1Ch 100: Fundamentals for ChemistryCh 7: Quantitative Composition of CompoundsLecture Notes(Sections 7.1 to 7.3)The Mole• The mole is a counting unit (analogous to the dozen unit)– A large unit used to describe large quantities such as number of atoms 1 mole = 6.022 x 1023units• 6.022 x 1023 is known as Avogadro’s number (NA)• Relationship between the mole & the Periodic Table– The atomic mass is the quantity (in grams)of 1 mole of that element– The units of atomic mass are grams/mole– Mass is used by chemists as a way of “counting”number of atoms/molecules of a substance• Mole calculations2Got mole problems? Call Avogadro at 602-1023.Answer: molasses (a mole of asses)What do you get if you have Avogadro's number of donkeys?Molar Mass• Molar mass is the mass in grams of 1 mole of a substance• Molar mass refers to both atoms & molecules1. Elements (atoms)Examples:1 mole of Na has a mass of 22.99 g1 mole of Cl has a mass of 35.451 mole of Cl2has a mass of 70.90 g2. Compounds (molecules)Examples:1 mole of NaCl has a mass of 58.44 g• Mass of Na (22.99 g) + Mass of Cl (35.45 g)1 mole of CO2has a mass of 44.01 g• Mass of C (12.01 g) + 2 x Mass of O (16.00 g)3Mole Calculations1. To convert from atoms (or molecules) to moles, divide the # of atoms (or molecules) by Avogadro’s #Example: How many moles are 1.0x1024atoms?2. To convert from moles to atoms (or molecules), multiply the # of atoms (or molecules) by Avogadro’s #Example: How many molecules are in 2.5 moles? 24231 mol(1.0×10 atoms) = 1.7 mol6.022×10 23246.022×10(2.5 mol) =1.5×10 molecules1 molMole-Mass Calculations1. To convert from moles to grams, multiply the # of moles by atomic massExample: How many grams in 2.5 moles of carbon?2. To convert from grams to moles, divide the mass in grams by atomic massExample: How many moles are in 2.5 g of lithium? 112.01 g(2.5 mol) = 30. g (or 3×10 )1 mol 11 mol(2.5 g) = 0.36 mol (or 3.6×10 )6.941 g4Percent Composition• Percent composition is the percentage of each element in a compound (by mass)• Percent composition can be determined from either:1. the formula of the compound2. the experimental mass analysis of the compoundNote: The percentages may not always total to 100% due to rounding part% Composition = ×100%wholePercent Composition CalculationsTo determine % Composition from the chemical formula:1. Determine the molar mass of compound2. Multiply the molar mass of the element of interest by the number of atoms per molecule then3. Divide this value by the molar mass of the compoundExample:The % Composition of sodium in table salt1. The molar mass of NaCl is 58.44 g/mol2. There is 1 atom of Na in each NaCl molecule3. The atomic mass of Na is 22.99 (# atoms of A)(atomic mass of A)% Composition of A= ×100%molar mass of compound 1×22.99% Composition of Na= ×100%=39.33%58.445Percent Composition CalculationsPerform the following % Composition calculations:1. The % composition of carbon in carbon monoxide2. The % composition of oxygen in water3. The % composition of chlorine in sodium hypochloriteAmadeo Avogadro(1743-1794)• Italian lawyer turned chemist• Major contributions included:1. Established difference between atoms & molecules:• Oxygen & nitrogen exist as molecules O2& N22. Reconciled the work of Dalton & Guy-Lussac3. Establishing Avogadro’s Principle: equal volumes of all gases at the same temperature and pressure contain the same number of molecules.• Note: Avogadro did not determine Avogadro’s number nor the mole (these concepts came later)1. Avogadro is honored because the molar volume of all gases shouldbe the same2. Much of Avogadro’s work was acknowledged after he died, by Stanislao
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