Chem Calcs

• Avogadro's number


  There are 6.02 x 10^23 particles of a substance in 1 mole
• Mole


  Unit used to measure the amount of a substance
• Moles


  Mass divided by relative atomic mass (M)
• Concentration in g/dm³


  Mass (g) divided by volume (dm³)
• Concentration in mol/dm³


  Moles (mol) divided by volume (dm³)
• 1 mole of any gas has a volume of 24dm³
• Converting from cm³ to dm³

  1dm³ = 1000cm³
• Percentage Yield


  Mass of product actually made divided by maximum theoretical mass of product, multiplied by 100
• Atom Economy


  Relative formula mass of desired product from equation divided by sum of relative formula masses of all reactants from equation, multiplied by 100
• Calculating energy changes


  Energy change in a reaction = Total energy needed to break the bonds in reactants - Total energy released when bonds in the products are formed
• Rate of reaction


  Mean rate of reaction = quantity of reactant used divided by time taken
• Chromatography


  R_f = distance moved by substance divided by distance moved by solvent
• Combustion of carbon


  Carbon + Oxygen → Carbon dioxide
• Combustion of hydrogen


  Hydrogen + Oxygen → Water
• Combustion of hydrocarbons


  Methane + Oxygen → Carbon dioxide + Water
• Oxidation of metals


  Copper + Oxygen → Copper (II) oxide
• Displacement of halogens


  Sodium bromide + Chlorine → Bromine + Sodium chloride
• Reaction of metal with acid

  Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
• Reaction of metal oxide with acid

  Copper (II) oxide + Hydrochloric acid → Copper (II) chloride + Water
• Displacement of metals


  Copper (II) sulfate + Magnesium → Magnesium sulfate + Copper
• The Haber process


  Nitrogen + Hydrogen → Ammonia
• Energy Reaction


  Info:

  
• Calculating the formula of a compound


  1. Write the element symbols

  2. Write the masses

  3. Write the Ar values

  4. Divide masses by Ar

  5. Divide by the smallest number
• Example


  • 3.2 g of sulfur reacts with oxygen to produce 6.4 g of sulfur oxide
• The Ar (relative atomic mass) of sulfur, S = 32, and the Ar of oxygen, O= 16
• Converting the empirical formula to a molecular formula


  1. Add up the atomic masses of the atoms in the empirical formula

  2. Divide the relative formula mass (Mr) by the mass of the atoms in the empirical formula

  3. Multiply the numbers in the empirical formula by the result
• Percentage mass


  A way of saying what proportion of the mass of the compound is due to atoms of that element
• 75% of the mass of methane is made up of carbon atoms, so if you have 100 g of methane, it will contain 75 g of carbon
• Calculating percentage mass


  1. A, number of atoms of that element

  2. M, mass of the compound

  3. Multiply by 100
• The formula to calculate percentage mass is: Percentage mass of an element in a compound = (A, number of atoms of that element / M, mass of the compound) x 100
• Calculating the mass of a product

  1. Write out the balanced equation

  2. Work out the relative formula masses (M) of the limiting reactant and the product you want to find the mass of

  3. Work out how many moles there are of the limiting reactant

  4. Use the balanced equation to work out how many moles there'll be of the product

  5. Use the number of moles, along with the relative formula mass of the product to calculate the mass
• 3 x 2.54 = 7.62 g
• As reactant' is an important part of a reaction
• You can calculate the mass of a product formed in a reaction by using the mass of the limiting reactant and the balanced reaction equation
• At the same temperature and pressure, two moles of chlorine will also take up the same volume
• 24 g of oxygen (O2) will occupy 24 x 24 = 18 dm³ at room temperature and pressure (r.t.p. = 20 °C and 1 atm)
• Volume of gas (dm³)

  Mass of gas (g) / M, of gas × 24
• Acid


  Substance that forms hydrogen ions (H+) when dissolved in water
• Alkali


  Substance that forms hydroxide ions (OH-) when dissolved in water
• Neutralisation reactions


  Acid reacts with alkali to form salt and water