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MOLE CONVERSIONS

"Conversions" are what you need to do to "convert" one unit to another unit. The conversion is

accomplished by multiplying the given unit by a conversion factor, which yields the wanted unit after

the math is done. The conversion factor is a ratio that relates the given and wanted unit. (see eq 1)

#.#given unit wanted unit wanted unit eq 1

given unit

Conversion equations are not sentences! They are never written left to right. First write the given

number (#.#) and the given unit on the left, followed by an empty set of (--)'s with the fraction bar in

it, then write the wanted unit on the right. Finally, fill in the ( )'s with the units: given unit on the

bottom so it cancels, wanted unit on the top so you have an equality. NO numbers are in the ( )'s at

this time!

The ratio of units tells you what numbers to write in the ( )'s. For mole conversions there are really

only three potential ratios of units that you will have.

g ) or (mol ) g : mol ratios are the units of molar mass. It does not matter which

1. ( mol g

unit is on the top or bottom the ratio is still molar mass. You calculate

molar mass from the periodic table, by summing the atomic masses of

all the atoms in the molecule. Molar mass is defined as the number of

grams that are in one mole of a substance, so the numerical answer that

you calculate always goes with the grams and a 1 always goes with the

moles.

For instance: molar mass of H2O = 18.02. In a conversion

factor this is written: (18.02 g H2O 1 mol H2O

1 mol H2O ) or (18.02 g H2O )

Note that the ratio is still the same ratio whether it is written as

g/mol or mol/g, still 18.02 g of water in 1 mol of water. Remember

that the given and wanted units dictate which unit is on top or bottom

in the conversion factor.

mol B ) mol : mol ratios relate the number of a molecule in a chemical

mol A ) or ( mol A

2. ( mol B

reaction to the number of another molecule in a chemical reaction. The

values that are plugged into the conversion factor are the coefficient for

each molecule, which are found in the balanced chemical equation.

For instance, consider 2NaN3 2Na + 3N2. The mole ratio

between NaN3 and N2 would be written as follows:

3 mol N2 )

( 2 mol NaN3 ) or ( 2 mol NaN3

3 mol N2

Note that the ratio is the same ratio whether it is written;

mol NaN3/mol N2 or mol N2 /mol NaN3, still 2 mol NaN3 to 3 mol N2.

Remember that the given and wanted units dictate which unit is on top

or bottom in the conversion factor.

mol ) The ratio of molecules to moles is by definition 6.022 x 1023

3. ( molecules) or ( molecules

mol

molecules in 1 mole. This ratio is written

( 6.022 1023 molecules ) or ( 1 mol )

1 mol 6.022 1023 molecules

Note that the ratio is the same ratio whether it is written;

molecules/mol or mol /molecule: 6.022 x 1023 molecules in 1 mol.

Remember that the given and wanted units dictate which unit is on top

or bottom in the conversion factor.

STOICHIOMETRY

Conversions become more difficult when no relationship between the given and wanted units exists in

nature. This is mainly found in STOICHIOMETRY when you convert grams of one substance to

grams of another substance. There is no ratio to express this! Molecules react by count (MOLES) not

by mass! Therefore the following sequence must be done to convert grams of one substance to grams

of another substance.

example How many grams of C are produced from a given number of grams of

compound A, # g A, in the equation aA + bB cC + dD?

This must be calculated using the following series of conversions,

# g A mol A mol C g C

This is a sequence of three conversions. The first and third are g:mol conversions of type 1

above and the second is a mol:mol conversion of type 2 above. All three may be written as

follows:

given # g A 1 mol A # mol B #.## g B

#.## g A # mol A 1 mol B

wanted g B

One thing to notice in stoichiometry is that the units all cancel on the diagonal. So, a quick

way to check your work is to see if all the units are the same diagonally. If they are not, then

one of your conversion factors is probably upside-down.

A second thing to notice is that whenever you need to relate an amount of one substance in

a chemical equation with an amount of another substance in that equation; you are really doing

a mole-to-mole conversion. You just have to do an extra step at the beginning to get to moles

and then an extra step at the end to get away from the moles.

Overall, the three ( )'s problem outlined above works anytime that you have a stoichiometry problem

that converts grams A grams C. You can use this scheme to convert grams grams of any 2

substances in a balanced chemical equation. (not just grams of a reactant to grams of a product).

What about grams A mol C conversions? And what about mol A grams C conversions?

These problems are really short stoichiometry problems. Check out the series that was listed above:

g A mol A mol C g C

Generally speaking identify where you are in the series (the given unit) and identify where you are to

end in the series (the wanted unit), and do those conversions.

For example,

grams A mol C

scheme g A mol A mol C g C

given g A wanted mol C

Overall 2 conversions (2 's): g A mol A mol C

So, do the first TWO ( )'s of stoichiometry.

mol A grams C

scheme g A mol A mol C g C

given mol A wanted g C

Overall 2 conversions (2 's): mol A mol C g C

So, do the last TWO ( )'s of stoichiometry.

How do you convert moles to formula units? How do you convert moles to formula units? Converting from moles to particles (atoms, molecules, or formula units): Multiply your mole value by Avogadroâ€™s number, 6.02Ã—10 23. Mole-to-mole conversions: Use the coefficients from your balanced equation to determine your conversion factor. Be sure your units cancel out so you end up with the ...

Title: MOLE CONVERSIONS

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