Warmup 10/26/15 We know that S + O 2  SO 2. How do you think scientists figured out this exact formula? Come up with a way you might find out if you were a scientist Objective Tonight’s Homework To learn how scientists figure out chemical equations pp 210: 56, 57 pp212: 58, 59

Notes on Empirical Formulae Let’s take our example from yesterday and look at it from a different angle. Imagine that a salt manufacturer has found just from trial and error that if he combines 79 tons of sodium with 121 tons of chlorine, he gets 200 tons of salt. From this, is it possible to get a chemical formula?

Notes on Empirical Formulae In any problem like this, we want to convert what we have into moles.

Notes on Empirical Formulae In any problem like this, we want to convert what we have into moles. 79 tons Na 9x10 5 g 1 mol Na_ 1 1 ton 23.0 g Na =3.1x10 6 mol Na

Notes on Empirical Formulae In any problem like this, we want to convert what we have into moles. 79 tons Na 9x10 5 g 1 mol Na_ 1 1 ton 23.0 g Na 121 tons Cl 9x10 5 g 1 mol Cl_ 1 1 ton 35.5 g Cl =3.1x10 6 mol Na =3.1x10 6 mol Cl

Notes on Empirical Formulae In any problem like this, we want to convert what we have into moles. 79 tons Na 9x10 5 g 1 mol Na_ 1 1 ton 23.0 g Na 121 tons Cl 9x10 5 g 1 mol Cl_ 1 1 ton 35.5 g Cl Often, we will get multiple different amounts of moles. Divide all of them by the lowest one and round to whole numbers. (Multiply by 2 if needed!) 3.1x10 6 mol Na 3.1x10 6 mol =3.1x10 6 mol Na =3.1x10 6 mol Cl =1

Notes on Empirical Formulae In any problem like this, we want to convert what we have into moles. 79 tons Na 9x10 5 g 1 mol Na_ 1 1 ton 23.0 g Na 121 tons Cl 9x10 5 g 1 mol Cl_ 1 1 ton 35.5 g Cl Often, we will get multiple different amounts of moles. Divide all of them by the lowest one and round to whole numbers. (Multiply by 2 if needed!) 3.1x10 6 mol Na 3.1x10 6 mol Cl 3.1x10 6 mol 3.1x10 6 mol =3.1x10 6 mol Na =3.1x10 6 mol Cl =1

Notes on Empirical Formulae This end result of 1 for Na and 1 for Cl means that our compound is made of 1 Na and 1 Cl:

Notes on Empirical Formulae This end result of 1 for Na and 1 for Cl means that our compound is made of 1 Na and 1 Cl: 1Na + 1Cl  NaCl

Notes on Empirical Formulae This end result of 1 for Na and 1 for Cl means that our compound is made of 1 Na and 1 Cl: 1Na + 1Cl  NaCl You may get problems that ask you to do this from percent composition as well. Just turn each percent number into grams. Literally just replace the “%” sign with “g”. The math will work out fine. Promise!

Notes on Empirical Formulae A special note. Think of these like recipes. 4 eggs + 1 flour + 2 sugar  1 cake If I try to do the following: 11 eggs + 2 flour + 5 sugar I’ll use up enough ingredients to make 2 cakes, but I’ll still have 3 eggs and 1 sugar left over. Think of empirical formulae like this. They tell us “recipes” to combine chemicals to make something new.

Practice with Empirical Formulae Open your book to page 218 and find the empirical formulae for each compound in problems 104 and 105. Like usual, the teacher will have the answers up front and we can go over a few before the end of class.

Exit Question Find the empirical formula of a substance with the composition: 87.5% Zn and 12.5% N a) ZnN b) ZnN 2 c) Zn 2 N d) Zn 2 N 3 e) Zn 3 N 2 f) Zn 3 N 4