Calculations The math of chem. rxns.. What is a mole, again? A mole is a counting unit, like a dozen. –1 dozen eggs = 12 eggs –1 mole eggs = ? –6.022.

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Presentation transcript:

Calculations The math of chem. rxns.

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 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!

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 ) –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!

Calculate number of atoms in two mol Kr. One mole = 6.02 x atoms 2 moles = ? atoms 2 mol Kr x 6.02x10 23 atoms Kr = 12.04x10 23 atoms 1 mol Kr

Calculate number of moles for x atoms of He x atoms = 1 mole x atoms = ? moles x atoms He x 1 mol He = 3 mol He 6.02x10 23 atoms He

Molar Mass Remember atomic mass? –Atomic mass of Na = 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 g/mol Cl (mass of Cl) –So, it is g NaCl/mol NaCl

State the number of moles in 58.4 grams of Carbon g C = 1 mol C 58.4 g C = ? mol C 58.4 g C x 1 mol C = 4.86 mol C g C

12.04 x molec. x 1 mol H 2 O x g H 2 O = 6.02x10 23 molec. 1 mol H 2 O g H 2 O 6.02x10 23 molec. = 1 mol –12.04x10 23 molec. = x mol 1 mol = grams on Periodic Table –x mol = y grams Calculate mass in grams of x molecules H 2 O

END

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

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 g O x molec. O 2 = 1 mol O x molec. O 2

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%

Calculate % comp for Na 2 O 2 Mass Na (in Na 2 O 2 ) = 2 x g Na = g Na Mass O (in Na 2 O 2 ) = 2 x g O = g O g tot. % Na = g x 100% = 58.96% Na g % O = g x 100% = % O g (or 100% % = 41.04% !)