Avogadro’s number Molar Mass Molar volume Notes on the Mole: part 2 K. Crowley DRHS 2014 Avogadro’s number Molar Mass Molar volume

This is a very small mass, which we obviously can't weigh on a scale. The mole: Review: As you already know, carbon-12, the isotope of carbon with six protons, six electrons, and six neutrons, is the "standard" for atomic masses. Carbon – 12 is the “standard.” We'd like to say that its mass is exactly 12 "somethings", and we call that something the atomic mass unit. This is a very small mass, which we obviously can't weigh on a scale.

Avogadro: the story behind the man and his number Born in Italy, grew up in a very important time in chemistry: John Dalton and Gay-Lussac were beginning to understand basic properties of atoms and molecules. Gay-Lussac’s law of combining volumes particularly interested Avogadro.

Law of combining volumes explained When two volumes of gases react with one another to create a third gas, the ratio between the volume of reactants and the volume of the product is always made of simple whole numbers. Two volumes of hydrogen gas combine with one volume of oxygen gas to form two volumes of water vapor with nothing left over, or: 2H2 + O2  2H2O

And thus: Avogadro’s law was created: Standard temp = 0oC Standard pressure = 1 atm (“atmosphere”) of pressure Avogadro deduced that in order for this to be true, equal volumes of any two gases at the same temperature and pressure must hold an equal number of particles (Avogadro's law). Remember: gases are squishy under pressure and expand when heated

Molar Volume of a Gas at STP (standard temperature and pressure) At STP the molar volume of ANY gas is 22.4L/mol This is called the “standard molar volume” of a gas.

Standard temperature = 273 K = 0oC Standard Pressure = 1.00 atm Molar Volume Standard temperature = 273 K = 0oC Standard Pressure = 1.00 atm 1 mole of gas at STP = 22.4L All examples have 6.02 x 1023 molecules and occupy 22.4 L 32.0 grams 2.0 grams 28.0 grams 1 mole O2 1 mole H2 1 mole N2

Avogadro’s number: Another history story How did chemists determine how many atoms are in a mole in the first place? The first rough estimate came courtesy of physicist Robert Millikan, who measured the charge of an electron. The charge of a mole of electrons, called a Faraday, was already known by the time Millikan made his discovery. Milikan

Faraday led to Avogadro’s number Dividing a Faraday by the charge of an electron, then, gives us Avogadro's number. The best estimate of the value of a Faraday, is 96,485.3383 coulombs per mole of electrons. The best estimate of the charge on an electron is 1.60217653 x 10-19 coulombs per electron. If you divide the charge on a mole of electrons by the charge on a single electron you obtain a value of Avogadro’s number of 6.02214154 x 1023 particles per mole.

The Mole Mass (g) Back to carbon - 12 Volume (of a gas at STP) Particles (atoms, molecules, or formula units Parts of a particle (atoms only) Mass (g) 22.4 L = 1 mole 1 mole = 22.4 L Molar mass = 1 mole 1 mole = molar mass 6.02 x 1023= 1 mole 1 mole = 6.02 x 1023

On your iPad, find “Mass, particles, volume & the mole” w/s Let’s try the first problem on “Mass and the Mole” What is the mass in g of 0.125 mole of copper (II) chloride? First, know what you’re given and what you plan on doing!

The Mole Mass (g) Back to carbon - 12 Volume (of a gas at STP) Particles (atoms, molecules, or formula units Parts of a particle (atoms only) Mass (g) You go to mass 22.4 L = 1 mole 1 mole = 22.4 L 1 mole = molar mass Molar mass = 1 mole Using molar mass 6.02 x 1023= 1 mole 1 mole = 6.02 x 1023 You start with moles

Before we start the problem: What is the balanced formula for Copper (II) chloride? Cu2+ Cl- What is the molar mass of copper (II) chloride? Cu = 63.5 g Cl = 35.5 g

Dimensional Analysis Time! Only 3 needed, so it rounds to 16.8 grams

Now let’s try the first problem on “particles and the mole” How many particles are in 2.55 moles of water? First, identify what the particle is that the question’s asking you about Water is a molecule!

1 mole of water = 6.02 x 10 23 “what” s? Only 3 sig figs, so the answer = 1.54 x 1024 molecules

Now let’s do the first problem on “Volume and the Mole” Find the number of mole of 23.8 L of helium gas (assume all these problems are at STP) Remember: 1 mole of ANY gas at STP = 22.4 L

3 sig figs needed, so final answer is: 1.06 moles