Transcript Spectroscopic Methods

Volumetric Measurements
In addition to this presentation, before coming to lab or
attempting the prelab quiz you must also:
 Read chapter IV in the “Laboratory Handbook…” by
Griswold et al. before coming to lab
 Watch the prelab videos on the use of the balances and
thermometers
 Read the introduction to the lab exercise in the
coursepack
Note: This presentation specifies the precision of all measuring devices
you will use in this lab (i.e., how many sig figs each instrument allows).
This is useful information to write in your lab notebook, near the front.
What’s the point?
• In this lab you will
• gain experience using volumetric glassware
- graduated cylinders, beakers, volumetric flasks
• gain a sense of the precision and accuracy of
different glassware
• learn to relate significant figures in reported
measurements to precision in measuring devices
• practice working with significant figures and units
- see Chapter 1, Brown, LeMay and Burstein
Background
• Different pieces of volumetric glassware have
different purposes and therefore different
precision and accuracy
this is true of all measuring glassware; compare a
measuring cup and a cake mixing bowl with volume marks
• The difference in precision and accuracy affect
three important things:
- how reproducible the measurements are
- how accurate the measurements are
- how many significant figures you can report
Graduated Cylinders
• Use: to measure an amount of liquid to transfer
• Depending on the cylinder size, they have
markings for every 1 mL, 2 mL, 0.1 mL
• Read the volume by finding the bottom of the
meniscus
- This is the curve formed by the liquid:
you must be eye-level with the meniscus
to read it properly
• In the figure, the markings are every 2 mL,
but you can read between the marks to get
+/- 1 mL precision (i.e., the volume here
would be reported as 100 mL, not 100.0 mL)
• To use a graduated cylinder:
• fill the cylinder to the desired mark
• position yourself eye-level with the meniscus
• read the volume from the bottom of the
meniscus
• now, you can be confident that you are
transfering the desired amount of liquid
Beakers
• Use: to transfer approximate amounts of liquids
• Beakers have markings that are widely
spaced. So, the volume readings are
approximate
• In the figure, you see two volume scales.
On the right is the amount in the beaker.
• Also in the figure, you can see this
beaker’s precision is given as +/- 5%.
• For a 250 mL beaker, that is:
(0.05
x 250 mL) = 12.5 mL = +/- 10 mL
• So, here we report the volume to the tens
place: 200 mL, or 190 mL, but not 195 mL
Volumetric Flasks
• Use: to contain an accurate amount of liquid
• Volumetric flasks have one mark that
indicates the calibrated volume at a
particular temperature (e.g., 20 oC)
• Virtually all are made to contain (see
introduction to the lab exercise)
• When the bottom of the meniscus is level
with the mark, volumetric flasks have the
following precisions:
5 mL: 5.00 mL
10 mL: 10.00 mL +/- 0.01 mL
25 mL: 25.00 mL +/- 0.01 mL
100 mL: 100.0 mL +/- 0.1 mL
• Volumetric flasks are very useful for making
solutions of known volume.
• In this lab, we will be using them simply to
investigate their accuracy and precision.
• To use a volumetric flask:
- fill the cylinder to the desired mark
- position yourself eye-level with the meniscus
- read the volume at the bottom of the meniscus
- now, you can be confident that the flask
contains the desired amount of liquid
Necessary Calculations
1) Relating volume and density
Suppose you have measured the mass of
H2O and have looked up its density in the
CRC Handbook at a measured temperature
density = mass / volume
Example: what is the volume if the mass of H2O
is 25.032 g and its density is 0.99976 g/mL?
• mass = 25.032 g
• density = 0.99976 g/mL
25.032 g x
1 mL
0.99976 g
= 25.038 mL
Necessary Calculations
2) Percent error
Suppose you have measured something for which
you know the true or expected value. How
accurate is your measurement? Is it too small
(negative error) or too big (positive error)?
Percent error is one way to express this accuracy.
% error = (measured - expected) x 100%
expected
Example: you have H2O, contained in a 25-mL
volumetric flask, whose volume you have
calculated to be 24.752 mL. What is the percent
error?
% error = (measured – expected) x 100%
expected
A 25-mL volumetric flask contains 25.00 mL, the
“expected” value.
% error = (24.752 – 25.00) mL x 100%
Note the critical
25.00 mL
sig figs in red
% error = -0.25 mL x 100% = -0.99%
25.00 mL
Pay attention to sig figs in this lab!!
For Everyone’s Safety
• your lab goggles and coat must be on
whenever anyone is working in the lab
• no open toed shoes are allowed in lab
• move around the lab carefully to avoid
bumping into other students