1.3: Empirical and Molecular Formulas. Ms. Kiely Coral Gables Senior High IB Chemistry SL

Transcription

1 1.3: Empirical and Molecular Formulas Ms. Kiely Coral Gables Senior High IB Chemistry SL

2 Practice How heavy are 1.20 x 10²⁵ atoms of potassium?

3 ANSWER How many grams in 1.20 x 10²⁵ atoms of potassium? 7.79 x 10² grams

4 Conversions Converting from moles to representative particles: -If it is a metal, convert from moles to atoms by multiplying by Avogadro s number -If it is a molecule or compound that contains nonmetal atoms only, convert from moles to molecules by multiplying by Avogadro s number -If it is a compound that contains a metal, convert from moles to formula units by multiplying by Avogadro s number Converting from moles to atoms specifically: -If it is a metal, convert from moles to atoms by multiplying by Avogadro s number -If it is a molecule or compound that contains nonmetal atoms only, convert from moles to molecules by multiplying by Avogadro s number, then multiply by the amount of atoms in one molecule -If it is a compound that contains a metal, convert from moles to formula units by multiplying by Avogadro s number, then multiply by the amount of atoms in one molecule

5 Conversions Converting from representative particles to moles: -If it is a metal, convert from atoms to moles by dividing by Avogadro s number -If it is a molecule or compound that contains nonmetal atoms only, convert from molecules to moles by dividing by Avogadro s number -If it is a compound that contains a metal, convert from formula units to moles by dividing by Avogadro s number Converting from atoms to moles specifically: -If it is a metal, convert from atoms to moles by dividing by Avogadro s number -If the sample is a molecule or compound that contains nonmetal atoms only, convert from atoms to molecules by dividing by the amount of atoms in one molecule, then divide by Avogadro s number -If it is a compound that contains a metal, convert from atoms to formula units by dividing by the amount of ions in one formula unit, then divide by Avogadro s number

6 Conversions Converting from moles to grams: -If it is a metal, convert from moles to grams by multiplying by the element s molar mass (g/mol) -If it is a molecule or compound that contains nonmetal atoms only, convert from moles to grams by multiplying by the molecule s molar mass (g/mol) -If it is a compound that contains a metal, convert from moles to grams by multiplying by the formula units molar mass (g/mol) Converting from grams to moles: -If it is a metal, convert from grams to moles by dividing by the element s molar mass (g/mol) -If it is a molecule or compound that contains nonmetal atoms only, convert from grams to moles by dividing by the molecule s molar mass (g/mol) -If it is a compound that contains a metal, convert from grams to moles by dividing by the formula units molar mass (g/mol)

7 ANNOUNCEMENT QUIZ ON CONVERSIONS AND EMPIRICAL FORMULAS/MOLECULAR FORMULAS ON FRIDAY, SEPTEMBER 7TH

8 Conversions review 1. How many moles are in 1.20 x 10²⁵ atoms of potassium? 2. How many atoms are in moles of zinc? 3. How many molecules are in moles of N₂O₅? 4. How many molecules in 28 grams of CO₂? 5. How many atoms in 28 grams of CO₂? 8

9 Answers 1. How many moles are in 1.20 x 10²⁵ atoms of potassium? 1.99 x 10 mol 2. How many atoms are in moles of zinc? 4.52 x 10²³ atoms 3. How many molecules are in moles of N₂O₅? 2.41 x 10²³ molecules 4. How many molecules in 28 grams of CO₂? 3.8 x 10²³ molecules 5. How many atoms in 28 grams of CO₂? 1.1 x 10²⁴ atoms 9

10 Combustion of a hydrocarbon There are different types of combustion reactions. The following are the rules for specifically the combustion of a hydrocarbon. A hydrocarbon undergoing combustion will always react with oxygen to yield carbon dioxide and water vapor. Hydrocarbon + O₂ CO₂ + H₂O A metal undergoing combustion will always react with oxygen to yield a metal oxide: metal + O₂ metal oxide For example: 2Mg + O₂ 2MgO 10

11 Empirical Versus Molecular Formulas Empirical Formula: formula showing the simplest ratio of numbers of atoms of each element in a compound Molecular Formula: formula showing all the atoms present in a molecule. The molecular formula of a compound is often a multiple of the empirical formula; however, in some cases the empirical formula and molecular formula of a compound are the same.

12 Empirical Versus Molecular Formulas Why are empirical formulas used? If we have an unknown substance that we know is a compound, we can experimentally determine its composition (what elements it is made of) by getting information on the different masses that exist in the sample. For example, if we notice there are many atoms that weigh approximately 12g and other atoms that approximately weight 1g then we can assume the sample is made up of carbon and hydrogen atoms, respectively, giving us an empirical formula of CH. CH, however, would be very unstable. The true molecular formula must be something more stable, such as CH₄, or C₂H₂, etc.

13 Solving for the Empirical Formula of a Compound If you know how much of each element is present in grams: 1. Convert the mass of each element to moles. (If you are given a percentage, then just immediately write the percentage and grams and convert the grams to moles.) 2. Divide each result by the smallest mole amount you calculated in step Approximate to the nearest whole number. That whole number is now the subscript of each respective element in the compound.

14 Example 1 part A only An organic compound, A, containing only the elements carbon, hydrogen and oxygen was analysed. (a) A was found to contain 54.5% C and 9.1% H by mass, the remainder being oxygen. Determine the empirical formula of the compound. INSTRUCTIONS: 1. Convert the mass of each element to moles. (If you are given a percentage, then just immediately write the percentage and grams and convert the grams to moles.) 2. Divide each result by the smallest mole amount you calculated in step Approximate to the nearest whole number. That whole number is now the subscript of each respective element in the compound.

15 Answer a) C₂H₄O

16 Molecular Formulas The molecular formula of a compound can be deduced from the empirical formula if the true molar mass of the compound is known: = Mass of molecular formula Mr of empirical formula Mr stands for molar mass To obtain the molecular formula, obtain the value of x and multiply it against the subscripts present in the empirical formula.

17 Example 1 Revisited - part B An organic compound, A, containing only the elements carbon, hydrogen and oxygen was analysed. (a) A was found to contain 54.5% C and 9.1% H by mass, the remainder being oxygen. Determine the empirical formula of the compound. (b) The mass of A is What is the molecular formula of compound A?

18 Answer a) C₂H₄O b) C₄H₈O₂

19 Example The molar mass of aluminium chloride is 267 and its composition by mass is 20.3% Al and 79.7% chlorine. Determine the empirical and molecular formulas of aluminium chloride.

20 ANSWER empirical formula AlCl3; molecular formula: Al2Cl6;

21 Example A hydrocarbon contains 81.7% by mass of carbon. Determine its empirical formula, showing your workings. [3 marks]

22 Answer C3H8