Stoichiometry (continued)

Purity Calculation

The purity calculation is done to determine the amount of impurities that exist in the substances. These calculations are widely used as not all substances are pure.

Example: A calcite sample containing 80% calcium carbonate decomposes upon heating according to the reaction:

What mass of calcium oxide is obtained from burning 800g of calcite?

x = 640g CaCO3

For the remainder of the calculation, only the pure CaCO3 value, ie 640g, is used.

x = 358.4g CaO

Yield Calculation

It is common in chemical reactions that the amount of product is less than the expected value. In this case, the yield was not total. This can happen for a variety of reasons, such as poor instrument or reagent quality, poor operator readiness, and so on.

The calculation of the yield of a chemical reaction is made from the quantity of product obtained and the theoretical quantity (which should be obtained). Where there is no reference to the reaction yield involved, it is assumed to have been 100%. Example:

In a process of obtaining iron from hematite ore (Fe2O3), consider the unbalanced chemical equation:

Using 480g of the ore and assuming a yield of 80% in the reaction, the amount of iron produced will be:

Balanced Equation:

Data: 1Fe2O3 = 480g
2Fe = x (m) at 80% yield
MM Fe2O3 = 160g / mol
MM Fe = 56g / mol

x = 336g of Fe

Income calculation:

x = 268.8g of Fe