Stoichiometry problems may be concerned with relating the masses of two substances that have a common atom. 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 interprete 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. In each problem you will create a plan as shown below before entering any numbers.
Identify the Known substance.
Assign symbols to the given data and to the unknown quantity.
Connect the Known and Unknown creating a plan in which any intermediate quantities are calculated.
Carry out the calculation
Example: Butane gas (C4H10) is commonly used as a fuel. During the combustion the carbon in butane is converted to CO2.
What mass of CO2 arises from 100 g butane?
What mass of CO2 arises from 100 g butane?
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. 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. In each problem you will 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).
The known above is C4H10 .
Identify the Unknown substance.The amount in moles can be calculated from the mass (m) of a pure substance using the molar mass (M).
| n = | m |
| M |
The known above is C4H10 .
The unknown above is CO2.
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.
Masses are given above so these are m(CO2) and m(C4H10).
Masses are given above so these are m(CO2) and m(C4H10).
| n(AxBy) | = | n(A) |
| 1 | x |
If the compounds (AxBy) contain a common atom, their amounts in moles can be related through the common atom using two relationships of the type shown at the right.
Note that the number in the denominator of the fraction containing the polyatomic substance is always 1.
Identify the mathematical relationships required to do each step of the calculation.Note that the number in the denominator of the fraction containing the polyatomic substance is always 1.
| Plan for problem above: | Known C4H10 | Unknown CO2 | ||||||||||||||||
| m(C4H10) | uses M(C4H10) | n(C4H10) |
| n(C) |
| n(CO2) | uses M(CO2) | m(CO2) | ||||||||||
The relationships (or rearranged forms of these) shown on this page are the most common for stoichiometry calculations.
Carry out the calculation