mass numbers and RFM

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• Created by: dav09
• Created on: 30-10-20 11:47

Mass number

• the relative mass of protons and neutrons is 1
• the mass number of an atom tells you the the total number of protons and neutron in the nucleus
• isotopes are atoms of the same elements with different numbers of neutrons

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Relative Formula Mass (RFM)

• The symbol for the relative atomic mass is Ar.
• This is calculated from the mass number and relative abundances of all the isotopes of a particular element.
• The symbol for the relative formula mass is Mr and it refers to the total mass of the molecule.
• To calculate the Mr of a substance, you have to add up the relative atomic masses of all the atoms present in the formula.
• they are always measured in grams (g).

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Conservation of mass

• The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.
• Mass is always conserved, therefore the total mass of the reactants is equal to the total mass of the products, which is why all chemical equations must be balanced.
• The sum of the relative atomic/molecular masses of the reactants will be the same as the sum of the relative atomic/molecular masses of the products.
• A precipitation reaction is one in which two solutions react to form an insoluble solid called a precipitate.
• If the reaction flask is closed and no other substance can enter or leave the system, then the total mass of the reaction flask will remain constant.
• For example, the reaction between calcium chloride and sodium sulfate produces a precipitate of calcium sulfate.
• If carried out in a closed system then the mass before and after the reaction will be the same.
• The balanced equation is:

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moles and their calculation

• This is the mass of substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of 12C.
• The mole is the unit representing the amount of atoms, ions, or molecules and its symbol is mol.
• One mol is the amount of a substance that contains 6.02 x 1023 particles (Atoms, Molecules or Formulae) of a substance (6.02 x 1023 is Known as the Avogadro Number).Example -  1 mole of Sodium (Na) contains 6.02 x 1023 atoms of Sodium.
• how many moles are there in 25g of sodium oxide, NaO
• calculate the RFM of sodium oxide
• RAM Na = 23 / RAM O =16
• so the RFM of NaO2 = (1 x 23) + (2 x 16) = 62g
• number of moles = mass of substance (g) ÷ RFM (g)
• 25 ÷ 62 = 0.403 moles NaO
• answer = 0.403 moles of NaO
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Balanced equations and their calculation

• make sure that the formulae are correct for the chemicals involved
• make sure that the equations balances using moles
• using a balanced equation, you van calculate the amount of product that you could theoretically get
• e.g  how many grams of calcium oxide do you get forom the thermal decomposition of 200g of cacium carbonate? CaCO3 = CaO + CO2
• 100g CaCO3 will produce 56g of CaO
• so 200g of CaCO3 will produce (2 x 56g) CaO
• 112g CaO
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