1. Did I learn that in Gen Chem I?
Some Review…
Did I learn that in Gen Chem I?
CHM PGCC Barbara A. Gage

2. 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 PGCC Barbara A. Gage

3. 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 PGCC Barbara A. Gage

4. Significant Figures
They don’t go away…
CHM PGCC Barbara A. Gage

5. 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 PGCC Barbara A. Gage

6. Significant Figures The graduated cylinder at the
right contains mL of liquid.
It is assumed that you can visually
divide the space between lines
into ten parts.
CHM PGCC Barbara A. Gage

7. 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 PGCC Barbara A. Gage

8. 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.
sig figs
X sig figs
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 PGCC Barbara A. Gage

9. 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 PGCC Barbara A. Gage

10. Volume Measuring Devices
Why do the numbers run different ways?
CHM PGCC Barbara A. Gage

11. Measurement Devices To Contain (TC) Designed to hold a given amount
Beakers, Erlenmeyer flasks, graduated cylinders
CHM PGCC Barbara A. Gage

12. Reading a graduated cylinder
Locate the bottom of the meniscus and determine the value
CHM PGCC Barbara A. Gage

13. Measurement Devices To Deliver (TD)
Designed to dispense a given amount
Burets, pipets
CHM PGCC Barbara A. Gage

14. Reading a buret
Locate the bottom of the meniscus and determine the value
CHM PGCC Barbara A. Gage

15. 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 g and are only used for small masses that require more accuracy.
g = 0.1 mg !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Using the wrong balance is a SERIOUS error. Be sure to read your activity carefully!
CHM PGCC Barbara A. Gage

16. 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 PGCC Barbara A. Gage

17. 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 g less than before.
% error can be positive or negative.
CHM PGCC Barbara A. Gage