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Moles

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

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Eg; One mole is… 27g of aluminium63.5g of copper 16 + 2 = 18g of water

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Calculating the number of particles Number of particles = moles x Avogadro Constant. Eg; How many particles are there in 0.1 moles of hydrogen? N = 0.1 x 6.02x10 23 = 6.02 x 10 22 molecules.

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Calculating the number of moles Moles = number of particles / Avogadro constant. Eg; How many moles of methane are there in 3.01 x10 23 molecules? Moles = 3.01x10 23 / 6.02x10 23 = 0.5 moles.

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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

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Eg; What is the molar mass of calcium carbonate? Write the formula; CaC0 3. M = RAM Ca + RAM C + 3x RAM O. M = 40 + 12 + 3x16 = 100g

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Calculating the number of moles N o 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

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Number of moles Mass (g) Relative atomic mass n m RAM Think of the equation as a triangle

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Number of moles = mass (g) Relative atomic mass n m RAM n = m / RAM Rearranging;

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

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Number of moles mass = n m RAM m = nRAM XRAM Rearranging;

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Calculating the mass from moles Mass = N o 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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