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The Mole Concept and Avogadro's constant

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The Mole-at first glace What is “the mole”? Dictionary definition: “a small burrowing mammal with dark velvety fur, a long muzzle, and very small eyes, feeding mainly on worms, grubs, and other invertebrates… Animal?- must be biology-not chemistry! Are we in the right class?

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The Mole-demystified Another definition-”Amount of substance…”- more like chemistry ! Ream of paper (500) Dozen of eggs (12) Pair of socks (2)

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The mole & coins! Suppose you had to count a vast number of coins What is the quickest method? If the mass of a certain number of coins is known, the total number can be weighed and then the amount can be calculated.

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The Mole-demystified 2 Quantity of 6.02x10 23 Why this magic number? It is the number of particles present in exactly 12 grams of the isotope carbon-12 Called the Avogadro Constant

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The mole If you weigh out 12g of carbon-12, you will get 6.02x10 23 of carbon atoms

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The mole All elements measured relative to carbon-12 Amount of substance (mol)= number of particles/6.02 x 10 23 mol -1 Number of particles= amount of substance (mol) x 6.02 x 10 23 mol -1 Mole is the same number regardless of the representative particle: atom, molecule, ion, electron

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The mole

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Example 1 Calculate the number of molecules of water in 0.01mol of water Number of water molecules=0.01mol x 6 x 10 23 mol -1 = 6 x 10 21 How many carbon atoms are there in 0.02 mol of carbon dioxide? How many oxygen atoms are there in 0.02 mol of carbon dioxide?

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Molar Mass Molar mass is the mass of 1 mole of the substance Number of moles=mass in g/mass of 1 mole in g A mole of carbon atoms, C, has a mass of 12g A mole of hydrogen atoms, H, has a mass of 1g BUT a mole of hydrogen molecules has a mass of 2g

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Molar mass- example Calculate the molar mass of aluminium sulfate, Al 2 (SO 4 ) 3 2 x Al= 2 x 26.98g/mol= 53.96g/mol +3 x S= 3 x 32.06g/mol= 96.18g/mol +12 x O= 12 x 16.00g/mol=192.00g/mol Molar mass= 342.14g/mol

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Relative atomic mass, relative formula mass Relative atomic mass- the weighted average mass (according to relative abundances) of all the naturally occurring isotopes of an element compare with an atom of the carbon-12 isotope, which has a mass of exactly 12 Formula= 12 x average mass of one atom of the element/mass of one atom of carbon-12 Relative molecular mass is the sum of the relative atomic masses of all the atoms in 1 molecule. Relative molecular mass=12 x average mass of one molecule of the element/ mass of one atom of carbon-12

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Molar mass vs relative atomic/molecular masses Same except molar mass has units of g/mol Relative atomic mass & relative molecular mass are dimensionless Find the molar mass of magnesium carbonate, MgCO 3 MgCO 3 = 24+12+3x16=84g/mol Find the relative molecular mass of carbon dioxide, CO 2. CO 2 = 12+2x16=44 (NO UNITS!)

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Amount of substance vs mass

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More problems Find the moles of water molecules present in 54g of water Moles of water molecules= 54g/18g mol -1 =3.0mol

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Everything combined! Calculate the number of calcium atoms present in 0.5kg 0f calcium Amount of calcium=500g/40g mol -1 =12.5mol Atoms of calcium=12.5 mol x Avogadro's constant= 12.5 x 6.02 x 10 23 = 7.53 × 10 24

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Everything combined more! Calculate the number of carbon and hydrogen atoms and the total number of atoms in 22g of propane, C 3 H 8. Molar mass of propane, C 3 H 8 =(3x12)+(8x1)=44g mol -1 1 mole of propane therefore has a mass of 44g and contains 6.02x10 23 molecules of propane. Amount of propane=22g/44g mol -1 = 0.50mol Molecules of propane=0.5 x 6.02x10 23 =3x10 23 Total number of atoms=11x3x10 23 =3.3x10 24 Total number of carbon atoms=3x3x10 23 =9x10 23 Total number of hydrogen atoms=8x3x10 23 =2.4x10 24

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1 By definition: 1 atom 12 C “weighs” 12 amu On this scale 1 H = 1.008 amu 16 O = 16.00 amu Atomic mass is the mass of an atom in atomic mass units (amu)

1 By definition: 1 atom 12 C “weighs” 12 amu On this scale 1 H = 1.008 amu 16 O = 16.00 amu Atomic mass is the mass of an atom in atomic mass units (amu)

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