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 1023 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 1023)– 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 1023 atoms

• 2 moles = ? atoms

2 mol Kr x 6.02x1023 atoms Kr = 12.04x1023atoms 1 mol Kr

Calculate number of moles for 18.06 x 1023 atoms of He.

• 6.02 x 1023 atoms = 1 mole

• 18.06 x 1023 atoms = ? moles

18.06 x 1023atoms He x 1 mol He = 3 mol He 6.02x1023 atoms He

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

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

• 12.04 x 1023 molec. x 1 mol H2O x 18.01 g H2O =

6.02x1023 molec. 1 mol H2O

36.02 g H2O

• 6.02x1023 molec. = 1 mol– 12.04x1023 molec. = x mol

• 1 mol = 18.01 grams on Periodic Table– x mol = y grams

Calculate mass in grams of 12.04 x 1023 molecules H2O

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 O2?

• How many atoms is that?

1.0 g O2 x 1 mol O2 x

32.00 g O2

6.02 x 1023molec. O2 =

1 mol O2

1.9 x 1022 molec. O2

• 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 Na2O2• Mass Na (in Na2O2) = 2 x 22.99 g Na = 45.98 g Na

• Mass O (in Na2O2) = 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% !)