The Mole

Historically, there was no way to know the mass of one atom or molecule WHY?

Because atoms are so small!

Experiments by a French chemist, Joseph Louis Gay-Lussac opened the door to making determining the mass of elements possible.

Volumes of Gases and the mole The experiment: equal volumes of gases at the same temperature and pressure reacted and the following occurred: B A+ →Solid forms ½ A ½ BA + → + solid B½ A +→ + solid ½ B

Gay-Lussac’s law “The law of combining volumes states that, when gases react together to form other gases, and all volumes are measured at the same temperature and pressure: The ratio between the volumes of the reactant gases and the products can be expressed in simple whole numbers” In other words: 2H 2 + O 2 → 2 H 2 O coefficients are whole number ratios and can express volume as well as numbers of atoms/molecules

Based on these results, what can be concluded about the number of molecules in equal volumes of gases measured at the same temperature and pressure??

Along came a guy named Amedeo Avogadro

Amedeo Avogadro determined that equal volumes of gases at the same temperature and pressure contain the same number of molecules/atoms. He himself did not determine the number of atoms in a mole, but his principle of gas volumes led to its discovery. This is known as Avogadro’s Hypothesis.

Why was this significant? If equal volumes had equal numbers of molecules, then you can compare the masses of equal volumes in a ratio. Mass 1 liter oxygen = 16 Mass 1 liter hydrogen 1 Mass 1 liter nitrogen = 14 Mass 1 liter hydrogen 1

What would happen if you compared 2 Liters of each gas? Mass 2 liter oxygen = ? Mass 2 liter hydrogen = 16 1 Mass 2 liter nitrogen = ? Mass 2 liter hydrogen =14 1

What would the ratio of one atom of oxygen to hydrogen be? 16 to 1! What would the ratio of one atom of nitrogen to hydrogen be? 14 to 1

This enabled scientists to standardize these relationships - atomic mass units If H = 1 amu Then: O = 16 amu N = 14 amu Look on the periodic table – the masses given on the table for each type of atom is given in amus

So what does this have to do with the mole??? Remember: atoms are small Scientists wanted to determine the number of atoms in a standard amount of substances. Originally they used hydrogen, then oxygen and finally carbon. They experimentally determined the number of atoms in 12.0g of carbon They called this a mole (molecular weight)

How many atoms are in 1 mole??? You guessed it: 6.02 x It is the number of particles (atoms) in 12.0 g Carbon

This is a very large number! Consider this: One mole of paper clips strung together would wrap around the world 400 trillion times!

More mole facts 6.02 X Watermelon Seeds: Would be found inside a melon slightly larger than the moon X Donut Holes: Would cover the earth and be 5 miles (8 km) deep X Pennies: Would make seven stacks that would reach the moon.

A Mole is a shorthand way of saying 6.02x When you go to a bakery and ask for a dozen donuts, how many donuts do you get? If I asked you to sit in pairs, how many students would sit together? “Pairs” and “dozens” are examples of counting numbers that you are familiar with and that you can easily count. A mole is known as a “chemist’s dozen”

So how can we easily find out the mass of one mole of anything? We use the periodic table The atomic mass of an element is not only equal to the mass in amus of one atom of the element, but also the mass in grams of one mole of that element

6 C amu is the mass of one atom of carbon or the mass in grams of one mole of Carbon atoms Now all molecular weights are based on carbon = 12 amu

An amu is defined as 1/12 the mass of a carbon-12 atom 1amu = 1.66 x g OR 1 gram = 6.02x amus

Going back to the balloons If I have 32.0g of O 2 and 2.0 g H 2, what could I say about the volumes they take up? They must be the same…. If a substance is in the gas state at 1.00 atm and 0 o C, then one mole of that substance takes up a standard volume of 22.4L –This is the molar volume at STP (standard temperature and pressure)

Therefore: 1 mole H 2 = 6.02 x molecules H 2 = 2.02 g H 2 = 22.4L 1 mole O 2 = 6.02 x molecules O 2 = 32.0 g O 2 = 22.4L 1 mole C = 6.02 x atoms C = 12.0 g C 1 mole H 2 O = 6.02 x molecules H 2 O= 18.0 g H 2 O = 22.4L Where did I get the 18.0 g from?

Look at your periodic table 2 x 1 mole H = 2 (1.01g H) = 2.02 g H 1 x 1 mole O = 1 (16.0 g O) = 16.0 g O = 18.0 g H 2 O

Mole relationships 1 MOLE MOLAR MASS 6.02 x particles 22.4L if a gas at STP

Use the periodic table to find the mass in grams of 1 mole of aluminum atoms 1 mole Al = 27.0 g Al What is the mass of two (2) moles of aluminum? USE Dimensional Analysis!! 2 moles Al x 27.0 grams Al = 54.0 grams Al 1 mole Al Can you figure out the mass of one mole of carbon dioxide (CO 2 )?

1 mole of C = 12.0 g 1 mole of O = 16.0 g What is the mass of one mole of CO 2 ? C = 1 X 12.01g = 12.0g O = 2 X 16.00g = 32.0g 44.0g The molar mass of CO 2 = 44.0g

How about 1.50 moles CO 2 ? 1 mole CO 2 = 44.0g CO mole CO 2 x 44.0g CO 2 = 66.0 g CO 2 1 mole

67.0 g H 2 O = ? moles 1 mole H 2 O = 18.0 gH 2 O 67.0 g H 2 O x 1 mole H 2 O= 3.72 moles 18.0 gH 2 O

How many molecules H 2 O are in 3.50 moles H 2 O? 1 mole H 2 O = 6.02 x molecules 3.50 moles H 2 O x 6.02 x molecules 1 mole H 2 O = 2.11 x molecules H 2 O

How many atoms of H are in 3.45 moles of H 2 O? Two approaches: 1. How many moles of H are in 1 mole of H 2 O? 3.45 moles H 2 O x 2 mole H x 6.02 x atoms H 1 mole H 2 O 1 mole H OR 2. How many atoms of H are in one molecule H 2 O ? 3.45 moles H 2 O x 6.02 x molecules H 2 O x 2 atoms H 1 mole H 2 O 1 molec H 2 O

So that is the story of the mole! Now we will try some problems in class