1.07 Accuracy and Precision

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1.07 Accuracy and Precision

• Virtual Laboratory

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The Plan

• Review how to read instruments with accuracy and
  precision
• Complete 1 Trial of each Part of the Virtual Lab
  via screen share/slides with teacher
• Use the data to calculate density using the rules
  of accuracy/precision and significant figures
• Discuss the analysis questions
• Complete the lab on your own

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Before we Begin

• Please open your virtual lab in the course

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Create/Download the Lab Form

• Copy and Paste the lab form into a document or
  download the pre-made form from the Chemistry
  Resource Center https//sites.google.com/site/chem
  istryflvs/

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READY?
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Why use Significant Figures?

• The benefit of this agreement is the consistency
  in the way all scientists read and record
  measurements.
• You will always know exactly how many digits your
  measurement should have based on the instrument
  used.
• You will be able to determine what the smallest
  graduations were on the instruments used by
  others when they share their data with you.

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Please Open the Virtual Lab
We will begin with Part I Density of an
unknown liquid
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How do you measure properly?

• When measuring you will determine the number of
  sig figs by estimating one digit past the
  smallest measurement, or graduation, on the
  measuring tool.
• How many digits should our measurement have here?

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What is this measurement?
Use the slides starting with the largest to make
the balance even.
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Make your measurement and record data
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Add liquid to the cylinder and read the amount
What do you think this volume is? Record in
your Notes
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Take the mass of the full cylinder

• Lets read this together. Record in your Notes

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Repeat!

• You will need to complete two more trials of this
  Part on your own
• Lets keep moving!

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Go back to the Menu
Move on to Part II Density of an irregular
shaped object
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Mass is EASY here
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Fill cylinder and record volume
How many digits should we have? What is the
volume?
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Place Solid into Liquid Read and Record

• This is called the displacement method

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NEXT!

• You will need to complete two more trials of this
  Part on your own.
• Lets move on to Part III

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Part III Density of a regular-shaped solid
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Mass the Object. Read and Record

• What is the mass?

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Use the ruler to measure length, width and height

• Remember the number digits is the same for the
  instrument regardless of the measurement

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What about width?

• What is this measurement?

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Complete length on your own!

• After you measure the length, click Data Table to
  review all of your information.
• How do your measurements compare?
• Did you use the correct number of digits?

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Trial 1 Data
Remember to go back and complete 2 more trials
for each Part
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Calculations

• We now need to use our data to complete the
  density calculations
• Density is mass divided by volume or
• D M/V
• If you have downloaded the pre-made lab form it
  is color coded and numbered to help you

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Part I Density

• Part I Density of Unknown Liquid
• Calculate the mass (g) of the liquid for each
  trial (A). (Subtract the mass (g) of the empty
  graduated cylinder 1 from the mass (g) of the
  graduated cylinder with liquid 3)
• mass of the liquid 3 - 1 (A) g
• Trial 1
• 35.58 g- 25.02g ? How many significant figures
  should our answer have?

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Remember Rules for Addition/Subtraction

• The final answer cannot have more places after
  the decimal than any of the given measurements.
• The final answer cannot have a final digit, which
  represents the uncertain or estimated place,
  farther to the right than any of the final digits
  in the measurements used.
• SO.. Our answer should have the same number of
  decimal places as our measurements when
  adding/subtracting.

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Calculate Density

• Calculate the density (g/mL) of the unknown
  liquid (B) for each trial. (Divide the mass (g)
  of the liquid calculated above (A) by the volume
  (mL) of the liquid 2)
• (A) / 2 (B) g/mL
• Trial 1 Using our mass we just calculated
  divide by volume from the data table.
• 10.56g/8.32 mL ? g/mL
• How many digits should our answer have?

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Remember Rules for Multiplication/Division

• Only given measurements affect the number of sig
  figs allowed in the final answer.
• If you are only given one measurement, the total
  number of sig figs in that measurement equal the
  total number of sig figs allowed in your final
  answer.
• If you are given more than one measurement to be
  used in a calculation, the final answer must be
  rounded to the same total number of sig figs as
  the measurement that has the least total sig
  figs.
• SO.. Our answer should have the same number of
  figures as the least significant figures in our
  data set when multiplying/dividing.
• How do we know how many significant figures our
  data has? Print your reference sheet! You will
  need it for the whole course

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Rules to Determine Sig Figs
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Whats our density?

• 10.56g/8.32 mL ? g/mL
• This is division so we follow the rules for
  division and use the least number of figures.
• 8.32 has 3 figures
• Our answer is 1.27g/mL when properly rounded
• Remember to include your units or your
  measurement is just a number!

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Rules for Rounding up or down

• In order to follow the significant figures rules
  for calculations, it is sometimes necessary to
  round your answer or add zeros to the end of the
  answer to give it the proper number of sig figs. 

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Density for Trial 2

• Part II Density of Irregular-Shaped Solid
• Calculate the volume (mL) of the irregular-shaped
  solid (C) for each trial. (Subtract the volume
  (mL) of the water 5 from the total volume (mL)
  of the water and solid 6)
• 6 5 (C) mL
• Trial 159.2 mL 54.9 mL ?
• Check your rules!!

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Whats the answer?

• Remember to follow the rules for subtraction.
• volume 4.3 mL
• Calculate the density (g/mL) of the
  irregular-shaped solid (D) for each trial.
  (Divide the mass of the solid 4 by the volume
  of the solid calculated above (C))
• 4 / (C) (D) g/mL
• Trial 1 38.718 g/4.3 mL ?
• 9.0 g/mL

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Part III

• -Calculate the volume (cm3) of the regular shaped
  solid (E) for each trial. (Multiply the length
  8 cm width 9 cm height10 cm for each
  trial to get the volume in the unit cm3.)
• 8 x 9 x 10 (E) cm3
• Trial 1 5.24cm x 3.00cm x 2.50cm ?
• 39.3 cm3

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Density for Part III

• Calculate the density (g/ cm3)of the
  regular-shaped solid (F) for each trial. (Divide
  the mass (g) of the solid 7 by the volume (cm3)
  calculated above (E))
• 7 / (E) (F) g/ cm3
• Trial 1 25.33 g/39.3 cm3
• 0.64 g/cm3

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Analysis Questions

• 1 How would you determine the proper number of
  significant figures of a liquid using a graduated
  cylinder?
• Hmm.. Well, what did we have to do to read our
  two different cylinders in this lab?

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Analysis Questions

• 2 Can just one measurement be considered
  precise? Can just one measurement be considered
  accurate? Explain your answers completely.
• - What is accuracy?
  Precision?

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Analysis Questions

• 3 In parts II and III of the lab you used
  different sized objects in each trial. Compare
  the density values that you calculated for these
  items, how do the three trials compare?
• What was our density for Part II?
• What about Part III?
• How did our density compare? What the object in
  Part II more or less dense then Part III?

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Congrats!!

• Remember to go back and complete two more trials
  and calculations for all three Parts of this lab
  before submitting to your teacher.
• Great job!

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Questions?