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Mass, density, Avogadro’s number and Moles

Mass, density, Avogadro’s number and Moles. Relative Atomic Masses. The mass of atoms in Grams is very small Ex. Oxygen = 2.67 X 10 -23 Need a better way to express these numbers Relative atomic mass Based on carbon-12 New scale needs a new Unit Atomic mass unit AMU. AMU.

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Mass, density, Avogadro’s number and Moles

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  1. Mass, density, Avogadro’s number and Moles

  2. Relative Atomic Masses • The mass of atoms in Grams is very small • Ex. Oxygen = 2.67 X 10-23 • Need a better way to express these numbers • Relative atomic mass • Based on carbon-12 • New scale needs a new Unit • Atomic mass unit • AMU

  3. AMU • Carbon-12 has exactly 12 AMU’s (surprise surprise) • All other atoms are based of this assumption. • Hydrogen-1 is about 1/12 the mass carbon -12 • Therefore • = 1.007825 amu • Oxygen-16 about 12/16 of carbon-14 • therefore • =15.994915

  4. Average atomic mass • Average atomic mass • The weighted average of the atomic masses of the naturally occurring isotopes of an element. • For example • Bag full of 100 marbles • 25% have mass of 2.00 g • 75% have mass of 3.00 g • Now we add the mass times the percent of each one • (2.00x.25)+(3.00x.75)= 2.75g

  5. Lets try some! • What is the average atomic mass of copper? • Copper-63 • amu=62.929599 • abundance(%) 69.17 • Copper-65 • amu=64.927793 • abundance(%) 30.83

  6. Solved • (69.17*.62929599)+(30.83*.6492793) • 63.546 • Compare to our periodic table • Is it close • Then we are good!

  7. Another one • Oxygen-16 • AMU=15.994915 • %=99.762 • Oxygen-17 • AMU=16.999131 • %=.038 • Oxygen-18 • AMU=17.999160 • %=0.200

  8. Solved • (15.994915*.99762)+(16.999131*.0038)+(17.999160*.00200) • Three four five six don’t matter do the same thing • 15.9994 • Check the table! • Are we close

  9. The mole • MOLE • The amount of a substance that contains as many particles as the are atoms in exactly 12g of carbon -12 • This is the SI unit for the amount of a substance • Like saying a dozen.

  10. The number • Avogadro’s number • The number of particles in exactly on mole of a pure substance • 6.0221367 x 1023 • round it to 6.022 x1023 • Obtained by experiments • Best modern value

  11. The mass of a mole • Molar Mass • The mass of one mole of a pure substance • Usually written in g/mol • Equal to the atomic mass of the element on the periodic table • Ex • He=4.00g • Li=6.94g • Hg=200.59g

  12. Gram Mole conversions • We can use this information to determine molar mass and vise versa • Ex. How many grams of He are in two moles of He?

  13. Lets try some • What is the mass in grams of 2.25 mol of the element Fe? • 126 g Fe • What is the mass in grams of 0.375 mol of K? • 14.7g K

  14. Some more • What is the mass in grams of 0.0135 Na? • 0.310g Na • What is the mass in grams of 16.3 mol Ni?

  15. Density • Density • The ratio of mass to volume • Mass divided by volume

  16. Try some • What is the density of a block with a mass of 25g and a volume of 25 cm3 • What is the density of a block with a mass of 10g and a volume of 100cm3

  17. Solutions • D=25/25 • 1.0 g/cm3 • D=10/100 • 0.1 g/cm3

  18. More problems • What is the mass of a coin with a volume of 3cm3 and a density of 50g/cm3? • What is the volume of a ball with a mass of 35.8kg and a density of 10.53 g/cm3?

  19. Solutions • Solve the formula for

  20. Solution #2 • Solve the formula for

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