M l and Moles d Masses M A mole is 6.022x1023 of anything; atoms, molecules, or even ppotato chips. p While this is an incrediblyy large g number, it is a useful number when you are dealing with atoms or molecules, because it represents roughly a handful of a solid material that you might use in an experiment. experiment Since doing chemistry by handfuls of material is inexact, messy, and dangerous, the preferred chemical technique is to measure all your chemicals h i l in i grams, andd then th use mathematical th ti l conversions i to t find out how many moles of material you had. Since you should already have conversions down pat, that means we aren’t really doing anything new, all we are doing is playing with a different set of conversion factors.
M l and Moles d Masses M Each element has a different number of protons and neutrons in its nucleus, so one mole of each element will have a different mass. The atomic mass of an element is defined as the mass of one mole of that element. You do not have to memorize these masses for my class. class On any test you will be given a periodic table that has the atomic mass for each element listed. I’m going to guess, however, that by the end off the th year you will ill have h six i or eight i ht off the th mostt common elements l t memorized anyway, just because you will have used them so many times.
G Grams off elements l t to t moles l Problem: I have 50.00 grams of lead. How many moles is this? ((I will treat this jjust like another unit conversion problem) p ) Starting units: grams Ending units: moles Conversion factor: The atomic mass of lead (Pb) look it up on the periodic table At i mass = 207.2 Atomic 207 2 g Pb = 1 mole l Pb Set up equation: grams x (moles/grams) = moles Plug in numbers and solve: 50.00 g Pb x (1 mole Pb/207.2g Pb) = .2413 moles of Pb
M l off element Moles l t to t grams Problem: I want to use .45 moles of carbon in a chemical reaction. How manyy ggrams of carbon must I use? Starting units: moles Ending units: grams Conversion factor: The atomic mass of carbon (C) look it up on the periodic table At i mass = 12.01 Atomic 12 01 g C = 1 mole l C Set up equation: mole x (grams/moles) = grams Plug in numbers and solve: .45 moles C x (12.01 g C/1 mole C) = 5.4 g of C
More complicated molar conversion i Now let’s try a more complicated problem. I have 5.00 ppounds of copper pp that I want to recycle. y How many y moles of copper is this? Starting Unit: 5.00 5 00 lbs Cu Ending Unit: moles of Cu Conversion factors needed: 1 lbs = 453.59g 1 mole Cu = 63.55 g (atomic mass)
How many moles is 5 lbs of copper?? Conversion equation: g of Cu mole of Cu lbs of Cu × × lbs of Cu g of Cu
Plug in the numbers: 45359 . g off Cu 1 mole off Cu 5.00 lbs lb off Cu C × × 1 lbs of Cu 6355 . g of Cu = 35.6879 moles of Cu = 35.7 mole of Cu rounded to 3 sigfig
M l l masses Molecular Since a molecule is a collection of atoms that are held together by chemical bonds, the molar mass of a molecule is simply the sum of the atomic masses of all the atoms that make up the molecule. What is the molar mass of water (H2O)? Atoms in the molecule 2 hydrogens, atomic mass 1.008g 1 oxygen, atomic mass 16.00 g Total = 2(1.008) + 16.00 = 18.016g
M l l masses Molecular Try to get the molar mass of these molecules C2H6O (ethanol) CH3CH2OH (Hint: this is the same molecule, just written a different way) H2SO4 (NH4)2SO4 (Hint: what does the subscript on the parenthesis mean?) CoC2O4.2H2O (Hint: the .2H2O means that two water molecules are part of the complete molecule and must be included in the molar mass)
M l l masses Molecular C2H6O
2(12.01) + 6(1.008) + 16.00 = 46.068 g
CH3CH2OH
H2SO4
2(12.01) + 6(1.008) + 16.00 = 46.068 g
2(1.008) + 32.07 + 4(16.00) = 98.086 g
(NH4)2SO4
2(14.01) + 8(1.008) + 32.07 + 4(16.00) = 132.154 g
CoC2O4.2H2O 58.93 + 2(12.01) + 4(16.00) + 4(1.008) + 2(16.00) = 182.982g
C bi ti Problem Combination P bl Let’s try a problem that combines all the elements you have learned so far in the tutorials. If I need 0.50 moles of sodium sulfate for an experiment, how manyy ounces of this material will I have to weigh g out?
C bi ti Problem Combination P bl If I need 0.50 moles of sodium sulfate for an experiment, how many ounces of this material will I have to weight out? y This problem p has several elements combined in it. Analysis: Let’s see if you can sort them out. Let’s start by identifying what you are given and what you need for an answer. Given: 0.50 moles of sodium sulfate Answer you want to find: ounces of sodium sulfate
C bi ti Problem Combination P bl Simply put: 0.50 moles of sodium sulfate » oz. of sodium sulfate So you are given moles and you want to change it to a mass. How do you do this? First we need a molecular mass conversion i factor f t to t go from f moles l to t grams, andd that th t brings bi us to the first part of the problem; you don’t have a molecular formula, you have a name. So your first step will be to use the name to come up with the correct molecular formula.
C bi ti Problem Combination P bl Simply put: 0.50 moles of sodium sulfate » oz. of sodium sulfate Once you have the correct molecular formula, then the next stepp is to find the molar mass so you y can do your y mole-to-gg conversion. Notice that this will leave us with units of grams, grams so it becomes clear that the last part of the problem is converting from grams to ounces.
C bi ti Problem Combination P bl So let’s put it all together 1. Name to formula sodium sulfate = Na2SO4 2. Formula to molecular mass 2(22.99) + 32.07 + 4(16.00) = 142.05g 3. Mole-to-g g conversion 0.50 moles x (142.05g/mole) = 71.025g 4 g-to-oz 4. g to oz conversion 71.025 g x (1 lbs/453.59g) x (16 oz/1 lbs) =2.50535 oz
C bi ti Problem Combination P bl Actually the preferred method is to put all the calculations into one long equation so you don’t have to round off between steps: 0 50 mole x (142.05g/1 0.50 (142 05g/1 mole) x (1 lbs/453.59g) lbs/453 59g) x (16 oz/1 lbs) =2.50535 oz And then, only at the end of the calculation, round off to the correct number of sig figs, 2, and put the correct label on the answer. =2.5 ounces of sodium sulfate
