1 Chemical Quantities Chapter 10

2 How to measure matter? Three ways to measure matter 1. By counting 2. By mass 3. By volume

3 Example of 3 ways to measure matter By counting: 1 dozen apples = 12 apples By mass 1 dozen apples = 2.0 kg apples By volume 1 dozen apples = 0.2 bushels apples

4 What is Dimensional Analysis? A tool for solving conversion problems To convert measurement into different units. Take given value and multiply it by conversion factor Example: 1 kg = ?? g 1 kg x 1000g = 1000g 1kg

5 Finding mass from count What is the mass of 90 average sized apples if 1 dozen apples has a mass of 2.0 kg? Information: 12 apples = 1 dozen 1 dozen = 2.0 Kg Mass of 90 apples = ?

6 What is a representative particle? Representative particles – refers to the species present in a substance Atoms, molecules, or formula unit SubstanceRepresentative particle Elementatom Diatomic moleculeMolecule Covalent CompoundMolecule Ionic compoundFormula unit Ion

7 What is a mole? Mole – (mol) amount of substance that has the same mass in grams as its atomic or molecular mass in atomic mass units Mole = 6.02 x representative particles of a substance SI unit for measuring the amount of a substance Developed by Amadeo Avogadro in the early 1800s x is Avogadro’s number

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9 Convert number of atom to moles How many moles of magnesium is 1.25 x atom of magnesium? mol Mg

10 Converting Moles to number of particles Two steps: 1. Convert number of moles of compound to the number of representative particle (molecule or formula unit) 2. Multiply the number of representative particles by the number of atoms (6.02 x )

11 Example How many molecules are in 2.12 mol of propane (C 3 H 8 )? Number of moles = 2.12 mol C 3 H 8 1 mol C 3 H 8 = 6.02 x molecules C 3 H 8

12 Review What does the atomic mass mean? The average mass of all the naturally occurring isotopes of that element Where do you find it on the periodic table? What are the units for atomic mass? Atomic mass units

13 Molar mass Atomic mass of an element expressed in grams is the mass of a mole of the element Molar mass – mass of one mole of a substance ElementAtomic mass (amu) Molar mass (g) Moles (mol) H111 C12 1

14 Molecular Mass Used for diatomic molecules MoleculeMolecular mass (amu) Molar mass (g) Moles (mol) H2H2 2 x 1amu = 2amu 2 x 1g = 2g1 F2F2 2 x 19amu = 38amu 2 x 19g = 38g 1 Cl 2 2 x 35.5amu = 71 amu 2 x 35.5g = 71 g 1

15 Binary compounds and formula mass Formula mass – number of atoms present is multiplied by the mass of each element. Then the mass of the different elements are added together. SubstanceFormula mass (amu) Molar mass (g) Moles (mol) NaCl1(23amu)+1(35.5amu) = 58.5amu 1(23g)+1(35.5g) = 58.5g 1 H2OH2O2(1amu)+1(16amu) = 18amu (2g)+1(16g) = 18g 1 Al 2 S 3 2(27amu)+3(32amu) = 150amu 2(27g)+3(32g) = 150g 1

16 Mole – mass relationship Use the molar mass of an element or compound to convert between the mass of a substance and the moles of a substance n = number of moles of a substance Example: How many moles are in a mass of H 2 O = 90g? 1. What is the molar mass? 2 x (1g) + 16g = 18g 2. 1 mole of water = 18g of water 3. n = mass of substance / molar mass n = 90 g / 18g = 5 moles How many molecules are in 5 moles? 5 moles x 6.02 x molecules/mole = 3.01 x molecules

17 What is Percent composition? Percent composition – percent by mass of each element in the compound Percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound multiplied by 100% Equation: Percent composition from Chemical formula Assuming 1 mole of compound % mass = mass of element in 1 mol x 100% Molar mass of compound

18 Example % mass = mass of element in 1 mole x 100% Molar mass of compound What is the % composition of Hydrogen in 1 mole of water? H 2 O Molar mass of H 2 O = 18g mass of 1 mol of H 2 = 2g % mass of element = 2g x 100% = 11.1% 18g

19 Taking it a step further Previous slide assumed 1 mole of compound How do you calculate % composition for amounts other than 1 mole? 2 steps 1. Need to determine what the % composition would be for 1 mole 2. Multiply this % times the amount of substance

20 Example What is the % composition of Hydrogen in 50 g of water? Step 1: H 2 O Molar mass of H 2 O = 18g mass of 1 mol of H 2 = 2g % mass of element = 2g x 100% = 11.1% 18g Step 2: Multiply percentage by amount (11.1/100) x 50g = 5.55 g of H 2

21 How to determine # of moles from chemical equation? Coefficients in front of molecule or element indicate how many moles are present. Example: 2 H 2 + O 2  2H 2 O How many moles of H 2 ? 2 moles of H 2 How many moles of O 2 ? 1 mole of O 2 How many moles of H 2 O? 2 moles of H 2 O

22 Proof of conservation of mass 2 H 2 + O 2  2H 2 O Calculate the molar mass for each side of this equation. Left side H 2 = 2g, O 2 = 2(16g)=32 g  total = 2(2g) + 1(32 g) = 36 g Right side H 2 O = 2(1g) + 16g = 18g  total = 2(18g) = 36 g Same amount of mass on each side even though not same number of moles

23 What do balanced chemical equations tell you? 1. Number and type of substances needed to cause a particular reaction 2. Number and types of substances produced in a particular reaction 3. # of moles of each reactant needed. 4. # of moles of each product produced. 5. Molar mass of each reactant needed. 6. Molar mass of each product formed. 7. Physical state of each of the reactants and products