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The Mole Chapter 11

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**Measuring Matter Section 11.1**

Chemists need a convenient method for counting the number of atoms, molecules or formula units in a sample of a substance. Atoms and molecules are so small that chemists created their own counting unit called the mole

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What is a mole? Mole- abbreviated mol is the SI base unit used to measure the amount of a substance A mole of anything contains 6.02 x1023 representative particles This number is known as Avogadro’s number

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**Conversion Factors Example: 4800g to kg Conversion factor: 1000g= 1kg**

Recall from chapter 2: A method of problem solving that focuses on the units to describe matter Conversion factor- a ratio of equivalent values used to express the same quantity in different units Example: 4800g to kg Conversion factor: 1000g= 1kg

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**One mole = 6.02 x1023 representative particles**

Conversion factor: # particles = # of moles x

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**Practice Determine the number of atoms in 2.50 mol of Zn.**

Calculate the number of molecules in 11.5 mol H2O.

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**How many moles contain each of the following:**

5.75 x1024 atoms Al 3.58 x1023 formula units of ZnCl2

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**Mass and the Mole Section 11.2**

Molar Mass- The mass in grams of one mole of any pure substance Units: grams/mol Example: Manganese has an atomic mass of amu there fore it has a molar mass of g/mol

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**Suppose you are in a chemistry lab and you need 3**

Suppose you are in a chemistry lab and you need 3.00 moles of manganese. What amount would you weigh out on the scale? What information do I need to know? What information is given in the problem?

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**Determine the mass in grams:**

3.57 mol Al 2.45 mol Zn 42.6 mol Si

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**Convert using molar mass**

Atoms or Molecules MOLES Grams Convert using 6.02 x1023 Convert using molar mass

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**How many atoms are in each of the following samples?**

55.2 g Li 0.120 kg Ti

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**Moles to Compounds Section 11.3**

Recall that the chemical formula of a compound indicates the types of atoms and the number of each contained in one formula unit of the compound Example: CCl2F2 (Freon) 1 carbon 2 chlorine 2 fluorine

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**Determine the number of moles of chloride ions in 2.50 mol ZnCl2**

What information is given in the problem? 2.50 mol ZnCl2 2 mol Cl- ions : 1 mol ZnCl2

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Practice Problem How many moles of oxygen atoms are present in 5.00 mol diphosphorus pentoxide?

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**The Molar Mass of Compounds**

Molar mass of a compound is determined by adding the masses of all elements present. # of moles x molar mass = number of grams Example: K2CrO4 2 mol K x g K = g 1 mol K 1 mol Cr x g Cr = g 1 mol Cr 4 mol O x g O = g 1 mol O 78.20 g 52.00 g g g K2CrO4

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Practice Problems Determine the molar mass of each of the following compounds: NaOH Sr(NO3)2 (NH4)PO4 C12H22O11 C2H5OH

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**Answers Determine the molar mass of each of the following compounds:**

NaOH g/mol NaOH Sr(NO3)2 g/mol Sr(NO3)2 (NH4)PO4 g/mol (NH4)PO4 C12H22O11 g/mol C12H22O11 C2H5OH g/mol C2H5OH

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**Mole- to- Mass Conversion**

Atoms or Molecules MOLES Grams Convert using 6.02 x1023 Convert using molar mass

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**Convert using molar mass**

Example What is the mass of 3.25 moles of sulfuric acid (H2SO4)? What am I given in the problem? What do I need to know? 3.25 mol H2SO4 Atoms or Molecules MOLES Grams Convert using 6.02 x1023 Convert using molar mass

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3.25 mol H2SO4 Determine the number of moles present in 22.6 g AgNO3

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**Converting the mass of a compound to number of particles**

Atoms or Molecules MOLES Grams Convert using 6.02 x1023 Convert using molar mass

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Practice Problem A sample of silver chromate (AgCrO4) has a mass of 25.8g. How many Ag+ ions are present? How many CrO4 2- ions are present? What is the mass in grams of one formula unit of silver chromate?

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**How many Ag+ ions are present?**

How many CrO4 2- ions are present? What is the mass in grams of one formula unit of silver chromate?

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**Empirical and Molecular Formulas Section 11.4**

The percent by mass of each element in a compound is called the percent composition of a compound mass of element x 100 = percent by mass of compound mass

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Practice Problem Calculate the percent composition of baking soda (NaHCO3) (Molar Mass= 84 g/mol) Na H C O

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**Which has the larger percent of sulfur, H2SO3 or H2S2O8?**

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Empirical Formula Empirical Formula- formula with the smallest whole number mole ratio of the elements Starting with the percent composition- drop the percentage sign and replace it with grams Multiply the mass of each element by the conversion factor that relates moles to grams (molar mass) Calculate the simplest whole number ratio of moles of the elements by dividing each number of moles by the smallest value in the mole ratio. If a fraction is present- multiply by 2 to produce a whole number

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Practice Problem Determine the empirical formula for a compound that contains 35.98% aluminum and 64.02% sulfur.

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Molecular Formula Molecular Formula- specifies the actual number of atoms of each element in one molecule or formula unit of the substance experimentally determined molar mass = n mass of empirical formula Molecular formula = (empirical formula) n

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Practice Problem A colorless liquid composed of 46.68% nitrogen and 53.32% oxygen has a molar mass of g/mol. What is the molecular formula?

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**The Formula of a Hydrate Section 11.5**

A hydrate is a compound that has a specific number of water molecules bound to its atoms. The number of water molecules bound to its atoms is indicated with a prefix Prefixes in Hydrates Number of Atoms Prefix 1 Mono- 6 Hexa- 2 Di- 7 Hepta- 3 Tri- 8 Octa- 4 Tetra- 9 Nona- 5 Penta- 10 Deca-

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**Determining the formula of a hydrate**

A mass of 2.50 g of blue, hydrated copper sulfate (CuSO4 ·x H2O) is placed in a crucible and heated. After heating, 1.59g white anhydrous copper sulfate (CuSO4) remains. What is the formula for the hydrate? Name the hydrate.

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**Known: Unknown: Determine the mass of water lost.**

Mass of hydrated compound: 2.50 g CuSO4 ·x H2O Mass of anhydrous compound: 1.59 g CuSO4 Molar mass of CuSO4: g/mol CuSO4 Molar mass of H2O: g/mol H2O Unknown: Formula of hydrate: Name of hydrate: Determine the mass of water lost. Calculate the number of moles of H2O and anhydrous CuSO4 using the molar mass. Determine the value of x= moles H2O ÷ moles CuSO4 The ratio indicates the number of water molecules bonded to CuSO4 Name the hydrate using the correct prefix

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Practice Problem A hydrate is found to have the following percent composition: 48.8 % MgSO4 and 51.2% H2O. What is the formula and name for this hydrate?

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