# Quantitative Chemistry

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• Quantitative Chemistry
• Atomic Mass
• Protons + Neutrons = Mass Number
• Isotopes
• Same number of protons (atomic number)
• Different number of neutrons (mass number)
• Same element
• Different number of neutrons (mass number)
• Same number of protons (atomic number)
• Relative Formula Mass (Mr)
• = sum of  all (Mr)s
• e.g. Mr for MgCl? = 95
• Moles
• Mr of substance in grams is known as one mole
• Number of moles = Mass in g of element ÷ Mr of element
• Percentage mass
• = (mass of element in compound ÷ total Mr of compound)  x 100
• e.g. % mass of sodium in sodium carbonate
• (23x2  ÷ 106) x 100 = 43.4%
• Empirical Formula
• List all elements in compound
• Write amount in question
• Divide by Ar for that element (gives moles)
• Divide each by smallest number if moles
• Mass of Reactants
• e.g.  If we have 50g of CaCO3, how much CaO can we make?
• Work out the Mr values for the two compounds
• Mr of CaCO3 = 100
• Mr of CaO = 56
• Percentage Yield
• = (amount made ÷ predicted yield) x 100
• Never 100%
• Impurities
• Other reactants
• Difficult to separate
• Reversible reaction
• Reversible reactions
• Products of reaction can react to produce original products
• e.g. ammonium chloride ? ammonia + hydrogen chloride
• Analysing Substances
• Chromatography
• Paper
• Gas (GC-MS)
• Separates mixture of compounds
• Substances travel through tube at different speeds so become separated
• No. of peaks on gas chromatograph shows no. of different compounds present
• Position of peaks shows retention time for each substance
• Can be linked to mass spectronometer
• Sensitive
• Fast
• Accurate
• Mass Spectronomy
• Finds Mr of moelcule
• 100 g of CaCO3 would yield 56 g of CaO
• Mr of CaO = 56
• Mr of CaCO3 = 100
• Therefore, 50g of CaCO3 makes 28g of Cao