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The Mole - Molar Calculations  A mole is the Avogadro number of particles.. rather like a dozen is twelve, the mole is just that, an Avogadro number of particles! The Avogadro number is a big number 6.02 x 10 23 = 602 000 000 000 000 000 000 000 = six hundred and two thousand, million, million, million! It is given the symbol NA (NumberAvogadro)
by physicists and L (why????) by chemists When we are dealing with atoms, having a large number like this to compute with is valuable. The link between mass and number of particles is simple: The mass of one particle (smallest unit - atom or molecule) expressed in grams contains NA particles. eg. If you have 12g of carbon (atomic mass 12) there are 6.02 x 10 23 atoms of carbon in your sample. You have one mole. If you have 2g of hydrogen (atomic mass 1, but diatomic molecules go round in pairs therefore the smallest unit has a molecular mass of 2) there are 6.02 x 10 23 molecules of hyrdogen in your sample. You have one mole. If you have 18g of water (atomic mass of hydrogen is 1 and of oxygen is 16, therefore the molecular mass of H2O is 18) there are 6.02 x 10 23 molecules of water in your sample. You have one mole. Try the following questions: How many atoms of lead are found in a 2mg sample
? (atomic mass of lead is 210g) What would be the mass of one hundred million million
(100 000 000 000 000 = 1014) atoms of Uranium 238 (atomic
mass is 238)? |
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