moles in equations

• Chemical equations can be used to calculate masses of reactants & products by:

  • First identify the KNOWN & UNKNOWN substances.
  • The KNOWN substance is the substance you know the mass of.
  • The UNKNOWN substance is the substance you need to find out.
• • Find the MOLES of the KNOWN substance, by DIVIDING the mass by the Mr of the KNOWN substance.
  • Look at the BIG numbers in front of the substances in the BALANCED equation.
  • Use the number IN FRONT of the KNOWN & UNKOWN to write the RATIO of them.
  • Known : Unknown
  • Use this ratio to find the MOLES of the UNKNOWN.
  • Find the mass by MULTIPLYING the MOLES of the UNKNOWN by its Mr.
• • Moles = 46 / 23 = 2 mol of Na

  Known : unknown

  • Na : Na2O
  • 4 : 2

  2 mol : 2/2 = 1 mole of Na2O

  • Mass = mass x Mr of Na2O = 1 x (23 + 23 + 16) = 62g

  
• • Moles Au = 0.175 / 197 = 0.000888

  
• • Mr CaO = 56
  • Mr CaSO = 120
  • 7/56 x 120 = 15g

  
• If you know the masses of the REACTANTS & PRODUCTS that took part in a reaction, you can work out the BALANCED SYMBOL EQUATION for it.
• To find the BALANCED SYMBOL EQUATION:

  • Find any unknown masses by using the CONSERVATION OF MASS.
  • Find the Mr for each reactant & product.
  • Divide the masses by the Mr for each reactant & product to find their MOLES.
  • Divide each of the moles by the SMALLEST NUMBER out of all of them.
  • If any of the values are NOT whole numbers, multiply all of them by a number that causes them to become WHOLE NUMBERS.
  • Put each of these numbers IN FRONT of their chemical formulas in the equation.