STOICHIOMETRY

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

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

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

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?

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

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

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

1L O22L H2 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?