STOICHIOMETRY
LETS REVIEW- MOLES AND PARTICLES

1 mole = 6.022 x 1023 particles (formula units/
molecules/atoms)
Moles
Multiply by
6.022 x 1023
Particles
Divide by
6.022 x 1023
Moles
Particles
ANOTHER WAY TO LOOK AT IT:
N = n x NA
N = Number of particles (molecules, formula
units, atoms)
 n = amount (in mol)
 NA = Avagadro constant (mol-1)

FOR EXAMPLE

N = n x NA
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?
There is 1 atom of
nitrogen and 2
atoms of oxygen in
every molecule of
NO2
7.52 x 1023 molecules x (3 atoms/ molecule)
= 2.26 x 1024 atoms in 1.25 mol of NO2
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
PARTICLES TO MASS

What is the mass of 0.750 mol of CO2 gas?
Divide by
6.022 x 1023
Multiply
by M
Mass
Moles
Particles
Multiply by
6.022 x 1023
Divide by
M
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 TO MASS

How many molecules of iodine chloride, ICl, are
in a 2.74 x 10-1 g sample?
Multiply
by M
Divide by
6.022 x 1023
Mass
Moles
Particles
Multiply by
6.022 x 1023



Divide by
M
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
MOLAR VOLUME
PRESSURE

What is pressure?
Collisions
 Depend on:

Temperature
 Volume


PV=nRT
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)

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
+
2L H2
1L O2
2L H2O
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.
MOLAR VOLUME
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:

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)
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!
EXAMPLE:

What is the volume of 3.0 mol of nitrogen dioxide
gas at STP?
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?