Chemical calculations
- Created by: firefled
- Created on: 12-04-15 10:48
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- 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)
- One mole- the amount of any substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12
- Masses of atoms
- 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
- Detection- ions with correct mass/charge ratio pass through slit and are detected. Signal amplified and recorded.
- Deflection- magnetic field deflects ions according to mass/charge ratio.
- Acceleration- electric fields acceleration +ve ions to high speed
- 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
- Because Cl exists as a molecule, some become molecular ions and others fragment.
- Ionisation- gaseous sample bombarded with high energy electrons, forms positive ions
- 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
- Where n=moles, c=concentration, v=volume.
- n = m / M
- 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.
- Equations tell us what substances react together and the amounts
- 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
- Give indication of the efficiency of a reaction
- In a reaction, the compounds formed, other than the product needed, are waste
- Atomic and molecular masses
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