What formulas give atom to atom mole to mole and atom to molecule ratios?

Answer 1

let's start with what a mole is...

think of "a dozen".. what is that? 12 right?

a dozen = 12 of anything.. 12 donuts. 12 people, 12 answers to this question.. agreed?

A pair = 2 of anything...a pair of aces = 2 aces. Pair of shoes = 2 shoes. Pair of contact lenses = 2 lenses. etc.. pair = 2

a gross = 144 of anything.... right?

bakers dozen = 13..... 13 cookies = a bakers dozen

dozen, pair,gross and bakers dozen are all "labels" we give to a quantity of something.... right?

a "mole" is just like dozen, pair, and gross etc. it is a label we give to a quantity of something. The number, though, is = 602,200,000,000,000,000,000,000... we usually write that as 6.022x10^23

a mole of donuts = 6.022x10^23 donuts

a mole of answers = 6.022x10^23 answers. take awhile to read all those...

a mole of people = 6.022x10^23 people...

a mole of 1 dollar bills = $6.022x10^23

and just like

2 dozen donuts = 2 x 12 donuts = 24 donuts

2 moles of donuts = 2 x 6.022x10^23 donuts = 1.204x10^24 donuts

got that down? Nothing mysterious. just a label. just like a dozen

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now that you know what it is, how do we use it?

First, we have a "mole" because the number of atoms of stuff that we can hold in our hand and see is HUGE. That is because atoms are tiny. In the lab, we would probably work with say a gram of something. probably about the size of a couple of aspirin. depending on the substance, that's probably about 1/10th of a mole = 6.022x10^22 atoms. so it's a convenient unit of measure. Rather than saying 60,220,000,000,000,000,000,000 atoms, we say 0.1 moles... ok?

Next, the atomic masses listed on a Periodic Table are in units called "AMU's".. Atomic Mass units. and can be read as AMU per atom.

but 1 amu = 1.66x10^-24 g

and there are 6.022x10^23 atoms in 1 mole of atoms right? that's what a mole is

now see if you can follow...

(1 amu / atom) x (6.022x10^23 atoms / mole) x (1.66x10^-24 g / amu) = 1g / mole of atoms...usually just written g/mole

notice what I've done? I've converted amu/atom to g/mole.. units of amu / amu cancel... atoms / atom cancel and we're left with g/mole

What that gibberish means is this. You can read atomic mass on a periodic table as either amu/atom or g/mole... They are equivalent units.

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Next.

now that you have atomic mass in grams / mole what does that mean?

for carbon 12, for example, atomic mass = 12.01 g / mole. that means if you have 12.01 g, you have 1 mole of Carbon 12. ie 6.022x10^23 atoms of carbon 12

if you have 3.01x10^23 atoms of carbon 12, you have 1/2 of a mole. since 1 mole weighs 12.01 g, you have 6.00 grams of Carbon..

See how that works?

let's try a couple of practice problems

a)

calculate moles of C in 24.02 g C

from the periodic table, the atomic mass of C = 12.01 amu/atom = 12.01 g/mole right?

24.02 g C x (1 mole C / 12.01 g C) = 2.000 moles C... g/g cancels..

b)

calculate mass of C in 3.00 moles C

3.00 moles C x (12.01 g C / mole C) = 36.03 g C... moles / moles cancel

c)

calculate moles NaCl in 116.8 g NaCl...

from periodic table mass Na = 23.0 g mass Cl = 35.4... so mass NaCl = 1x23 + 1x35.4 = 58.4 g/mole

116.8 g NaCl x (1 mole NaCl / 58.4 g) = 2.00 moles NaCl.. g/g cancels... leaves moles.. do you see that?

d)

how many atoms are in 2 moles of C?

2 moles x (6.022x10^23 atoms C / mole C) = 1.204x10^24 atoms

NOTE: this takes practice. which is why you get assignments to do these conversions. Ok? practice.

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when you start working in the lab and start working problems with chemical equations, moles are important. this is why...

the coefficients of balanced chemical equations are mole ratios. not mass...

2 H2 + 1 O2 ---> 2 H2O... the 2,1,and 2 are MOLES (or you could read it as molecules also)

2 moles H2 reacts with 1 mole O2..ok?

it ISN'T,2 grams of H2 reacts with 1 gram O2 to form 2 grams of H2O.

so we convert masses to moles to use the balanced equations to calculate how much reactants we need and products we produce.

whew...

ask questions if you need to.. I'll check back in the morning.

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as to the actual definition, a mole is technically defined as "the number of elemental entities in 12 g of carbon 12" = 6.022x10^23.