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Some Review…. Did I learn that in Gen Chem I?. Data and Results. Data = any measured values recorded in the course of an activity temperature, mass from balance, volume in a graduated cyclinder

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some review

Some Review…

Did I learn that in Gen Chem I?

CHM 1030 PGCC Barbara A. Gage

data and results
Data and Results
  • Data = any measured values recorded in the course of an activity

temperature, mass from balance, volume in a graduated cyclinder

  • Results = any value that is the result of manipulating the data (subtracting, converting, finding the log of…)

volume dispensed from a buret, mass of solid that was massed in a container

CHM 1030 PGCC Barbara A. Gage

how many decimal places do i need
How many decimal places do I need?
  • Don’t ask this question!
  • You need to determine this from the data you are using and any conversions or constants
  • The answer will depend on the number of significant figures…remember those?

CHM 1030 PGCC Barbara A. Gage

significant figures

Significant Figures

They don’t go away…

CHM 1030 PGCC Barbara A. Gage

significant figures1
Significant Figures
  • When making a measurement you must record all digits you are sure of and one that is a reasonable estimate (regardless of where the decimal place falls)
  • The object below is 4.86 cm. You can be certain of the 4 and 8. The 6 is an estimate and can vary +/- 1 between measurements.

CHM 1010 PGCC Barbara A. Gage

significant figures2
Significant Figures
  • The graduated cylinder at the

right contains 38.34 mL of liquid.

It is assumed that you can visually

divide the space between lines

into ten parts.

CHM 1030 PGCC Barbara A. Gage

significant figures addition and subtraction
Significant Figures –Addition and Subtraction
  • When you are adding and subtracting numbers you only count the columns where you are sure of all the values in that column.

CHM 1030 PGCC Barbara A. Gage

significant figures multiplication and division
Significant Figures –Multiplication and Division
  • When multiplying or dividing two or more values, the answer should contain the number of digits in the value with the least number of significant figures.

0.003570 4 sig figs

X 23.4 3 sig figs

0.083538 5 sig figs (calculator answer)

You can only trust the answer to 3 sig figs so the correct answer is 8.35 x 10-2.

CHM 1030 PGCC Barbara A. Gage

exact numbers
Exact Numbers
  • Some numbers are “exact” such as the 50 in 50 drops (if you count exactly 50) or 60 s = 1 min
  • Exact numbers do not affect the number of significant figures in an answer

CHM 1030 PGCC Barbara A. Gage

volume measuring devices

Volume Measuring Devices

Why do the numbers run different ways?

CHM 1030 PGCC Barbara A. Gage

measurement devices
Measurement Devices
  • To Contain (TC)
  • Designed to hold a given amount
  • Beakers, Erlenmeyer flasks, graduated cylinders

CHM 1030 PGCC Barbara A. Gage

reading a graduated cylinder
Reading a graduated cylinder

Locate the bottom of the meniscus and determine the value

CHM 1030 PGCC Barbara A. Gage

measurement devices1
Measurement Devices
  • To Deliver (TD)
  • Designed to dispense a given amount
  • Burets, pipets

CHM 1030 PGCC Barbara A. Gage

reading a buret
Reading a buret

Locate the bottom of the meniscus and determine the value

CHM 1030 PGCC Barbara A. Gage

balances
Balances
  • Top loaders in PGCC labs generally measure to the nearest 0.01g. These should be used when you do not require data to more than 0.01g.
  • Analytical balances measure to the nearest 0.0001g and are only used for small masses that require more accuracy.
  • 0.0001 g = 0.1 mg !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
  • Using the wrong balance is a SERIOUS error. Be sure to read your activity carefully!

CHM 1030 PGCC Barbara A. Gage

do fingerprints make a difference
Do fingerprints make a difference?
  • Because the analytical balances are so sensitive, your fingerprints, hand lotion, hand sanitizer or breakfast residue may affect the reading.
  • Measured objects should be placed on and taken off the balance with a lab wipe.
  • Did your fingerprints make a difference?

CHM 1030 PGCC Barbara A. Gage

some miscellaneous things
Some Miscellaneous Things…
  • The analytical balance has some degree of variability in the last digit. If you did not transfer any material from your fingers, there is a chance that the mass after touching will be 0.0001 g less than before.
  • % error can be positive or negative.

CHM 1030 PGCC Barbara A. Gage