Prelab for Week 2

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Prelab Reading for Week 2
Chemistry 201A
Cuesta College


  1. The balances are to be used for weighing items as described in the lab manual--not your pen, mail, or textbook.  Misuse of these balances could result in significant damage.  Balances are expensive!

  2. Never put chemicals on the balance pans alone--always use a piece of weighing paper, plastic weighboat, or glassware.

  3. Each balance has a TARE feature that will "zero" the digital readout. For example, if you just want to know the weight of a chemical, put the weighing paper or glassware on the balance pan and push the "zero" (or "tare") button. The digital display will read "0.00". Adding any other weight will result in only the weight of what you are adding being displayed. Just make sure you give the balance time to stabilize on a particular value.

  4. Make sure you only use the TARE feature just before measuring the mass of the desired object.  If you go back to your bench, someone may tare something else in your absence.

  5. Keep the balance pans clean. If you spill a chemical on the balance or balance pan, take the time to clean it off for the next person.  If dirty, clean the balance BEFORE you use it!

  6. NEVER weigh anything hot on the balances--this could damage the balance and your measurement. Wait until the object is only warm to the touch before weighing it.

  7. NEVER transfer liquids or solids inside the balance cover. Take the container off the balance pan, add the solid or liquid over the bench top, and then return to the balance

  8. Try to use the same balance each time during the course of a particular lab.

  9. ALWAYS bring an ink pen and data table with you to the balances. Write down your data as you collect it. Do not rely on memory and don't shout data values to your partner across the room.


  1. Always read the volume of a liquid in a container so your eye level is at the same level as the surface of the liquid.  Can you see a car's speedometer accurately from the passenger seat?

  2. When the volume of water and water-solutions are read in a container, you will notice that there is a curved surface at the top of the liquid. This is called the MENISCUS. Always read the bottom of the meniscus.


  1. Beakers are very convenient, but often not very accurate or precise.

  2. Many beakers are labeled with the accuracy of the beaker listed as a percentage.  Typical beakers are only ±5%

  3. Beakers are a great tool for when accuracy isn't an issue, but not to be used when volumes need to be known to more than 2 significant digits.

Graduated Cylinders:

  1. Determine the volume contained in a graduated cylinder by reading the bottom of the meniscus at eye level.

  2. Read the volume using all certain digits and one uncertain digit.

  3. Certain digits are determined from the calibration marks on the cylinder. The uncertain digit (the last digit of the reading) is estimated.  It is your job as a junior apprentice chemist to always estimate one digit past what is known for certain.

  4. Depending on the size of the graduated cylinder and the graduations, the uncertain digit may be to the milliliter ( X ), the tenth of a milliliter ( 1.X ), or the hundredth of a milliliter ( 1.1X ). You will have to look at the graduated cylinder to determine how many digits to record!

  5. Make sure that you carefully empty the graduated cylinder, catching the last drop from the funnel opening.

  6. DO NOT dry a graduated cylinder by stuffing paper towels down to the bottom.  Rinse a graduated cylinder with DI water, then rinse 2-3 times with small volumes of the liquid you are about to measure.

Here is a picture of a graduated cylinder.  Note the curved meniscus.  To read the volume of the liquid, first make note of the divisions.  There appear to be lines for increments of 1 and 0.2 mL.  The bottom of the meniscus is certainly above 6 but less than 7 mL.  It is also certainly above the 6.6 mL mark, but below the 6.8 mL mark. 

Your job is to determine where between 6.6 and 6.8 mL.

I would estimate the volume to be 6.60  mL.  An answer of 6.6 is not complete, and the volume is certainly not 6.65 mL.  An answer of 6.62 mL is fine, but the limit of accuracy is probably more like ± 0.05 mL for this device.

An answer of 6.600 mL has too many digits.


  1. Pipets come in two main types, volumetric (holds and delivers one volume only) and graduated (like the grad. cylinder). 

  2. We will use 10.00 mL volumetric pipets this week.  They are designed to deliver 10.00 mL, ± 0.02 mL.

  3. Great pictures are here:

Thank you, Olivery Seely from CSU DH

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