dakota state university page 36 of 232 basic

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Unformatted text preview: beaker and contents and subtracting the mass of the beaker. This is known as “mass by difference.” In this case, start by checking to see that the balance is level, and press the “tare” button to set the balance to zero. Place your container on the balance pan, and record the mass of the container. Once the container has the material in it, repeat the procedure (check to see that the balance is level, press the “tare” button to set the balance to zero, place your container on the balance pan, and record the mass of the container and material.) Dakota State University Page 36 of 232 Basic Laboratory Procedures General Chemistry I and II Lab Manual Finally, just a couple of hints to improve your results when using a balance. First always use the same balance. If the balance is off slightly, thee errors will usually cancel themselves out if you are always using the same balance. Secondly, avoid drafts, vibrations or anything the that might give you an erroneous reading. The best way to do this it avoid motion near the balance when it is in use, avoid weighing objects when they are hot, and do not lean on the counter when the balance is in use. Graduated Cylinders: Graduated cylinders are used to measure volume. They are the most commonly used devices for volume measurement in the lab because of their accuracy, speed and ease of use. NEVER use the graduations on a beaker or flask for volume measurement; their accuracy is not sufficient for laboratory use. Most graduated cylinders are accurate to three significant figures (as opposed of flasks and beakers that are accurate to only two significant figures, and burettes and pipettes that are accurate to four significant figures). Before we begin, it is important to note that graduated cylinders are to be used for measuring volume only. NEVER use the graduated cylinder to mix reagents or to heat a substance! With a liquid in the graduated cylinder, always read the bottom (or top) of the meniscus. A meniscus is a curvature to the liquid caused by intermolecular forces between the liquid and the glass. If you have attractive forces between the glass and the liquid, such as water, the liquid will “creep up” the sides of the glass slightly to cause the normal downward curvature. If these forces are repulsive, then the liquid will not move up along the walls as far as the liquid, creating an inverted meniscus. Always look past the wall of the glass, and read the volume at the center of the liquid. Remember to estimate the last significant figure when reading the volume. This means that you simply guess how far in between the two closest graduation lines the top of the meniscus is. If it looks to you like the top of the meniscus is right on one of the graduation lines, then record an extra “0” it the end of the recorded value so the reader knows that this is the case. Dakota State University Page 37 of 232 Basic Laboratory Procedures General Chemistry I and II Lab Manual Pipette: Like the graduated cylinder, the pipett...
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