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Using Moles to Count Atoms

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Because chemists often deal with large numbers of small particles, they use a large counting unit—the abbreviated mol. Mole -the SI base unit used to measure the amount of a substance whose number of particles is the same as the number of atoms of carbon in 12 g of carbon-12 (video 1)

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A mole is a collection of a very large number of particles About ! This number is usually written as 6.022x10 23 /mol and is referred to as AVOGADRO’S CONSTANT

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The constant is named in honor of the Italian scientist Amedeo Avogadro. Avogadro’s constant is defined as the number of particles, x10 23, in exactly 1 mol of a pure substance. One mole of gumballs is x10 23 gumballs. One mole of popcorn is x10 23 kernels of popcorn

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Avogadro’s constant equals /mol; the number of particles in 1 mol (video 2) You might wonder why represents the number of particles in 1 mol. The mole has been defined as the number of atoms in grams of carbon-12. Experiments have shown that x10 23 is the number of carbon-12 atoms in g of carbon-12.

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One mole of carbon consists of x10 23 carbon atoms, with an average atomic mass of amu. An ATOMIC MASS UNIT (AMU)- is unit of mass that describes the mass of an atom or molecule, is equal to one twelfth of the mass of carbon-12 atom (C12) Moles and grams are related. The mass in grams of 1 mol of a substance is called its MOLAR MASS

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molar mass the mass in grams of 1 mol of a substance (video 4) Because the amount of a substance and its mass are related, it is often useful to convert moles to grams, and vice versa. You can use conversion factors to relate units. (video 5)

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Ex: The shopkeeper knows that exactly 10 gumballs have a combined mass of 21.4 g. This relationship can be written as two equivalent conversion factors, both of which are shown below 10 gumballs/21.4g 21.4g/ 10 gumballs

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There is a relationship between the amount of an element in moles and its mass in grams. This relationship is graphed for iron nails Because the amount of iron and the mass of iron are directly related, the graph is a straight line.

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Ex: Determine the mass in grams of 5.50 mol of iron. ( molar mass of Fe = g/mol Fe)

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Ex: What is the mass in grams of 2.50 mol of sulfur, S ( molar mass of S= ________ )

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Ex: Determine the amount of iron present in 352g of iron. (molar mass of iron= _______ )

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