Calculations

Calculations

Relative Formula Mass
1. Relative atomic mass, Ar
  1. The same as the mass number of the element.
  2. All elements have two number - bigger one is mass number or relative atomic mass.
2. Relative formula mass, Mr
  1. Is all the relative atomic masses added together.
  2. For MgCl2 it would be:
    1. Mg = 24
    2. +
    3. Cl2 = (35.5x2)
    4. = 95
3. "ONE MOLE" of a substance is equal to its Mr, in grams
  1. The relative formula mass (Ar or Mr) of a substance in grams is known as one mole of that substance.
  2. NUMBER OF MOLES =
  3. Mass in g (of element or compound) ------------------------------ Mr (of element or compound)
Two formula mass calculations
1. Calculating % mass of an element in a compound
  1. percentage mass OF AN ELEMENT IN A COMPOUND
    1. =
      1. Ar X no. of atoms (of that element) ------------------------------------ Mr (of whole compound)
        X 100
2. Finding the empirical formula (from masses or percentages)
  1. 1) list all the elements in the compound.
  2. 2) underneath them write their experimental masses of percentages.
  3. 3) divide each mass or percentage by the Ar, for that particular element.
  4. 4) turn the numbers you get into a nice simple ratio by multiplying and/or dividing them by well-chosen numbers.
  5. 5) get the ratio in its simple form, and that tell you the empirical formula of the compound.
Percentage yield compares actual and predicted yield
1. 1) the predicted yield of a reaction can be calculated from the balanced reaction equation.
2. 2) percentage yield is given in the formula:
  1. percentage yield =
  2. actual yield (grams) -------------- predicted yield (grams)
  3. X 100
3. 3) percentage yield is always somewhere between 0 and 100%.
4. 4) a 100% percentage yield means that you got all the product you expected to get.
5. 5) a 0% yield means that no reactants were converted into product.
Calculating masses in reactions
1. 1) write out the balanced equation.
2. 2) work out the Mr - just for the two parts you want.
3. 3) apply the rule: divide to get one, then multiply to get all.

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