MASS-MASS STOICHIOMETRY
We use the mass-mass stoichiometry when getting the unknown mass of a substance by using the given mass of the other substance in a chemical reaction.
For example, given the chemical equation FeO + Al --> Al2O3 + Fe, find the mass of the Aluminum if the mass of Iron (II) oxide is 14.4g.
Here are the following steps in solving this problem: (Ratio and Proportion)
1. Balance the equation.
FeO + 2Al --> Al2O3 + 3Fe
2. Get the formula mass of the substance with unknown mass and the substance with given mass.
Al=26.9g/mol and FeO=71.5 g/mol
3. Represent the missing mass using “x”. Then, construct two ratios – one is the unknown mass over the product of its formula mass and moles, the other is the given mass over the product of its formula mass and moles.
4. Calculate the mass of the unknown.
x g Al=3.60 g Al
* Using dimensional analysis:
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We will use the given mass of FeO and multiply in the reciprocal of its formula mass. Remember that the formula mass of a substance is equal to one mole of it.
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Then, we will multiply it to the ratio of the substances to cancel the unit of FeO mole.
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To cancel the unit ‘mol Al’ we will multiply the equation in the formula mass of Al over 1 mole.
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The only remaining unit is the unit of mass of the unknown. We can now solve the problem using the equation.
MASS-VOLUME STOICHIOMETRY
We use mass-volume stoichiometry to get the volume of the unknown substance using the given mass of another substance in a chemical reaction. Remember that every 1 mol of the unknown substance is equal to 22.4 L or 22400 mL at STP condition.
Given the equation Zn + HCl --> Zn Cl + H2, find the volume of hydrogen produced if there is 4.65 g of Zinc.
Ratio and Proportion:
1. Balance the equation.
Zn + 2 HCl --> Zn Cl + H2
2. Get the formula mass of the substance with given mass.
Zn=65.38 g/mol
3. Construct two ratios – one is the given mass of Zn over the product of its mole and formula mass, the other is the unknown volume of hydrogen over 22.4 L at STP condition.
4. Calculate the volume of the unknown.
x L H2=1.59 L H2
* Dimensional Analysis:
VOLUME-VOLUME STOICHIOMETRY
We use volume-volume soichiometry to find the unknown volume of a substance using the given volume of another substance in a chemical equation.
For example, given the equation SO2 + O2 --> SO3, find the volume of the oxygen that reacted with 37.5 L of SO2.
Ratio and Proportion:
1. Balance the equation first.
2 SO2 + O2 --> 2SO3
2. Multiply the given volume to the ratio of the moles of the two substances (use the unit L instead of mol) and cancel the units.
3. We now have the unknown volume of the substance.
