Chapter 10 Final Review

Chapter 10: Chem Review

The Mole

• The Mole can refer to # of atoms, # of molecules, OR # of particles.
• 1 mole = 6.02x10^23
• Ex) 2 mol of Na is 1.204x10^24
• Molar Mass: the # of grams in 1 mol of a material
• Ex) 1 mol of C is 12.01 grams

Percent Composition

• Definition: % of Mass of each element in a compound
• Steps:
• 1: find the molar mass of each compound
• 2: Add those all up
• 3: Divide the mass of each element by the total mass
• 4: Multiply by 100

Mass %

• Definition: % of polyatomic ion or only one part of the compound
• Steps:
• 1: Follow the same steps as % composition
• 2: Add percentages of whatever part of the compound you need

Backward

• Definition: using % composition to find the mass of an element present in a sample
• Steps:
• 1: Change % to decimal
• 2: Multiply by mass of sample

Practice Problems

• Percent Composition:
• Find the % composition of NaNO3
• Mass %:
• FInd the mass % of CO3 in MgCO3
• Backward:
• If CaCl2 is 64% Cl, how many grams of Cl are in a 270 gram sample of CaCl2?

Empirical Formula

• Def: compound formula with subscripts in lowest WHOLE NUMBERS
• From % Composition Steps:
• 1: Make % the # of grams ad convert to moles
• 2: Divide through the smallest mole value. Answers are subscripts
• 3: Put elements together and use subscripts (If 1 or more subscripts are 1/2 double ALL subscripts)

Emperical Formula (Cont.)

• From actual masses:
• 1: Subtract to find the mass of the other element
• 2: Convert grams to moles
• 3: Divide through smallest mole value
• 4: Put elements together and use subscripts

Molecular Formula

• Def: A multiple of an empirical formula where subscripts are not in lowest whole numbers
• Steps
• 1: Find the empirical formula if not already given
• 2: Calculate the molar mass
• 3: Divide the mass of the larger compound by empirical formula mass
• 4: Multiply empirical formula by your step three answer

Practice Problems

• Empirical formula:
• A compound was analyzed and found to contain 32.4% Sodium, 22.65% Sulfur, and 45% Oxygen. What was the empirical formula?
• A 170 g sample contains 29.84 g Sodium, 67.49 g Chromium, and Oxygen. What was the empirical formula?
• Molecular Formula:
• A compound with an empirical formula P205 has a molar mass of 284 grams/mol. Find the molecular formula

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