Applying the basic laws of chemistry is not just reciting each law. The main task is reading the data: does it preserve total mass, lock the composition of one compound, compare two compounds made from the same elements, or use gas volumes under the same conditions?
OpenStax Chemistry 2e describes stoichiometric calculations as calculations that use factors from balanced equations to relate amounts of substances in Reaction Stoichiometry. So before calculating, identify the balanced equation, the substance being asked for, and the unit in the data.
Start from the type of data given.
If every substance in the system is counted, use conservation of mass.
If one pure compound is being analyzed, use constant composition.
If two different compounds are made from the same two elements, use multiple proportions.
If all compared substances are gases at the same temperature and pressure, use combining volumes.
Calcium oxide, CaO, can be checked with two different samples. The first sample has mass 2.8g and contains 0.8g of oxygen. The second sample has mass 3.5g and contains 1.0g of oxygen.
Water and hydrogen peroxide are both made from hydrogen and oxygen, but their formulas are different.
H2OandH2O2
With Ar(H)=1 and Ar(O)=16, the hydrogen mass in both compounds can be made the same: 2g.
H2OH2O2mO:2gH+16gO:2gH+32gO:16:32=1:2
Because the hydrogen mass is already the same, compare the oxygen mass. The ratio 1:2 is a simple whole-number ratio, matching the law of multiple proportions.
For gases at the same temperature and pressure, the coefficients in a balanced equation can be read directly as volume ratios. For example, ammonia decomposition:
2NH3(g)→3H2(g)+N2(g)
If 600mL of ammonia decomposes, then 2 parts equal 600mL and 1 part equals 300mL.
One problem can use more than one law. Product mass and leftover reactant use conservation of mass, but if the problem then asks for element composition in the same compound, that part uses constant composition. Do not force one law to answer every question.
Define the system boundary, then choose the part of the data that is actually being compared.
If the problem asks about total mass, define the system boundary first.
If it asks about one compound's composition, compare the masses of its elements.
If it compares two compounds from the same elements, hold one element fixed before comparing the other.
If it asks about gas volume, confirm that all gases are measured at the same temperature and pressure.
Methane burns according to this balanced equation.
CH4(g)+2O2(g)→CO2(g)+2H2O(g)
If 50mL of methane gas reacts completely and every gas is measured at the same temperature and pressure, what volume of O2 is needed and what volume of CO2 forms?
The coefficients give the volume ratio CH4:O2:CO2:H2O=1:2:1:2. Therefore:
VO2VCO2=2×50=100mL=1×50=50mL
The answer is 100mL of oxygen gas and 50mL of carbon dioxide gas. If the water product has condensed into liquid, its volume is not read directly from the gas ratio.
OpenStax Chemistry: Atoms First 2e summarizes definite and multiple proportions as mass patterns supporting the idea that atoms combine in fixed ratios in Early Ideas in Atomic Theory.
