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Published byBethanie Stanley Modified over 8 years ago
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Calculations The math of chem. rxns.
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What is a mole, again? A mole is a counting unit, like a dozen. –1 dozen eggs = 12 eggs –1 mole eggs = ? –6.022 x 10 23 eggs! A mole is the same, no matter if it’s counting eggs or atoms. BUT it is SO large that the ONLY things we count with it are atoms and molecules!
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Would you like a mole of $?? If I gave you one dollar a second, how long do you think it would take for me to give you a mole of dollars? (i.e. $6.022 x 10 23 ) –It would take over 19,100,000,000,000,000 years. –The Earth was formed only 4,600,000,000 years ago! A mole of dollars (each.0043 in thick) would reach over 7000 light-years!
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Calculate number of atoms in two mol Kr. One mole = 6.02 x 10 23 atoms 2 moles = ? atoms 2 mol Kr x 6.02x10 23 atoms Kr = 12.04x10 23 atoms 1 mol Kr
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Calculate number of moles for 18.06 x 10 23 atoms of He. 6.02 x 10 23 atoms = 1 mole 18.06 x 10 23 atoms = ? moles 18.06 x 10 23 atoms He x 1 mol He = 3 mol He 6.02x10 23 atoms He
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Molar Mass Remember atomic mass? –Atomic mass of Na = 22.98977 g/mol (22.99 grams of sodium per mole of sodium) –This is from the periodic table Now, we learn molar mass: –What is the molar mass of NaCl? –22.99 g/mol Na + 35.45 g/mol Cl (mass of Cl) –So, it is 58.46 g NaCl/mol NaCl
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State the number of moles in 58.4 grams of Carbon. 12.01 g C = 1 mol C 58.4 g C = ? mol C 58.4 g C x 1 mol C = 4.86 mol C 12.01 g C
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12.04 x 10 23 molec. x 1 mol H 2 O x 18.01 g H 2 O = 6.02x10 23 molec. 1 mol H 2 O 36.02 g H 2 O 6.02x10 23 molec. = 1 mol –12.04x10 23 molec. = x mol 1 mol = 18.01 grams on Periodic Table –x mol = y grams Calculate mass in grams of 12.04 x 10 23 molecules H 2 O
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To change atoms to grams OR to change grams to atoms, first calculate the number of moles. atoms --> mol --> gram OR gram --> mol --> atoms NO atom --> gram or gram --> atom
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Ex. Gram -> Mol -> Molecule How many molecules are there in 1.0 grams of O 2 ? How many atoms is that? 1.0 g O 2 x 1 mol O 2 x 32.00 g O 2 6.02 x 10 23 molec. O 2 = 1 mol O 2 1.9 x 10 22 molec. O 2
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Sometimes, you have to calculate what percentage part of a compound is: –% part= part x 100% total What percent of the total box is one strawberry? –1/5 x 100% =20%
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Calculate % comp for Na 2 O 2 Mass Na (in Na 2 O 2 ) = 2 x 22.99 g Na = 45.98 g Na Mass O (in Na 2 O 2 ) = 2 x 16.00 g O = 32.00 g O 77.98 g tot. % Na = 45.98 g x 100% = 58.96% Na 77.98 g % O = 32.00 g x 100% = 41.04 % O 77.98 g (or 100% - 58.96% = 41.04% !)
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