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24 Cards in this Set

  • Front
  • Back
Which is more accurate--the transfer or measuring pipette?
transfer pipette
absolute uncertainty...
describes the amt of uncertainty associated with an actual measurement
relative uncertainty...
takes the absolute uncertainty and relates it to the magnitude of the measurement

relative uncertainty = abs uncertainty / magnitude of measurement
As the magnitude of the measurement increases, the relative uncertainty _________ (assuming a constant absolute uncertainty).
If you were to report the measurement in a lab report, you would include the uncertainty and use the following format: 12.35 +/ 0.02 mL. This is the _____ uncertainty.
absolute uncertainty
(not relative!)
You can tell uncertainty in pipettes and graduated cylinders, but for tolerances for ______ you consult tables (in Harris) that show common uncertainties.
volumetric glassware like volumetric flasks and transfer pipettes
How do you know when a result is "good" or "bad"? Knowing about the error associated with a measurement and the propagation of that error in the experiment can help you make an informed decision about the _____ of the reported result.
If several measurements are used to calculate a result, than the _______ uncertainty must be determined and reported.
Formula for result obtained from addition/subtraction

Formula for result obtained from multiplication/division
error = sqrt(sum of abs errors^2)

error = sqrt(sum of percent rel uncertainty^2)
The total # of decimal places in the reported figure should be...
the same as the those in the uncertainty.
Types of errors that can occur during an experiment are _____ and _____.
Systematic error can be easily _______ and ______. An example would be if an experiment were done...

Performing multiple trials ______ minimize systematic error. It will affect the ______ of a result, but not the ______.
identified and corrected.

with an improperly calibrated thermometer. If the thermometer always reads 1 degree C low, then all measurements can be corrected.

Random or indeterminate error is usually called _______ error. It affects the _____ of a result and ______ be corrected.
An example would be the error associated with...
Random error ____ be minimized by performing multiple trials, because the magnitude of the error...
human error
reading a scale. Different ppl may read the scale differently.
fluctuates in both the negative and positive direction with equal probability.
Arithmetic mean (x) is defined as...

Standard deviation (s) measures... and is defined as...

The square of the stdev (s^2) is often called _______.
x = sum of (x_i) / n
x is the measured quantity and n is the # of measurements

measures how clustered the data are about the mean
s = sqrt(sum of (x_i - x)^2 / (n-1))
n-1 indicates the # of degrees of freedom in the system

An ______ # of measurements would theoretically allow us to determine the true ______ and the true _____.

It is impossible to determine this in a finite data set.
Confidence limits...

Confidence interval...

How big this interval is depends on...
infinite, population mean, stdev

define the interval around an experimentally determined mean in which there is a high probability that that the true mean exists.

is the actual numerical magnitude of the CL.

how similar the experimental stdev (s) is to the true stdev (ρ)
Because we cannot know the true stdev for small sets of data, we rely on a statistical table developed by ______ to help determine CL.
The formula is...
W.S. Gossett
CL for μ = x plus/minus ts/sqrt(n)
t is the student t-value (from a table of values for t based on degrees of freedom)
The simplest way to reject data points that seem to be outliers is to utilize the ______.
Objective Exp 1:
Exercise A: Determination of ________ in ________
B: Intro to error: Determining _____
C: Intro to error: _______ a micropipette
To introduce the concept of experimental error and how to quantify it

A: uncertainty in volumetric glassware
B: density
C: calibrating a micropipette
Exp 1A:

Absolute uncertainties for:
vol. transfer pipette (25mL)
vol. flask (250 mL)
beaker (400 mL)
grad. cylinder (50mL)
buret (50 mL)
erlenmeyer flask (250 mL)
vol. transfer pipette (25mL)- 0.03 mL
vol. flask (250 mL)-0.12 mL
beaker (400 mL)-13 mL
grad. cylinder (50mL)-0.3 mL
buret (50 mL)-0.03 mL
erlenmeyer flask (250 mL)-10 mL
Do not dry glassware with _______, because they may contain contaminants. Glassware is clean when _______ during rinsing.
forced air spigots
no water droplets cling to it during rinsing
Exp 1B:

The most accurate way to determine mass of an aliquot of liquid is to ______.

Methods 1-5:
weight by difference. (subtract mass of beaker from mass of beaker with liquid)

1) 50 mL buret to deliver 5 mL
2) 25 mL vol. transfer pipet to deliver 25 mL
3) 10 mL measuring pipet to deliver 5 mL
4) 10 mL measuring pipet for 2,4,6,8,10 mL aliquots
5) 10 mL graduated cylinder to deliver 10,20,30,40,50 mL aliquots
Expt 1B:

For methods 1-3, determined avg mass, stdev, and 95% CL out of the 5 trials.
Then density and uncertainty (via propagation of error):

For methods 4-5, determined density by plotting...
Calculated slope and intercept using __________ and comparing it to Excel plot.

Which method was most accurate?
uncertainty for density = sqrt(rel.error for mass * rel. error for vol)
rel. uncertainty for mass = stdev/mass
rel. uncertainty for volume = abs.un/volume of aliquot

multiply % uncertainty by density to get uncertainty in grams

plotting mass v. volume. Density is slope.
linear least squares

Method 1--buret
Exp 2: Spectrophotometry

A: Quantifying Protein Concentration: Bradford Assay
What is it?

Protein studied:
It does not absorb light, so it is bound to a dye ________ that normally absorbs at ____
When bound to protein in acidic solns, the absorbance shifts to the red (____ energy) and the complex absorbs at ____.

equation and colors of protein-dye complex:

In acidic solns, _____ form of the dye is stable. Interactions b/w dye and protein mainly _____ and _____, including hydrophobic interactions b/w aromatic _______.
popular protein assay method (simple, rapid, sensitive) that uses spectrophotometry to measure protein conc.

bovine serum albumin (BSA)
Coomassie Brilliant Blue G250 (CBBG), 495 nm
lower energy, 595 nm

CBBG (greenish-brown) + BSA (colorless) --> CBBG-BSA complex (blue)

weak, non-covalent
amino acid residues
When the protein binds, the absorbance is ______ to the amt of protein binding. Therefore, the higher this absorbance, the _______ the protein content in sample. Because this relationship is _____ w/in a given range of conc., a __________ can be constructed using camples w/ known protein conc. This standard curve can then be used to determine _________ of an unknown sample.
Beer's Law calibration curve
protein conc.