Planning is important!
Stoichiometry problems are commonly about two substances involved in a chemical reaction. In one type you are asked to find the mass of one substance using information given about the second substance. These problems typically involve several steps where mathematical relationships between mass, molar mass and amount in moles are used.
It is important to develop general approaches to problem-solving that help you analyse and interpret the words of any problem in terms of mathematical relationships that you know A useful general approach for solving any problem is given below. You will be asked to use this approach in solving the problems in this module and to create a plan as shown below before entering any numbers.
Plan for example
Identify the mathematical relationships required to do each step of the calculation.
Carry out the calculation
Example problem: The first step in isolating copper from its ore CuS is 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)
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)It is important to develop general approaches to problem-solving that help you analyse and interpret the words of any problem in terms of mathematical relationships that you know A useful general approach for solving any problem is given below. You will be asked to use this approach in solving the problems in this module and to create a plan as shown below before entering any numbers.
| Identify the Known substance. Enough information is given to calculate the amount in moles (n) of the Known. The amount in moles can be calculated from the mass (m) of a pure substance using the molar mass (M). Known in example CuS. |  |
| Identify the Unknown substance. | |
| Assign symbols to the given data and to the unknown quantity. Be sure to include the substance to which each symbol refers, for example m(NaOH). This helps you to connect data to mathematical relationships. Symbols in example m(CuS) and m(SO2) | |
| Connect Known and Unknown by a plan to calculating intermediate quantities. 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 |  |
| Known substance A | Intermediate calculated values | Unknown substance B | ||||
| m(A) mass of A | uses M(A) molar mass A | n(A) moles of A | | n(B) moles of B | uses M(B) molar mass B | m(B) mass of B |
| Known CuS | Unknown SO2 | |||||
| m(CuS) | uses M(CuS) | n(CuS) | | n(SO2) | uses M(SO2) | m(SO2) |
Identify the mathematical relationships required to do each step of the calculation.
The relationships (or rearranged forms of these) shown on this page are the most common for stoichiometry calculations.
Carry out the calculation