know that the behavior of a given solution changes linearly with concentration, we
can make an x.y plot for the solution. This plot is called a standard curve. The “X”
component is often concentration, and the “y” component can be any variable that changes
linearly with concentration changes, such as density or color intensity. Once a standard
curve is established, the “y” variable for the unknown can be analyzed, and the
concentration of the unknown determined by solving for “x”. It is important to note that
the unknown must fall within the range of the standards – we can only guarantee that the
equation of the line is valid within the range we’ve analyzed
SOS
Absorbance at 350 nm
02
0.15
0.1
0.05
0
5
10
15
25
30
20
concentration of protein ug/mL)
Keep in mind during this analysis that the concentrations of the standards may not be
exactly the same concentration as your unknown, but you can still determine the
unknown’s concentration if you generate a curve-fit line. The equation in a “y=mx+b”
fomat) that we generate from the standard curve allows us to find the concentration of any
similar solution within the range of the standards. A computer program or graphing
calculator can plot a graph and generate an equation of the curve-fit line.
The standard curve yields a good fit as seen by the R close to 1. The absorbance of an
unknown protein sample was measured at 350 nm and found to be A =0.203. This is a
valid measurement because it falls between the absorbance values measured to make the
standard curve. Using the line for the best-fit on the standard curve the concentration can
be calculated
y (absorbance) =0.0154 (x-concentration) + 0.0111 The measured value for y = 0.203
can be substituted into the equation and solved for x. The final answer is x = 12.5 ug/mL.
Another piece of valuable information that a computer or graphing calculator can provide
is a statistical assessment of the quality of the standard curve. This will be shown as the
“pt” value, or correlation coefficient, and represents an overall assessment of how closely
cach data point matches the curve-fit line. A perfect fit is represented by an r value of
1.000; the lower the value, the more scatter in the data points. We can speak of the r
value as a measurement of precision because it measures scatter in the data of a graph. The
quality of equipment a chemist uses and the quality of the chemist’s technique will be
noticeable in the r’ value. An experienced chemist using volumetric pipets and volumetric
flasks and typical lab instrumentation can expect to obtain a standard curve with an r value
of at least 0.98 for most experiments
QUESTIONS RELATED TO SKILL BUILDER #5 ON BLACKBOARD.
Department of Chemistry & Biochemistry, University of San Diego, last updated August 2018
, 12
Department of Chemistry & Biochemistry, University of San Diego, last updated August 2018
,
13
2
Date
Experiment/Subject
Quantitative Analysis
0912512019
Course &
Section No.
Lab Partner
Locker/
Desk No.
Experiment #28
The density and concentration of
Goalor An unknow aqueous
Solution will be determined.
| Part B: Density of the solution
with cap 34.5930
2. Pipet 10ml of the unknown
Solution and re-weigh to obtain
mass of the solution us. 9 281
46. 19/2
20c
may
Safety. In this experiment it is 1. Weigh a labled empty bottle
should be relaxed when operating
3. Take the temperature of the
the volumetric pipet as it
break
Solution
in your hands if you’re too tense. 4. Determine the density of the
solution
The unknown solutions may be toxic.
5. Repeat 1-4 twice so have
you
Waste: Some of the unknowns three trials in all
are toxic check with the
6. As with all glassware, please
be sure to clean the bottles and
professor on ways to pipet well with di. before
returning them to the Common
Procedure 8
trial 1:45.9281
I obtain 50 mL of your unknown solution
unknown solution trial 2
Use areas,
4/6.1912
461030
from the instructor to use for the entire
trial3
experiment
Part As Practice Pipet Technique
Practice pipetting with water
several times show the professor
that you’re capable to pipet properly
before confhuing Part B.
Date
Witness/TA
Signature
Date
THE HAYDEN-MCNEIL STUDENT LAB NOTEBOOK
Note:

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