Calculations

Calculations

1. Relative Formula Mass

   • Relative atomic mass, Ar
     • The same as the mass number of the element.
     • All elements have two number - bigger one is mass number or relative atomic mass.
   • Relative formula mass, Mr
     • Is all the relative atomic masses added together.
     • For MgCl2 it would be:
       • Mg = 24
       • +
       • Cl2 = (35.5x2)
       • = 95
   • "ONE MOLE" of a substance is equal to its Mr, in grams
     • The relative formula mass (Ar or Mr) of a substance in grams is known as one mole of that substance.
     • NUMBER OF MOLES =
     • Mass in g (of element or compound) ------------------------------ Mr (of element or compound)

2. Two formula mass calculations

   • Calculating % mass of an element in a compound
     • percentage mass OF AN ELEMENT IN A COMPOUND
       • =
         • Ar X no. of atoms (of that element) ------------------------------------ Mr (of whole compound)
           • X 100
   • Finding the empirical formula (from masses or percentages)
     • 1) list all the elements in the compound.
     • 2) underneath them write their experimental masses of percentages.
     • 3) divide each mass or percentage by the Ar, for that particular element.
     • 4) turn the numbers you get into a nice simple ratio by multiplying and/or dividing them by well-chosen numbers.
     • 5) get the ratio in its simple form, and that tell you the empirical formula of the compound.

3. Percentage yield compares actual and predicted yield

   • 1) the predicted yield of a reaction can be calculated from the balanced reaction equation.
   • 2) percentage yield is given in the formula:
     • percentage yield =
     • actual yield (grams) -------------- predicted yield (grams)
     • X 100
   • 3) percentage yield is always somewhere between 0 and 100%.
   • 4) a 100% percentage yield means that you got all the product you expected to get.
   • 5) a 0% yield means that no reactants were converted into product.

4. Calculating masses in reactions

   • 1) write out the balanced equation.
   • 2) work out the Mr - just for the two parts you want.
   • 3) apply the rule: divide to get one, then multiply to get all.