Stoichiometry
Stoichiometry is the process of using the principles of molar quantities to predict the theoretical amounts of reactants and products in a chemical reaction.
- Why – All chemical reactions involve real amounts of reactants and products. Based on knowing only one amount (one reactant or one product), the theoretical amounts of all other reactants and products can be determined.
- How – All stoichiometry is based on the mole ratios of the substances in a chemical reaction. A balanced chemical equation identifies the number of moles of each reactant and product in that chemical reaction. It is the coefficients of substances that determine the mole ratios of the substances.
- Real-world Connections:
- Stoichiometry might be used for finding the amount of pollutants produced in a specific chemical process such as those produced from driving a car.
- Stoichiometry might be used to predict the amount of a valuable product produced from a certain chemical reaction such as the recovery of silver from silver ore.
- Stoichiometry might be used to determine how much energy is produced from the chemical reactions that occur inside a battery.
Mole Ratios
- The coefficients indicate the number of moles of each substance in the reaction. This is why balancing chemical equations is so important
- It is the ratio based on the coefficients that are to be compared
Example:
2 H2 + O2 2 H2O
- Based on the balanced chemical equation, for every two moles of hydrogen gas and 1 mole of oxygen gas that are combined in the chemical reaction, 2 moles of water are produced.
Image used courtesy of CK-12 Foundation 2014
- The coefficients are 2, 1, 2 in this reaction
- There are two moles of hydrogen gas used for every 1 mole of oxygen gas used. This means that the mole ratio of hydrogen gas to oxygen gas is 2:1
- There are two moles of water produced for every 1 mole of oxygen gas used to produce the water. This means that the mole ratio of oxygen gas to water is 1:2
Mole to Mole Conversions (Mole-Mole Stoichiometry)
- The mole ratio from a balanced chemical equation to convert between moles of two different substances.
- The diagram below demonstrates the conversion process from moles of one substance to moles of a different substance in a chemical reaction.
Example:
According to the following equation, how many moles of water would be produced from 3.5 moles of oxygen gas?
2 H2 + O2 2 H2O
Known values:
- For every 1 mole of oxygen gas, 2 moles of water are produced (the mole ratio using the coefficients from the balanced chemical equation)
- There are 3.5 moles of oxygen gas that are completely consumed in the reaction.
Perform the conversion:
The known amount of one substance is placed first, the mole ratio is placed second so that the moles of the first substance will cancel out.
3.5 moles O2 2 moles H2O = 7.0 moles H2O
1 mole O2
Perform the math, (3.5 x 2)/1 = 7.0 moles. The units of the substance to be found are the units remaining, moles of water.
Visualizing Stoichiometry
Consider the following reaction:
3 Ca(NO3)2 + 2 K3PO4 → 6 KNO3 + Ca3(PO4)2
Given: 48.4 g Ca3(PO4)2 are produced
What mass of calcium nitrate must react with sufficient potassium phosphate to produce 48.4 g of calcium phosphate?
Step 1 – The mole ratio of calcium phosphate to calcium nitrate is 1 : 3
Step 2 – The number of moles of calcium phosphate is the mass of calcium phosphate divided by the molar mass of calcium phosphate.
Molar Mass of calcium phosphate = 310.174 g mol-1
Number of moles = 48.4 g/310.174 g mol-1 = 0.156 mole
Step 3 – Based on step 1, the reaction involves three times as many moles of calcium nitrate than the calcium phosphate; therefore the number of moles of calcium nitrate is 3 x 0.156 moles = 0.468 moles of calcium nitrate.
Step 4 – The question asked for the mass, not moles, of calcium nitrate: 0.468 mole x 164.086 g mol-1 = 76.8 g.