Transcript 10.2 Mole-Mass and Mole

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Objectives
◦ Describe how to convert the mass of a substance to
the number of moles of a substance, and vice versa.
◦ Identify the volume of a quantity of gas at STP.
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A mole is a unit of measurement
◦ Kind of like a dozen = 12, or a ream = 500
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A mole is a set number of particles,
molecules, atoms, or just whatever it is we
are talking about.
◦ That number is 6.02 x 1023
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If we wish to find the moles or particles of
something we follow two easy set ups.
◦ First if we are looking for moles:
 (Number of Particles Given) x ( 1 mole/ 6.02 x 1023
particles)=
The number of Moles
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Second, if we are looking for the number of
particles:
 (Number of Moles Given) x (6.02 x 1023 particles/1
Moles)=
The number of Particles
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If we wish to convert between the moles and
mass of a compound, we need the molar mass.
◦ For Example:
 If we wanted to see how many grams are present in 3
moles of NaCl, we simply do the following
 (Number of Moles) x (Molar Mass/ 1 mole)
 3 moles of NaCl x (58.5 g/mole of NaCl) = 176 grams of NaCl
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To find moles from mass:
◦ (Mass of Compound Given) x (1 mole/Molar Mass)
= Number of Moles
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Find the mass of :
 4.5 x 10-3 moles of C20H42
 2.5 moles of Fe(OH)2
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Calculate the Number of moles of:
 3.7 x 10-1 grams of Boron
 75.0 grams of N2O3
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Avogadro’s Hypothesis states that equal
volumes of gases at the same temperature
and pressure contain equal number of
particles.
When we try and calculate using a gas, we
need to confirm one item. That is STP, or
standard temperature and pressure. At these
perfect conditions we can progress in the
calculations
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As we learned before, 1 mole of anything is
equal to 6.02 x 1023
For gases, 1 mole is also equal to 22.4 L
◦ In other words a mole of ANY gas will occupy 22.4 L
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If we have the following:
 0.375 moles of O2 gas, what volume will it occupy
 Volume of Gas = (Moles of Gas) x (22.4 L/1 mole)
 With this we get the following
 Volume of O2 = (0.375 mole) x (22.4 L /1 mole)
 Volume of O2 = 8.40 L
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Calculate Volume of the given gases
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3.2 x 10-3 mol of CO2
3.70 mol N2
1.25 mol He
0.335 mol C2H6
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Describe how to calculate the percent by
mass of an element in a compound
Interpret an empirical formula
Distinguish between empirical and molecular
formulas.
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How many people are in the room right now?
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How many are Male? Female?
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What percentage of the whole is Male?
Female?
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The percent by mass of an element in a
compound is the number of grams of the
compound multiplied by 100%
Percent Composition is the percent by mass
of each element in a compound.
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K2CrO7
How many Potassiums, Chromiums, and
Oxygen do we have?
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Whats the total molar mass of the compound?
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What is the percent composition of the
elements compared to the whole?
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When a 13.60 gram sample of a compound
containing magnesium and oxygen is
decomposed,5.40 grams of oxygen is
obtained. What is the percent composition of
this compound?
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We use the subscripts to tell us the amount of
each element in a mole of compound, which
in turn can tell us the mass, and percent
composition.
Similar to when we solved for K2CrO7, let us
try and find the percent composition for
propane, C3H6.
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When we looked at propane we could see a
few things:
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◦
◦
◦
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The
The
The
The
The
molar mass is 44.0 grams
mass of carbon was 36 grams
mass of hydrogen was 8 grams
Percent composition of carbon is 81.8%
Percent composition of hydrogen is 18.2%
But what would the masses be if we had 82
grams of propane?
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An empirical formula is the lowest whole
number ratio of the atoms in a compound.
◦ CH
◦ CH2O
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A molecular formula is either the same as the
empirical formula, or it appears as a multiple
from the empirical formula.
C2H2
CH2O
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You are told the empirical formula of a
substance is CH4N, and that you have 60
g/mol of substance.
What is the molecular formula?