Dimensional Analysis - Assessment Reminder

First, tomorrow (Friday, September 3rd) is your first assessment of the semester. As stated in the previous post, you will be performing a lab practical based upon the Density of Pennies lab. All practices associated with that lab will be assessed.

• You will be required to find the density of a material accurately.
• You will be given roughly 15 minutes to collect data in the lab area by yourself.
• There will be no lab partners involved, so you must know how to operate and read all lab equipment used.
• Your procedure must maximize the number of significant figures in each measurement.
• Your procedure must minimize error, thus the order in which measurements are made must be considered. *Note, you will not be required to record the steps of your procedure. Doing steps in the wrong order will cause your calculated value for density to be inaccurate.
• Masses must be found to the 1/1000 of a gram and the volumes to the 1/10 of a mL.
• You must present your measurements in a Data Table in a neat and orderly manor. You will lose points for not including the units associated with each measurement. *Do not perform any calculations of substance (i.e. density) in the data table. You can calculate the mass and volume of the material by subtracting an initial amount from a final amount. That is the only type of calculation allowed in a data table.
• When calculating the density, significant figure rules MUST BE FOLLOWED!!
• If you do not include units in your final calculated value, you will lose points.
• Every year, a student or two forgets the most basic part of the lab, and that is how to find density. Please remember that it is mass divided by volume. If you have to be reminded, you will lose points.
• Finally, you will be asked to read a scenario based upon a lab procedure and ask questions associated with knowing proper lab procedure and calculation procedures involving measurements. You will need to answer most of the questions in complete sentences.

**Rules For Performing Dimensional Analysis**

1. Start out with what you know. (Or put another way, what value is given in the problem.)
2. Set-up what you know as a fraction if not already a fraction.
3. Find the correct conversion factor (or factors) needed to convert from one unit to another.
4. Set-up the conversion factor as a fraction so the unit(s) that are not desired factor (cancel) out and the desired units remain.
5. Perform the needed calculations. Almost all conversion factors will be treated as absolutes (counted numbers are absolutes), so the conversion factors should not affect significant figures. THE NUMBER OF SIGNIFICANT FIGURES STARTED WITH IN A CONVERSION IS HOW MANY SIGNIFICANT FIGURES THE ANSWER SHOULD HAVE!

The rule written below in big, bold letters should always be applied to metric conversion.

**Always convert to the base unit!!**

The base units for the metric system are meters (m) for length, liters (L) for volume, and grams (g) for mass. A copy of the conversion table given out in class can be found on the Events Calendar under the date September 2nd. A copy of the conversion packet can also be found on that date.