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Unformatted text preview: olecular weight of 82.15 g/mol and
density of 0.7231 g/mL, what volume in mL do we need to get?
Answer: Two step problem here, and one of the types of problems that tends to give
students much grief. The answer is 56.80 mL...how'd you do? Dakota State University page 225 of 232 Basic Laboratory Statistics General Chemistry I and II Lab Manual Basic Laboratory Statistics
In chemistry, we work very hard to control the environment to ensure that our
results are well understood. We make sure that our equipment is clean to avoid
extraneous reactions, we control temperature, heck, sometimes, we even control the
amount of light in the room. It is not uncommon to find organic laboratories that happen
to have windows to have aluminum foil over those windows to block any extraneous
light, since this is typically an un-controllable and un-accounted for factor. Most
importantly, we do many repetitions of the same experiment, to ensure that the results we
get are not one-time flukes. To be a valuable piece of information, all experiments must
As you get further ahead in your studies of chemistry, you will undoubtedly learn
more about probability and statistics, but this is a good time to start with some basic
concepts. Although many of the formulas I will show you here are often standard in
software packages such as Excel, it is important to understand the principle behind them
to be able to interpret them correctly. Before we begin, though, we need to understand
the two major categories of errors that can arise, and the results these will have on our
Types of Errors
There are two broad categories of errors that arise, referred to as “random” or
“systematic.” Any specific error you can name should fit into one of these two
categories, and each has a unique impact on the results.
Random error is just that, random. These are often referred to as “human error,”
although in truth these errors are far more common than that. For example, if you fail to
get all of the reagent off of a piece of weighing paper, or if you accidentally get an
unnoticed piece of dust in your reagent, these errors are random. They are unpredictable,
and can have a variety of consequences, including either overestimating the value that
you seek, or underestimating it.
Systematic errors are typically associated with the instruments that we use. For
example, if your balance is not calibrated correctly, all of it’s readings may be off by
some fixed amount, say, for example, 0.11 g. These errors will always result in errors in
one direction only; for example, all of our readings may be 0.11 g too high, or 0.11 g too
These errors impact “accuracy” and/or “precision.” These terms are often used
synonymously, but in fact, they each have very special and unique definitions.
“Precision” is a measure of how close all of the measurements are with one
another. That is, in our misaligned balance, we may get a series of readings that are all
very close to one another, and yet they are not very close to what the true value ought to
be. For example, five masses of 100.00 mL of water might read 100.09, 100.11, 100.10,
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