3. Composition of substances & solutions • 3.1: Formula mass &mole concept • Calculate formula masses (MW) • Define the mole & Avogadro’s number; explain the relationship between moles, atoms & molecules and convert one to another
Formula mass (aka molecular weight) Formula mass (aka MW): the sum of all the atomic masses in a molecule or compound Chloroform, CHCl3, was used as a surgical anesthetic & is now used as a building block for creating teflon, tetrafluoroethylene. NaClis common table salt. Chemistry Openstax
Try these molecular compounds • Calculate the formula masses (aka molecular weights) of: • Aspirin, C9H8O4 (acetylsalicylic acid) • Ibuprofen, C13,H18O2 1 C 9 12.01 108.09 H 8 1.01 8.08 O 4 15.99 63.96 180.13 amu C 13 12.01 156.13 H 18 1.01 18.18 O 2 15.99 31.98 206.29 amu
Try theseionic compounds • Calculate the formula masses (aka molecular weights) of: • Aluminum sulfate, Al2(SO4)3, used to purify water & make paper • Calcium phosphate, Ca3(PO4)2, used to inject DNA into cells 2 Al 2 26.98 53.96 S 3 32.06 96.18 O 12 15.99 191.88 342.02 amu Ca 3 40.08 120.24 P 2 30.97 61.94 O 8 15.99 127.92 310.10 amu
The mole • Mole: the ‘counting’ unit for matter on the atomic scale. • It’s like a dozen, or gross; equal to a number of objects. • From the Latin ‘moles’: mass, massive, barrier 1 mole = 6.02 E23 atoms or ions or molecules 1 mole 6.02 E23 ________ called Avogadro’s number when expressed as a conversion factor Consider:If you had one mole of marbles— 6.02x1023 marbles — and you spread them all over the entire surface of the Earth, how thick would the layer of marbles be? A layer produced with a mole of marbles would be about three miles thick!
Enormous numbers are hard to ‘get’ This droplet of water containsabout 0.002275 moles of water. 2.275 E-3 moles 6.02 E23 molecules = 1.369 E21 molecules 1 mole Global population = 7.6 billion = 7.6 E9 1.369 E21 molecules= 1.80 E11 > 100 billion X as many 7.6 E9 people Chemistry Openstax
Compare a mole Each is a mole. So, replace amu with grams/mole. Chemistry Openstax
Compare a mole Again, each is a mole. C8H17OH octanol MW 130.2 g/mol HgI2 mercury (II) iodide MW 454.4 g/mol S8 Elemental sulfur MW 256.5 g/mol CH3OH methanol MW 32.0 g/mol Chemistry Openstax
Converting mass to moles Atomic mass or formula mass (MW) can be used to convert mass to moles (or vice versa). 3 USDA nutritional guidelines suggest a daily intake of 4.7 g of potassium. What’s the requirement in moles of K? 4.7 g 1 mole K = 0.12 mol of K 39.10 g Beryllium, a very light element, is used to make transparent x-ray windows for imagining devices. How many moles of Be are in a thin window that weighs 3.24 g? 4 3.24 g 1 mole Be = 0.360 mol of Be 9.01 g
Converting mass to atoms or molecules Add Avogadro’s number to convert masses of atoms or molecules into moles, an then numbers of atoms or molecules. Copper is used to make electrical wire. How many copper atoms are there in 5.00 g of wire? 5 Strategy: mass moles atoms 5.00 g 1 mole Cu 6.02 E23 atoms = 4.74 E22 atoms Cu 63.55 g 1 mole Cu A prospector collects 15.00 g of pure gold from a river. How many gold atoms (Au) are there? 6 Strategy: mass moles atoms 15.00 g 1 mole Au 6.02 E23 atoms = 4.584 E22 atoms Au 197.0 g 1 mole Au
Let’s start building a mole map By the end of this course you’ll be using a bunch of conversion factors to make the change of chemistry happen. It helps to have a map to guide your choice of conversion factors. atoms mass (g) moles formula molecules atomic or formula mass (g/mol) Avogadro’s 6.02 E23 (____/mol) Flip conversion factors to solve for your ‘destination’ units.
First test drive Vitamin C has the molecular formula C6H8O6. The recommended daily dose for kids aged 4-8 years is 1.42 E-4 moles. How many grams should you give them? 7 MW = 176.12 g/mol Strategy: mol g g .mol 176.12 1.42 E-4 mol g. mol = 0.0250 g 1
Longer trip Our bodies make amino acids, the building blocks of proteins. The simplest amino acid is glycine, C2H5O2N. How many moles of glycine molecules are there in 28.35 g? How many hydrogen atoms in the 28.35 g? 8 MW = 75.07 g/mol Strategy: g moles molecules atoms atoms molecule mol g molecules mol 1 5 6.02 E23 • 28.35 g mol molecules atoms . • g mol molecule 1 1 75.07 0.3776 mol 1.137 E24atoms H
Long trip back Saccharin, C7H5NO3S, is the old ‘pink’ sugar substitute. How many g of sacchirin have 9.20 E21 carbon atoms? 9 MW = 183.18 g/mol Strategy: atoms molecules moles g g . mol formula ratio mol. molecule 1 1 183.18 9.20 E21 C atoms molecule mol g C atoms molecule mol 7 6.02 E23 1 = 0.400 g Chemistry Openstax
Can you? (1) Calculate a formula mass or molecular mass (aka molar mass or molecular weight, MW)? (2) Understand just how massive Avogadro’s number is? (3) Understand how and why a mole of different elements and compounds can have radically different masses? (4) Use atomic mass, molar mass, Avogadro’s number and formula subscripts as conversion factors to convert mass to moles to molecules to atoms? (5) Use a ‘mole map’ to help you choose conversion factors and the number of steps for conversion problems?