Purpose: To practice taking measurements and use the sig figs rules in order to correctly calculate measurements.

Materials:  ruler, meter sticks provided, balance, 100ml graduated cylinder, thermometer, copper wire, other equipment will be out.

Accuracy of measuring tools:

1.  Measure the width of a chemistry textbook using the rules of sig figs and Rulers A-D and record below.
• How are these rulers alike?
• How are they different?
• What ruler is most accurate?_________________
• What ruler is least accurate? _________________

Using sig figs while measuring:

1. What is the rule about estimating with measuring tools like rulers, graduated cylinders, etc?

(How did you know what to write in the second column in the table above?)

At your table you will find equipment to use for taking measurements:

Rules for calculations with significant figures:

1. When you add or subtract numbers in chemistry, before you round your answer you should count the number of ________________________________ in the numbers you’re adding or subtracting.  Then you round your answer to the ______________________ number of decimal places in your problem.
• When you multiply or divide numbers in chemistry, before you round your answer you should count the number of _____________________________________ in the numbers you’re multiplying or dividing.  Then round your answer to the _______________________ number of significant figures in your problem.
• Measure the perimeter of this sheet of paper in cm.  Look carefully at your ruler, so you can follow the sig fig rules when you write down your measurements!  (Your ruler is marked to the 1/10 of a centimeter, so YOUR measurements should all have _____ decimal places.)

___________cm + ____________cm + ___________cm+ ____________cm=  _____________________cm

• Measure the area of this sheet of paper in cm2. Area is Length X Width.

_____________ cm x__________________cm =  ____________________cm2

• Measure the volume of the object at your table in cm3.  Volume is Length x width x height

____________cm x _________________cm x _________________cm = ________________cm3

• What is the density of the copper wire?

First find Mass= ______________g         (use balance – record ALL numbers from the balance – don’t round)

Next find Volume.  Use water displacement method.  Record the initial water level:                  __________ ml

Add the wire.  Record final water level.               __________ ml

Now find the difference between the two water volumes.  THAT is the volume of the wire.             _________ ml

Finally, WRITE your density formula, SUBSTITUTE in your mass and volume values, and SOLVE.

Post-lab questions:

1. What is the elemental symbol for copper?___________
• Does it matter if each group gets a different size copper sample to determine density?  Explain your answer.
• Using reference table S, look up the accepted density of copper.________________________
• Calculate the percent error of the density of copper using your measurement and the accepted value.  Show all work, including the formula, substitute in the numbers, and solve)
• A student is calculating the density of an object.  She measure the mass as 5.45 g and the volume as 2.23 ml.

She reports that the density is 2.444 g/ml and her teacher marks it wrong, because she didn’t round to the correct answer.  What SHOULD her answer be, and WHY?

• Students using different rulers are measuring the length of a piece of wire. Mark says that the wire has a length of 11.5 cm.  Sarah says the wire has a length of 11.55 cm.  If they are both RIGHT, what can you tell me about the rulers that they used?

Mark’s :                                                                                                    Sarah’s:

Conclusion: Tell me why we use significant figures.  Tell me how significant figures can help you in lab.  Which rules are hard to remember? Discuss the equipment and which are more accurate and which are less accurate.

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