Moles
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Moles. Definition. A mole is the mass of a substance which contains the same number of particles as 12 grams of the isotope carbon 12 . These particles could be atoms, ions or molecules. 12 grams of carbon 12 contain 6.02 x10 23 atoms

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Presentation Transcript

Definition
Definition

  • A mole is the mass of a substance which contains the same number of particles as 12 grams of the isotope carbon12.

  • These particles could be atoms, ions or molecules.

  • 12 grams of carbon12 contain 6.02 x1023 atoms

  • This number is the Avogadro Constant, and is given the symbol L.


Eg; One mole is…

16 + 2= 18g ofwater

63.5g of copper

27g ofaluminium


Calculating the number of particles
Calculating the number of particles

  • Number of particles = moles x Avogadro Constant.

  • Eg; How many particles are there in 0.1moles of hydrogen?

  • N = 0.1 x 6.02x1023 = 6.02 x 1022molecules.


Calculating the number of moles
Calculating the number of moles

  • Moles = number of particles / Avogadro constant.

  • Eg; How many moles of methane are there in 3.01 x1023 molecules?

  • Moles = 3.01x1023 / 6.02x1023 = 0.5 moles.


Calculating the molar mass
Calculating the molar mass

  • One mole of an element has a mass equal to the relative atomic mass in grams.

  • Eg; the mass of one mole of sodium is 23g.

  • The mass of one mole of a compound is found by adding together the masses of all its elements in the correct ratio.

  • Units gmol-1



Calculating the number of moles1
Calculating the number of moles

  • No moles = mass / RAM

  • Eg; How many moles are there in 4g of calcium.

  • Look up the mass number of calcium in the periodic table.

  • The atomic mass is the mass number in grams.

  • N = 4 / 40 = 0.1 moles


Think of the equation as a triangle

Mass (g)

m

Relative atomic mass

RAM

n

Number of moles


Rearranging;

n = m / RAM

mass (g)

m

Number of moles =

Relative atomic mass

RAM

n


Calculating the number of moles of a compound
Calculating the number of moles of a compound.

  • No moles = mass / Mr

  • Eg; How many moles are there in 15g of calcium carbonate?

  • Look up the Ars, then add them together to find the Mr.

  • Mr = 40 + 12 + (16 x 3) = 100

  • One mole has a mass of 100g.

  • N = 15 / 100 = 0.15 moles.


Rearranging;

m = nRAM

mass =

m

n

RAM

Number of moles

X

RAM


Calculating the mass from moles
Calculating the mass from moles

  • Mass = No moles X Ar

  • Eg; What is the mass of 0.75 moles of sodium?

  • Ar = 23

  • Mass = 0.75 x 23 = 17.25g

  • Similarly for a compound;

  • Mass = No moles X Mr

  • Eg; What is the mass of 0.8 moles of sodium chloride?

  • Mr = 23 + 35.5 = 58.5

  • Mass = 0.8 x 58.5 = 46.8g


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