# Chemical calculations

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• Created by: firefled
• Created on: 12-04-15 10:48
• Chemical calculations
• Atomic and molecular masses
• Masses of atoms
• Carbon-12 is the standard of reference.
• Relative atomic mass- average mass of one atom of the element relative to 1/12 the mass of one atom of carbon-12
• Use relative isotopic mass when referring to isotopes.
• Masses of molecules
• Relative molecular mass- the average mass of a molecule relative to one-twelfth the mass of an atom of carbon-12
• Amount of substance
• One mole- the amount of any substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12
• Mass per mole is the molar mass
• Avogadros constant is 6.02*10^23. (Number of atoms in 12g carbon-12)
• Mass spectrometer
• Ionisation-  gaseous sample bombarded with high energy electrons, forms positive ions
• Acceleration- electric fields acceleration +ve ions to high speed
• Deflection- magnetic field deflects ions according to mass/charge ratio.
• Detection- ions with correct mass/charge ratio pass through slit and are detected. Signal amplified and recorded.
• High vacuum to prevent collision with air molecules
• Relative atomic mass is weighted average of the masses of all the atoms in the isotopic mixture.
• Mass spectrum of chlorine
• Because Cl exists as a molecule, some become molecular ions and others fragment.
• Ratio of atomic ions- 35Cl+ : 37Cl+ is 3:1
• Ratio of molecular ions: mass 70:72:74 is 9:6:1
• Empirical and molecular formulae
• Empirical formula- simplest formula showing simplest whole number ratio of elements.
• Molecular formula- shows actual number of atoms of each element
• Moles, mass and concentration
• n = m / M
• Where n=moles, m=mass, M=molar mass
• n=c x v
• Where n=moles, c=concentration, v=volume.
• Concentration tells you how much solute is dissolved in the solvent.
• Remember to convert volumes to dm^3
• Equations for solids and gas reactions
• Equations tell us what substances react together and the amounts
• Number of moles given in balanced equation are called stoichiometric amounts
• For solids- moles calculated from masses
• For gases, moles calculated from volumes using molar gas volume.
• Atom economy and percentage yield
• In a reaction, the compounds formed, other than the product needed, are waste
• Atom economy- mass of required product / total mass of reactants * 100%
• Percentage yield- mass of product obtained / maximum theoretical mass * 100%
• Give indication of the efficiency of a reaction
• Atom economy- mass of required product / total mass of reactants * 100%
• The higher the atom economy, the more efficient the process