# Atomic Weight Molecular Weight Formula Weight and Molar

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Atomic Weight, Molecular Weight, Formula Weight and Molar Mass loosely based on Chap 4 Sec 1 of Jespersen 6 th edition Dr. C. Yau Spring 2014 1

Atomic Weight or Atomic Mass Examine the periodic table. What is the atomic weight (AW) of carbon? The atomic weight of C is or 12. 01 u/atom It means that each C atom weighs 12. 01 u. (In this class you should make it a habit to look up AW to 4 sig. fig. ) 2

Molecular Weight or Molecular Mass The AW of C is 12. 01 u/atom The AW of O is 16. 00 u/atom. Therefore, the mass of one molecule of CO 2 would be 12. 01 + 2(16. 00) = 44. 01 u/molecule. (Always round to 4 sig. fig. ) We call this the molecular weight (MW) or molecular mass. What is the MW of methane (CH 4)? Ans. 16. 04 u/molecule 3

Formula Mass If we add up the atomic mass of Na and Cl we get 22. 99 + 35. 45 = 58. 44. What is wrong with saying… the molecular weight of Na. Cl is 58. 44 u/molecule? 4

Formula Mass For molecular substances, the smallest unit is the molecule. For ionic substances, the smallest unit is the "formula unit" abbreviated f. u. e. g. If you pluck out the smallest unit out of water, you would have a water molecule. 5

Formula Mass When you pluck out the smallest unit of table salt you would have a formula unit of Na. Cl. Na+Cl Cl Na+Cl Na+Cl- We often simply write Na. Cl, but it is understood that Na is Na+ and Cl is Cl-. There are no molecules in an ionic compound! 6

AW, MW and FW Summary: Atomic Weight (AW) = mass of one atom (for atoms) in units of amu/atom Molecular Weight (MW) = mass of one molecule (for molecular substances) in units of amu/molecule Formula Weight (FW) = mass of one f. u. (for ionic compounds) in units of amu/f. u. 7

Calculations of AW, MW and FW Calculate the formula weight of… (and give the proper units) cobalt(III) sulfide ammonium sulfate Calculate the molecular weight of… (CH 3 CH 2)3 N 8

Calculate the formula weight of… Na 2 SO 4 2 H 2 O This is known as a hydrate. The dot in the formula does NOT mean you multiply anything. The dot means there are 2 water molecules attached to each f. u. of Na 2 SO 4. What is its formula weight? 9

The Mole Concept One mole is defined as the number of atoms in exactly 12 g of C-12 atoms. Remember that C-12 refers to the isotope…. exactly 12 g C 6. 022 x 1023 C atoms It turns out that there are 6. 022 x 1023 C atoms in those 12 grams. So…. 1 mole = 6. 022 x 1023 MEMORIZE THIS NOW! (not an exact number, but in 4 sig. fig. ) 10

The Mole Concept 1 mole = 6. 022 x 1023 This number is known as Avogadro's number Note: Mole is abbreviated mol NOT m (which stands for meters) Note: It is a NUMBER. It is NOT a mass. It does NOT have the unit of grams. 11

The Mole Concept We all know… 1 dozen eggs = 12 eggs 1 dozen atoms = 12 atoms In the same way, we use the word "mole"… 1 mole eggs = 6. 022 x 1023 eggs Obviously that is a VERY big number of eggs. There would be no occasion to be talking about a mole of eggs. When do you we use "moles? " 12

The Mole Concept 1 mol atoms = 6. 022 x 1023 atoms 1 molecules = 6. 022 x 1023 molecules 1 mol electrons = 6. 022 x 1023 electrons The "mole" is not a mass itself, just a number of items. Why do we bother with such a strange number? Let us take a look at the periodic table. 13

The Mole Concept The atom is a very, very small particle. You can fit 7000 atoms on the dot of an i. It is too small to see with the naked eye, and too small to hold in your hand or weigh on the balance. We must use a much larger quantity of atoms: a mole of atoms. 14

The Mole Concept From the periodic table, we can get the atomic weight for Ag. This mass can be given in two units: 107. 9 u per atom or 107. 9 g per mole of atoms One atom of silver weighs 107. 9 u, which is 0. 000000000001079 gram Get the picture? You cannot weigh a single atom of silver, but you CAN weigh a mole of Ag atoms. 15 107. 9 g is about 3. 8 ounces.

The Mole Concept From the periodic table we see that… Ag is 107. 9 g/mol Cu is 63. 55 g/mol Cu Ag What does 107. 87 g of Ag have in common with 63. 55 g of Cu? They both contain the same number of atoms: 6. 022 x 1023 Ag atoms and 6. 022 x 1023 Cu atoms, in other words, 1 mole Ag and 1 mol Cu. 16

The Mole Concept Earlier we were talking about the AW, MW and FW. Now we are going to add one more…. MM = molar mass Whenever you see "molar" you should think "per mole. " Molar mass = mass per mole (for 1 mol) with units of g/mol KNOW THIS WELL! SEE THIS IN YOUR DREAMS…. AND NIGHTMARES! 17

Molar Mass (MM) The atomic mass from the periodic table can be used with two units: amu/atom or g/mole (or g mol-1) REMEMBER: Whenever you see a fractional unit (such as amu/atom, g/mol, g/m. L) you should think "conversion factor"! The g/mol gives you the link between MASS AND # of MOLES. 18

Molar Mass (MM) The FW of Co 2(SO 4)3 is 406. 7 amu/f. u. Its MM = 406. 7 g/mol 2(58. 93)+3(32. 07)+12(16. 00) = 406. 7 We had calculated the MW of… (CH 3 CH 2)3 N to be 101. 2 amu/molecule Its MM = 101. 2 g/mol 2 x 3 C + 5 x 3 H + N = 6(12. 01) + 15(1. 008) + 14. 01 = 101. 2 19

Molar Mass (MM) Note that we no longer need to distinguish between f. u. and molecules in the units of MM. It is understood that for Co 2(SO 4)3 its MM = 406. 7 g/mol of f. u. and for (CH 3 CH 2)3 N its MM = 101. 2 g/mol of molecules. 20

Use of MM in Dimensional Analysis Conversion factors from MM: e. g. MM (N) = 14. 01 g/mol e. g. MM (HCl) = 36. 45 g/mol 21

Use of MM in Dimensional Analysis When do we use MM? Whenever you see a fractional unit (such as g/mol or g mol-1) you should think "conversion factor"! MM is the link between mass & moles mass moles of particles moles of atoms moles of f. u. moles of molecules e. g. mol Cu. Cl 2 mol PBr 3 22

Use of MM Example 4. 1 p. 109 Titanium(IV) oxide is one of the best sunscreens because it completely blocks ultraviolet radiation from reaching the skin. In an experiment to prepare Ti. O 2, we start with 23. 5 g sample of titanium. How many moles of Ti do we have? 23

Example 4. 2 p. 110 We need 0. 254 mol of iron(III) chloride for a certain experiment. How many grams do we need to weigh? Do Practice Exercises 4. 1 & 4. 2 on p. 110 (Note: Ans to Pract. Exercises are at the back of the book. ) 24

Use of Avogadro's Number in Dimensional Analysis Conversion factors from Avogadro's number: Note that X can be any. THING, such as CO 2, K 2 SO 4, electrons, Na+, etc…. as long as it is EXACTLY the same on top as on the bottom. BUT it cannot be a unit, such as g or m. L. 25

Use of Avogadro's Number in Dimensional Analysis You use Avogadro's number ONLY if you have to go between the macroscopic and the particulate (sub-microscopic) levels. particulate level (conceptual, non-observable) atoms f. u. molecules u macrosopic level (observable) moles of atoms moles of f. u. moles of molecules g, mg, lbs, oz 26

Use of Avogadro's Number Example 4. 3 p. 111 Tungsten wire is the filament inside most incandescent light bulbs. In a typical light bulb, the tungsten filament weighs 0. 635 g. How many atoms of tungsten are there in such a light bulb flame? First ask yourself. . . “Do I need Avog. #? ” 27

Example 4. 4 p. 112 Carbon tetrachloride was used as a drycleaning fluid until it was found to be carcinogen. What is the average mass in grams of one molecule of carbon tetrachloride (CCl 4)? “Do I need Avog. #? ” 28

Use of Avogadro's Number Example What is the mass in grams of 2. 01 mole of Na. Cl? Example How many anions are in 7. 82 g of Mg. Cl 2? Example What is the weight in grams of 158 molecules of CO 2? REMEMBER! ASK YOURSELF… DO I NEED AVOGADRO'S NUMBER IN THIS PROBLEM? Do Practice Exercises 4. 3 & 4. 4 on p. 112 29

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