X Chemistry Unit 8 The Mole Problem Solving

X Chemistry Unit 8 The Mole Problem Solving involving Chemical Compounds

Vocab: n Percent composition – mass of the part divided by mass of the whole times 100 n Empirical Formula – the simplest whole number ratio of atoms in a compound n Molecular Formula- the true whole number ratio of atoms in a compound

Vocab: n Molar Mass- the mass of 1 mole of a substance ¨Equal to the atomic mass for elements ¨For a compound, the sum of the atomic masses for all the atoms in a compound ¨Units = grams/mole = g/mol n Mole - the amount of substance that is equal to 6. 02 x 1023 particles of that substance

Vocab: n Representative particle- the smallest particle of a substance; defined by the type of substance: ¨Element = atom ¨Covalent compound = molecule ¨Ionic compound = formula unit ¨Charged atom = ion

Vocab: n Avogadro’s Number- the number of particles in 1 mole of any substance ¨Avogadro’s n # = 6. 02 x 1023 Molar Volume- the volume of 1 mole of ANY gas at STP (Standard Temperature and Pressure = 0°C and 1 atm), ¨ 22. 4 Liters

What is a Mole? n Amedeo Avogadro (Italian Mathematician) ¨ used Carbon as the basis for the masses of the other elements ¨ devised a counting relationship between grams and moles ¨ 1 mole = 6. 02 x 1023 atoms of any element ¨ 1 mole of carbon = 12. 0 g Carbon n (matches the atomic mass on the P. T. )

What is a Mole? (REAL WORLD EXAMPLE) Relationships: ¨ 1 dozen = 12 donuts just like 1 mole = 6. 02 x 1023 atoms BUT: ¨ 1 dozen feathers = 0. 015 grams and ¨ 1 dozen bricks = 32, 400 grams!

What is a Mole? ¨ The mass of 1 mole of any element will be different than 1 mole of any other element n Practice: ¨ What are the masses of 1 mole of the following elements? n Magnesium = ¨ 24. 31 g/mol n Nitrogen = ¨ 14. 01 g/mol

One-mole Amounts

Determining Molar Mass: n Steps in Solving: ¨ Write the element symbols in the formula down the left hand side of the paper ¨ Multiply the number of atoms by the molar mass of that element according to the P. T. and rounded to the tenths place ¨ Sum the products from step 2 ¨ the units on your final answer are g/mol (grams per mole)

Determining Molar Mass: n Examples: ¨CO 2 ¨(NH 4)2 SO 4

Mole Conversions Moles! Use Avogadro’s # (6. 02 x 1023) Representative particles (atoms, molecules, formula units, ions) Mole Conversions Use molar mass Mass

Converting between Moles and Grams: n Mole-Mass Conversions ¨Dimensional Analysis!! Use molar mass as conversion factor! n Moles to grams: ¨# n of moles molar mass (g) 1 mole = # of grams 1 mole Molar mass (g) = # of moles Grams to moles: ¨# of grams

Converting between Moles and Grams: Examples: n How many moles of CCl 4 are their in 523. 4 g?

Converting between Moles and Grams: Examples: (cont. ) n How many grams of Na are there in 12. 3 moles of Na?

Conversions Between Moles & Particles: Use Avogadro’s Number!! Examples: How many atoms of carbon are contained in 0. 230 moles of C?

Conversions Between Moles & Particles Examples: How many molecules of sodium chloride are contained in 0. 145 moles of Na. Cl?

Conversions Between Moles & Particles Challenge!! How many molecules of barium chloride are contained in 1. 07 grams of barium chloride?

Percent Composition Problems: n 2 types: Using data measured in grams to find percentage of element in some given sample (More specific) 2. Using a chemical formula to find percentage of each element in a compound (More general) 1.

Percent Composition Problems: n Type 1: (just like the separation lab!) ¨% of element in a specific sample

Percent Composition Problems: Examples: If 20. 55 g of sodium combines completely with 31. 75 g of chlorine to form a compound, what is the % composition of each element in this compound?

Percent Composition Problems: n Type 2: ¨ Mass of element in 1 mole of a compound

Percent Composition Problems: Example: What is the percent composition of each element in C 12 H 22 O 11?

Empirical Formula Calculations Percent to mass Mass to moles Divide by Small Multiply ‘til whole % g moles Divide by smallest mole value empirical formula

Multiply ‘til Whole Hints Common Possible Endings: . 33 x 3. 25 x 4. 67 x 3. 50 x 2

Determining the Empirical Formula of a Compound: Steps for Solving: n Convert grams of each element to moles using molar mass n Inspect your answers and find the smallest number of moles of all the answers n Divide each number of moles for each element by the smallest determined in step 2 ¨ Your answers to step 3 have no units since it was moles/moles they cancel out!! n IF the ratios do not come out to a whole number, multiply all values by some whole number to get all whole numbers n These numbers are the subscripts for each of the elements in the empirical formula

Determining the Empirical Formula of a Compound: Example: Find the empirical formula for a compound composed of 78% iron and 22% oxygen.

Determining the Empirical Formula of a Compound: A compound is 81. 1 % boron and 18. 9% hydrogen. What is the empirical formula of this compound?

Determining the Molecular Formula of a Compound: n Steps for Solving: ¨ Find the Empirical Formula ¨ Find the molar mass of the Empirical Formula ¨ Divide the Molecular molar mass by the Empirical molar mass ¨ The answer to step 3 is the factor you multiply each subscript in the Empirical Formula by to get the Molecular Formula

Determining the Molecular Formula of a Compound: A compound of B and H is 81. 10% B and 18. 9% H. Its empirical formula is B 2 H 5. What is its molecular formula if the molar mass of the compound is 53. 3 g/mol?