# Calculations and results of quantitive analysis

please answer and calculate the following questions.

Results & Calculations

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

YOU SHOULD NOW BE READY TO ANSWER

QUESTIONS RELATED TO SKILL BUILDER #5 ON BLACKBOARD.

Department of Chemistry & Biochemistry, University of San Diego, last updated August 2018

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Department of Chemistry & Biochemistry, University of San Diego, last updated August 2018

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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

important to be alert when

dealing with flames. Your hands

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

dispose your unknown solution.

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: