The Mole

Measuring Matter If you are buying eggs, you wouldn’t say you bought 12 eggs- you would say you bought a dozen eggs. Similarly, you might buy a pair of gloves or a ream of paper for a computer A dozen, pair and ream all stand for a specific number of items (12, 2, 500) These terms makes counting objects easier

Measuring Matter When counting, you aren’t looking at the individual size of the object- small gloves come in pairs just like large gloves do and a dozen eggs can be small or large also… instead you are counting the number of individual pieces The number that unit represents is always constant

Measuring Matter Chemists also need a convenient method for counting accurately the number of atoms, molecules or formula units in a sample of a substance. As you know, atoms and molecules are extremely small There are so many of them in even the smallest sample that it is impossible to actually count them ourselves without help

Measuring Matter Chemists created their own counting unit called the mole The mole, commonly abbreviated mol, is the SI base unit used to measure the amount of a substance The mole represents the number of particles in a sample

Measuring Matter Though years of experimentation, it has been established that a mole of anything contains 6.022136 x 1023 particles. This number is called Avogadro’s number in honor of the Italian physicist who determined its value The number is usually rounded to three significant figures 6.02 x 1023

Measuring Matter Avogadro’s number is an enormous number, as it must be in order to count extremely small particles. As you can imagine, Avogadro’s number would not be so convenient for measuring marbles… it would be enough marbles to cover the surface of the whole earth 6km thick!

Measuring Matter Moles not only measure atoms- but, also molecules. You can have a : Mole of water Mole of carbon dioxide Mole copper Mole of hydrogen

Converting Moles to Particles and Particles to Moles Suppose you buy three and a half dozen eggs… how many eggs do you have? Lets do it on the board using conversion factors

Converting Moles to Particles and Particles to Moles Now suppose you want to determine how many particles of sucrose are in 3.5 moles of sucrose. Lets do the problem on the board:

Converting Moles to Particles and Particles to Moles Lets do this problem: Calculate the number of moles that contain 4.50 x 1024 atoms of zinc.

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Mass and the Mole You wouldn’t expect a dozen limes to have the same mass as a dozen eggs. Eggs and limes have different sizes and compositions, so it is not surprising that they have different masses Moles of substances also have different masses for the same reason- the substances have different compositions

Mass and the Mole If you put a mole of a carbon on a balance, it would have a completely different mass than a mole of copper. Carbon atoms are different than copper atoms- different protons, neutrons and electrons and therefore different masses

Mass and the Mole Mass of hydrogen is 1 amu, mass of helium is 4 amu and the mass of carbon is 12 amu. Therefore hydrogen is ¼ the mass of helium and 1/12 the mass of carbon Helium has 4 times the mass of hydrogen and ¼ the mass of carbon…

Mass and the Mole The mass in grams of one more of any pure substance is called its molar mass Molar mass of any element is numerically equal to its atomic mass and has the units g/mol An atom if manganese has an atomic mass of 54.94amu, therefore the molar mass of manganese is 54.94 g/mol

Using Molar Mass Imagine your class bought jellybeans in bulk to sell by the dozen at a candy sale. You soon realize that it is too much work counting out each dozen, instead you decide to measure the jellybeans by mass. You find that 1 dozen jellybeans has a mass of 35g. What mass of jellybeans should you give to a customer who wants 5 dozen? Lets do it on the board.

Mole to Mass Conversion Calculate the mass in grams of 0.0450 moles of chromium

Mass to Mole Conversion How many moles of calcium are in 525g of calcium (Ca)?

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Moles of Compounds Recall that the chemical formula for a compound indicates the types of atoms and the number of each contained in one unit of the compound For example, Freon has a formula CCl2F2; the subscripts in the formula tell you that the molecule consists of one atom of carbon, two atoms of chlorine and 2 atoms of fluorine

Moles of Compounds But, suppose you had a mole of Freon. The representative particles would be molecules of Freon. A mole of Freon would contain Avogadro’s number of Freon molecules… a mole of carbon, two moles of chlorine and 2 moles of fluorine in ONE mole of FREON.

Moles of Compounds Lets draw 5 moles of Freon on the board:

Molar mass of Compounds The mass of your backpack is the sum of the mass of the backpack, plus the mass of pencils, books, lunch box, and other items you have in there…You could find its mass by determining the mass of each item separately an adding them together…

Molar mass of Compounds Similar to the backpack analogy, the mass of a mole of a compound equals the sum of the masses of every particle that makes up the compound. Lets find the molar mass of one mole of potassium chromate (K2CrO4) BOARD NOTES:

Converting moles of a compound to mass Suppose you need to measure a certain number of moles of a compound for an experiment: First you must calculate the mass in grams that corresponds to the necessary number of moles Then that mass can be measured on the balance

Converting moles of a compound to mass I often use copper II sulfate, in our chemistry classes. What is the mass of 1.00 mole of Copper II sulfate? BOARD NOTES I often don’t use 1.0 molar solutions, but less. What is the mass of .25 mole of copper II sulfate?

Converting the mass of a compound to moles Imagine that the experiment you are doing in the laboratory produces 5.55g of a compound. How many moles is this? To find out you calculate the molar mass of the compound and determine it to be 185.0g/mol. The molar mass relates grams and moles, but this time you need the inverse of the molar mass as the conversion factor

Converting mass of a compound to moles Calculate the number of moles of Calcium hydroxide Ca(OH)2 are in 325g. BOARD NOTES

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Percent Composition It is the analytical chemist’s job to identify the elements a compound contain and determine their percent by mass For example, a 100g sample of a new compound contains 55g of element X and 45g of element Y. The percent by mass of any element in a compound can be found by dividing the mass of the element by the mass of the compound and multiplying by 100

Percent Composition Percent composition: the percent by mass of each element in a compound Formula:

Percent composition Lets calculate the percent composition of H2O. Break down the molecule into its components: H, H and O Find the mass of each element Calculate the molar mass of the molecule Find the mass of each element in a mole of water Divide the mass of hydrogen in the molecule by the total mass of the water molecule Multiply by 100 BOARD NOTES

Determine the percent composition of sodium hydrogen carbonate NaHCO3

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