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:
- Measure the width of a chemistry textbook using the rules of sig figs and Rulers A-D and record below.
| RULER USED | WHAT IS THE SMALLEST UNIT MARKED ON THIS RULER? (100’s, tens, ones, tenths, hundredths, etc.) | TO WHAT PLACE SHOULD YOU ESTIMATE? | WIDTH OF TEXTBOOK (cm) |
| A | |||
| B | |||
| C | |||
| D |
- Answer the following questions:
- How are these rulers alike?
- How are they different?
- What ruler is most accurate?_________________
- What ruler is least accurate? _________________
Using sig figs while measuring:
- 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:
| SMALLEST MARKED DECIMAL PLACE ON EQUIPMENT | SO… Your measurement should be to the… (whole number, 1/10, 1/100, etc) | YOUR ESTIMATED MEASUREMENT (use the sig fig measurement rule, so one decimal place MORE than what is marked) | |
| Volume of water in the 100 mL beaker | |||
| Volume of water in the 100 mL graduated cylinder | |||
| Volume of water in the buret (make sure you have me check when you measure using the buret!) | |||
| Volume of water in the Erlenmeyer flask | |||
| Mass of your pen/pencil using balance | |||
| Volume of water in the 10 mL graduated cylinder |
Rules for calculations with significant figures:
- 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
(Check: Did you follow your addition rule when you rounded your final answer? ________)
- Measure the area of this sheet of paper in cm2. Area is Length X Width.
_____________ cm x__________________cm = ____________________cm2
(Check: do your length and width measurements have TWO decimal places? ________Did you follow your multiplication rule when you rounded off your final answer?________)
- Measure the volume of the object at your table in cm3. Volume is Length x width x height
____________cm x _________________cm x _________________cm = ________________cm3
(Check: do your cm measurements have the right number of decimal places? ______ Did you follow your multiplication rule when you rounded off your final answer?______)
- 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
(Did you follow your subtraction rule before rounding your answer? ________ )
Finally, WRITE your density formula, SUBSTITUTE in your mass and volume values, and SOLVE.
density of the wire = _____________g/cm3 (did you follow your division rules before rounding your answer? _____)
Post-lab questions:
- 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.
