Systematic errors are typically associated with the

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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'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 be reproducible. 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 findings. 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 low. 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, 10...
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