This document discusses several chemistry concepts including:
1. Using the lowest number of significant figures when performing calculations to maintain precision.
2. The importance of labeling answers with appropriate units.
3. Using dimensional analysis to check calculations and cancel out units.
4. Calculating density, mass, volume, and laps completed using dimensional analysis in a word problem about refueling a race car.
5. Defining a mole as 6.02 x 1023 particles and using moles to calculate quantities of substances.
2. Calculating with significant digits When performing any calculations with significant digits, always use the number with the lowest number of significant digits. This allows us to maintain the proper degree of precision and accuracy when calculating our yields and points to the sources of error and ways to improve.
3. Calculations Example: 100g+299g=400g 100 has 1 significant figure, 299 has 3. Use the number with the lowest sig figs. Why??? Because we do not know the degree of precision was obtained. Let’s think about the bean lab. One group could have measured 47.99g and another gave us 50g. Therefore, 47.99g+50g=100g
4. Labeling When performing any calculation it is also of extreme importance to label answers. Remember our base units of measure. Grams=mass Liters=volume Meters=length An answer without a label will have points deducted!!!
5. Dimensional Analysis We will use this as a way to check our work and cancel out units. You may already know how to do this, but do not recognize what it is called. We have done something like this in our density lab.
6. Example problem You have a substance with a density of 63g/L. How will you find out the number of grams in 500ml?
7. Dimensional Analysis In NASCAR racing we did not have a fuel gauge in the race car. Instead we had to estimate how much fuel was consumed and when we would have to stop again. Fuel has a density of 800.00g/L The race car holds 51.2kg of fuel You are at a race track 1.0 mile per circuit and the race car will run 4.5 miles on 4.0L of fuel. Problem: how many circuits before we stop?
8. Setting up the problem How do we set up the problem??? First, let’s determine the volume of fuel we have. We do that based on density and the mass.
9. Setting up the problem Now that we have a volume we need to calculate our consumption. 64L of fuel How many laps (miles) can we run? 4.5mile/4.0L
10. Setting up the problem 1.125 mile/L Now given the volume of 64L at what point do we need to stop and re-fuel? 64Lx1.125mile/L= 72miles x 1.0lap/mile= Lap 72
11. Moles Wasn’t that fun???????? Just wait until we really get to practice this stuff every day!!! Avagadro’s Number is 6.02 x 1023 This represents one Mole of any given substance.
12. Moles We use moles to calculate how many molecules, concentrations, particles, etc… It represents very large quantities of items. Molecules and atoms are very small objects so we need any easy way to count them. How large is a mole? A mole of pennies stacked would the sun and back 500 million times! A mole is almost a trillion trillion!!!!
13. Atomic Mass and Moles The atomic mass of an atom is the molar mass in grams. For example, helium has an atomic mass of 4. It’s molar mass is 4g. If you have .3mol of Helium, how many grams?
14. Diatomic Molecules Remember with diatomic molecules (2 or more atoms) you must account for both! F has a mass of 19g/mol, but F2 has a mass of 38g/mol!
15. Compounds Count all the atoms in a compound!!! Let’s look at lithium hydroxide. What is the formula? LiOH 1 Li, 1 O, 1H Ans: 7+16+1=24g
