Chemists think in terms of molecules or more

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Unformatted text preview: Manual describe these features, but also the basics of maintenance that must be observed by all users and, finally, how to correctly read the display to avoid errors. Let’s begin with maintenance. Careful use of the balance is critical because of three factors that cross to make the balance, perhaps, the single most critical piece of equipment in the chemistry laboratory today. First, you will learn that there is a close relationship between the mass of a substance, and the number of molecules present. Chemists think in terms of molecules (or, more precisely, moles of molecules), but there is no instrument capable of counting the exact number of atoms or molecules. The next best option is to measure mass, which can easily be converted to and from the number of molecules. This in and of itself makes a good balance, to a good extent, the life blood of a chemist. The second factor is one of simple economics. A good quality balance easily will cost several thousand dollars, while high end (“analytical”) balances will cost tens of thousands of dollars. Finally, balances are extremely precise instruments. This means that balances are very easily damaged, susceptible to both mechanical and chemical damage. Some forms of damage are obvious, such as mechanical damage. If you drop or hit the balance, you can quickly and easily damage the mechanical components that do the work in a balance, especially the “knife edge”. Modern electronics balances are also designed to word on a level, draft free surface. These high tech devices still rely on the good old-fashioned low-tech “bubble” leveling device. It is good practice to check the level bubble to ensure that the balance is level before you begin. Balances are also very susceptible to corrosion. For the reason, you should never weigh any reagent directly on the weighing pan, even if it is a solid. Always use a piece of weighing paper, or a piece of laboratory glassware such as a beaker, flask or a watch glass. If you should inadvertently spill something on the balance, clean it up as soon as possible. Liquids must be prevented from getting inside the balance (paper towels will he near the balances) and solids must be removed as well (a brush works well for this, and will be located near the balance). To use the balance, first you must decide what the plan is. This may sound odd, but there are a couple of ways that the balance can be used. Always cheek the bubble to be sure the balance is level. If you have both the reagent and a container ready that you want to measure the reagent in, then put the container on the balance and press the “tare” button. This will set the balance to “zero”, even with the container on the weighing pan. Now, pour your reagent into the container; the mass shown on the scale is the mass of the reagent alone. Remember to add the reagent slowly, so you don’t have to remove excess reagent (remember, if you do have to take some of the reagent out, do not put it back into the original container). Sometimes, you will need the mass of the container so you can take it back to your bench, put something in it, and weigh it again. You can then get the mass of the contents by taking the mass of the...
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This note was uploaded on 09/18/2012 for the course CHEMISTRY 1010 taught by Professor Kumar during the Fall '11 term at WPI.

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