Chem Class- September 15, 2009

Unit 1 has begun! After having a brief summary of the safety rules in the Chemistry Laboratory, we've headed straight into measurement. This includes how to make accurate measurements and what factors affect measurement, the SI System of measurement, significant digits and scientific notation to communicate results, experimental error and applying dimensional analysis to chemistry problems.

Right. So, the most common system in use today is the SI System (abbreviated from the French le System internationale d'unites).

There are 7 Fundamental units:

1) kg (Kilogram-mass)

2) m (Metre-distance)

3) s (Second-time)

4) K (Kelvin- temperature)

5) mol (Mole-amount of substance)

6) A (Ampere-current)

7) cd (Candela-luminosity)

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This is very helpful when converting from one unit to another. Here's a table of SI prefixes and symbols I got off the web:

And now here's a cute Chemistry cartoon that deals with SI prefixes that I got from a Chemistry book:

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 We also learned about significant figures. They are measured or meaninful digits. Unless you are counting a small number of objects, it can be difficult to find the EXACT value of a property such as mass, time, volume or length. You may be quite precise, but not exact. The number of significant figures is equal to all the certain digits plus the first uncertain digit.

Things to note:
- Digits 1-9 are always significant
- If no decimal point is shown, any zeros at the end do not count
- Zeros are significant if they are to the right of a decimal

Examples:
365.249 -6 signifcant digits
150 - 2 significant digits
0.002 - 1 significant digit
2.010 - 4 significant digits

When you are multiplying or dividing numbers, round to the least number of significant figures.
When you are adding or subtracting numbers, round to the least number of decimal places.
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Next, we learned about experimental error. There are usually three reasons for error:
1) Physical errors in the measuring device
 2) 'Sloppy' measuring
 3) Changing ambient conditions
-Error is taken to be half the smallest division on your measuring device.

- There are two different possibilities of error (well, these are the two we learned for now anyways):

Absolute Error and Percent Error  (most common)

Here's how to calculate both:

Absolute Error = Measured value - Accepted Value (A postive number means you are over the accepted value and a negative number means you are under the accepted value)

Percent Error = Absolute error/ Accepted Value
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Finally, we ended off with Dimensional Analysis. This is the easiest way to change from one unit to another. We were instructed of the four basic steps.

1) Find a unit equality

2) Find the conversion factors

3) Apply the conversion factor

4) Cancel units

Yes! It's that easy!

If you need the extra help, have a look online. Youtube even has some step-by-step instructions on Dimensional Analysis. Here's one example:

Check it out.