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STOICHIOMETRY
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LETS REVIEW- MOLES AND PARTICLES
1 mole = 6.022 x 1023 particles (formula units/
molecules/atoms)
MolesParticles
Multiply by 6.022 x 1023
ParticlesMoles
Divide by 6.022 x 1023
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ANOTHER WAY TO LOOK AT IT:
N = Number of particles (molecules, formula units, atoms)
n = amount (in mol) NA = Avagadro constant (mol-1)
N = n x NA
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FOR EXAMPLE
A sample contains 1.25 mol of nitrogen dioxide. How many molecules are in the sample?
Molecules of NO2 = 1.25 mol x (6.022 x 1023
molecules/mol) = 7.52 x 1023 molecules
How many atoms are in the sample?
7.52 x 1023 molecules x (3 atoms/ molecule) = 2.26 x 1024 atoms in 1.25 mol
of NO2
N = n x NA
There is 1 atom of nitrogen and 2 atoms of oxygen in every molecule of NO2
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LETS REVIEW- MOLAR MASS
A mole of an element has a mass in grams that is numerically equivalent to the elements average atomic mass.
Molar Mass (M)= mass of one mole of a substance (g/mol) Molar mass of an element can be found by
looking at the periodic table Molar mass of a compound can be found by
totalling the mass of all elements in the compound What is the molar mass of beryllium oxide?MBeO = MBe + MO
= 9.01 g/mol + 16.00 g/mol
=25.01 g/mol
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PARTICLES TO MASS
What is the mass of 0.750 mol of CO2 gas?
1. MCO2 = 2 x (16.00g/mol) + 12.01 g/mol
= 44.01 g/mol 2. m = (0.750 mol)(44.01 g/mol) = 33.0g
Particles
Moles Mass
Divide by 6.022 x 1023
Multiply by 6.022 x 1023
Multiply by M
Divide by M
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PARTICLES TO MASS How many molecules of iodine chloride, ICl, are in
a 2.74 x 10-1 g sample?
1. MICl =126.90 g/mol + 35.45 g/mol = 162.36 g/mol 2. n = (2.74 x 10-1 g)/(162.36 g/mol) = 1.69 x 10-3
mol 3. (1.69 x 10-3 mol)(6.022 x 1023 molecules/mol)
= 1.01 x 1021 molecules
Particles
Moles Mass
Divide by 6.022 x 1023
Multiply by 6.022 x 1023
Multiply by M
Divide by M
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MOLAR VOLUME
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PRESSURE What is pressure?
Collisions Depend on:
Temperature Volume
PV=nRT
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STANDARD TEMPERATURE AND PRESSURE
Since V depends on P and T, you must state both when describing a gases V.
STP: Average atmospheric pressure at sea level
(101.3 kPa) Freezing point of water (OoC, 273K)
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MOLAR VOLUME
Joseph Louis Gay-Lussac (1778-1853) Law of combining volumes: when gases
react the volume of reactants and products (measured at equal T and P) are always whole number ratios
1L O22L H2 2L H2O
+
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MOLAR VOLUME
Amedeo Avogadro (1776-1856) Realized he could relate the volume of a
gas to the amount that was present (from mass)
Avogadro’s Hypothesis: equal volumes of all ideal gases at the same temperature and pressure contain the same number of molecules
In other words: 1 mole of any gas has the same volume as 1 mole of any other gas at STP.
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MOLAR VOLUME
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THOUGHT LAB- MOLAR VOLUME OF GASES
Two students in a lab decided to calulate the molar volume of carbon dioxide, oxygen and methane gas. They measured the mass of an empty,150 mL syringe and then the mass of the syringe + gas. They repeated this procedure for each gas.
The experiment was carried out in a room maintained at STP (273 K and 101.3 kPa). Their results are in the table on the next slide:
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THOUGHT LAB- MOLAR VOLUME OF GASES
Gas: Carbon Dioxide
Oxygen Methane
Volume of gas (V) 150 mL 150 mL 150 mL
Mass empty syringe
25.081 g 25.085 g 25.082 g
Mass gas + syringe
25.383 g 25.304 g 25.197 g
Mass of gas (m)
Molar mass of gas (M)
Number of moles of gas (n = m/M)
Molar Volume of gas at STP (V/n)
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MOLAR VOLUME
It turns out that the molar volume of any gas at STP is 22.4 L/mol!
Well, actually, that’s not entirely true…. The molar volume of 22.4 L/mol is assumed for
ideal gases- which are hypothetical gases that don’t take up space and do not attract one another
Real gases do take up space and attract each other somewhat- so the molar volume would vary slightly from 22.4 L/mol….but it’s close
We will always assume the molar volume of a gas is 22.4L/mol at STP!
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EXAMPLE:
What is the volume of 3.0 mol of nitrogen dioxide gas at STP?
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TRY THIS:
Suppose you have 44.8 L of methane gas at STP. 1. How many moles are present?
2. What is the mass of the gas?
3. How many molecules of the gas is present?