A) List four things to avoid doing when using a balance. B) You are in a hurry t

Question

A) List four things to avoid doing when using a balance.

B) You are in a hurry to dry some glassware. comment on the advisability of the following methods

1) using paper towels

2) Use compressed air from the lab airline

3) use acetone of uncertain cleanliness

4) heat in oven

5) use clean acetone and then heat in an oven

C) What effect wiill there be on the accuracy of a measurement if you use a volumetric pipet that is not completely dry but instead contains a few drops of the same liquid that you wil be drawing into the pipet?

D) Define meniscus. State how you use the meniscus in making volumetric measurements.

E) why is not safe to pipet by mouth?

Explanation / Answer

A) Pour the substance bit by bit....ie very little a time Wait for the physical balance to get stable.Then proceed Dont ever put excess weight over it....i.e shouldn't exceed limits While the need of tapping arise we should do it very gently. B) You can use cleaning in acetone and then heat in an oven...this is the best procedure. C) The titration that you will do might not be correct due to the incorrect volume of the sample being added from a glassware that is not fully dry D) Check this image for meniscus http://0.tqn.com/d/chemistry/1/G/P/b/meniscus.jpg When you read a scale on the side of a container with a meniscus, such as a graduated cylinder or volumetric flask, it's important that the measurement accounts for the meniscus. Measure so that the line you are reading is even with the center of the meniscus. For water and most liquids, this is the bottom of the meniscus. For mercury, take the measurement from the top of the meniscus. In either case, you are measuring based on the center of the meniscus. E) Pipetting by mouth risks ingesting the chemical you are trying to suck into the pipet. Even if the solution is harmless, you can't be sure that the pipet itself is clean. Who knows what chemical was in it last? Instead, use a pipet bulb or syringe to draw up the solution. These are generally available in any laboratory where you might need to use a pipet.

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