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GPS & Essential Questions:

This lesson explores the concepts of atomic mass, molar mass, and the mole in relation to chemical composition and calculations. Topics covered include average atomic masses, counting by weighing, atomic masses, and calculating mole and molar mass. Students will learn how to convert between moles, mass, and number of atoms and calculate the mass percent of an element in a compound.

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GPS & Essential Questions:

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  1. Atoms & MolesSC2 (c): Students will relate how the Law of Conservation of Matter is used to determine chemical composition in compounds & chemical reactions. (c): Apply concepts of the mole and Avogadro’s number to conceptualize and calculate mass, moles and molecules relationships molar volumes of gases empirical/molecular formulas What are the similarities of atomic mass and molar mass? How is the molar mass calculated for various compounds? How do you convert between moles, mass, and number of atoms? How is the mass % of an element in a compound calculated? GPS & Essential Questions:

  2. Average Atomic Masses Review: • Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations. • Average mass is calculated from the isotopes of an element weighted by their relative abundances.

  3. Average Atomic Masses of Elements • Most elements exist naturally as mixtures of isotopes. • Average atomic mass = the weighted average of the atomic masses of the naturally occurring isotopes of an element. • Depends both on the mass and the relative abundance of each of the element’s isotopes. • Multiply the atomic mass of each isotope by its relative abundance (expressed in decimal form) and adding the results. • Ex: Cu

  4. A. Counting by Weighing • Objects do not need to have identical masses to be counted by weighing. • All we need to know is the average mass of the objects. • To count the atoms in a sample of a given element by weighing we must know the mass of the sample and the average mass for that element.

  5. A. Counting by Weighing Averaging the Mass of Similar Objects • Example: What is the mass of 1000 jelly beans? • Not all jelly beans have the same mass. • Suppose we weigh 10 jelly beans and find: • Now we can find he average mass of a bean. • Finally we can multiply to find the mass of 1000 beans!

  6. A. Counting by Weighing Averaging the Mass of Different Objects • Two samples containing different types of components (A and B), both contain the same number of components if the ratio of the sample masses is the same as the ratio of the masses of the individual components.

  7. B. Atomic Masses: Counting Atoms by Weighing • Atoms have very tiny masses so scientists made a unit to avoid using very small numbers. 1 atomic mass unit (amu) = 1.66 10-24 g • The average atomic mass for an element is the weighted average of the masses of all the isotopes of an element.

  8. Objectives • To learn to convert between moles and mass • To understand the definition of molar mass • To learn to calculate the mass percent of an element in a compound

  9. Relating Mass to numbers of Atoms: • Three very important concepts provide the basis for relating masses in grams to numbers of atoms. • mole • Avogadro’s number • molar mass

  10. Mole: • SI unit for the amount of substance • Abbreviated mol • The amount of a substance that contains as many particles as there are atoms in exactly 12 g. of carbon-12.

  11. Avogadro’s Number: • Tells you the number of particles in exactly one mole of pure substance (mole, particle, or molecules) • Value means that exactly 12 grams of Carbon-12 contains 6.0221367 * 10 23 carbon 12 atoms. • Value is 6.0221367 * 10 23 atoms, particles, or molecules mole • Allows you to convert between atoms and moles, atoms and particles, or atoms and molecules. • See EX:

  12. C. THE MOLE & AVOGADRO’S NUMBER

  13. The molar mass of an element is numerically equally to the atomic mass of the element in atomic mass units. Written in units of grams/mol Ex. Lithium 6.94 g/mol (molar mass) 6.94 amu (atomic mass units) A molar mass of an element contains one mole of atoms. EX: Helium-4 = 4.00 amu = 4.00 g/mol = 1 atom of helium EX: Lithium-6 = 6.94 amu = 6.94 g/mol = 1 atom of helium Molar Mass:

  14. A. Molar Mass • A compound is a collection of atoms bound together. • The molar mass of a compound is obtained by summing the masses of the component atoms.

  15. A. Molar Mass • For compounds containing ions the molar mass is obtained by summing the masses of the component ions.

  16. Calculations Using Molar Mass • You can use molar mass (g/mol) as a conversion factor in chemical calculations. Moles of a compound = EX: How many moles are in 2.00 g of helium given the molar mass of helium is 4.00 g/mol. 2.00 g * mole = 0.500 moles 4.00g

  17. Calculations Using Molar Mass EX: How many grams are in 2.00 mole of helium given the molar mass of helium is 4.00 g/mol. 2.00 moles * 4.00g = 8.00 g mole • Mass of a sample (g) = (moles of sample)(molar mass of compound)

  18. B. Percent Composition of Compounds • Percent composition consists of the mass percent of each element in a compound: Mass percent =

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