Stoichiometry problems are commonly about two substances linked by a chemical equation in which they both appear. You may, for example, be asked to calculate the mass of product produced from a given amount of reactant as in the example.
Example: The first step in isolating copper from its ore CuS is the reaction with oxygen to produce copper oxide.
The air pollutant SO2 is also produced in this reaction. What mass of SO2 is produced from 100 g of CuS?
CuS(s) + O2(g)
CuO + SO2(g)
This involves using mathematical relationships between mass, molar mass and amount in moles. It is important to develop general approaches to problem-solving that help you analyse and interprete the words of any problem in terms of mathematical relationships that you know.
As described earlier, it is useful to construct a plan for solving the problem. The plan for solving the example problem is shown below. This is very similar to the plan described earlier for linking the masses of a compound and the atoms which the compound contains EXCEPT that the numbers used in the denominators of the fractions relating the two substances are now the coefficients on those substances in the balanced equation (not the subscripts on the atoms in the compound's formula).
Stoichiometry problems usually involve relating amounts in moles (n) of two substances.
Consider the example where substance A and B react according to aA + bB cC + dD
Example: The first step in isolating copper from its ore CuS is the reaction with oxygen to produce copper oxide.
The air pollutant SO2 is also produced in this reaction. What mass of SO2 is produced from 100 g of CuS?
CuS(s) + O2(g)
CuO + SO2(g)This involves using mathematical relationships between mass, molar mass and amount in moles. It is important to develop general approaches to problem-solving that help you analyse and interprete the words of any problem in terms of mathematical relationships that you know.
As described earlier, it is useful to construct a plan for solving the problem. The plan for solving the example problem is shown below. This is very similar to the plan described earlier for linking the masses of a compound and the atoms which the compound contains EXCEPT that the numbers used in the denominators of the fractions relating the two substances are now the coefficients on those substances in the balanced equation (not the subscripts on the atoms in the compound's formula).
Stoichiometry problems usually involve relating amounts in moles (n) of two substances.Consider the example where substance A and B react according to aA + bB
cC + dD| General plan: | Known CuS | connecting the masses using amounts in moles | Unknown SO2 | ||||||||
| m(CuS) mass of S | uses M(CuS)  | n(CuS) | uses equation for reaction
| n(SO2) | uses M(SO2) m = nM | m(SO2) mass of SO2 | |||||