Chemical Quantities at Home, at Work and in Industry

Chemistry with a calculator, everywhere

Every day, someone in your community is doing a mole calculation or a ratio calculation that keeps people safe: a pharmacist checking a child's dose, a water-treatment operator adjusting chlorine, a farmer working out fertilizer for a field, an engineer tuning a smokestack scrubber. When those numbers are right, nobody notices. When they are wrong, the consequences can be serious. This lesson looks at quantitative chemistry in the real world (expectations D1.1 and D1.2).

At home: medicines

Many medicine labels give doses by age or by body mass, because the right amount depends on the size of the body. Doctors and pharmacists often calculate doses in mg per kg of body mass, and some medicines have a maximum dose no matter how heavy the person is.

Liquid medicines also need a concentration: for example, a label may say a liquid contains 160 mg in every 5 mL. That is a ratio you can use exactly like a molar mass.

Important: the examples below use an imaginary "Medicine X" for practice only. Real doses must always come from the package, a pharmacist or a doctor, and should be measured with the dosing cup or syringe that comes with the medicine, not a kitchen spoon.

At home: cleaning products

Cleaning products are concentrated chemicals. Their labels give dilution ratios (such as "30 mL per litre of water") because the right concentration cleans well without wasting product, damaging surfaces or leaving harmful residues. More is not better.

Some cleaners must never be mixed, because they react to form toxic gases. Bleach (which contains sodium hypochlorite) reacts with ammonia to form chloramine gases, and with acids (such as some toilet-bowl cleaners or vinegar) to release chlorine gas. Both gases damage the lungs. This is why labels say "Do not mix with other chemicals".

Pollen counts and the air we breathe

Allergy forecasts report a pollen count: the number of pollen grains in one cubic metre of air, measured by collecting pollen on a sticky surface from a known volume of air and counting it under a microscope. It is a quantity per volume, just like the concentrations you will meet in Solutions and Solubility. People with allergies use the count to plan medicine and outdoor time; tree pollen is often a problem in spring, and ragweed in late summer.

In the environmental sector: scrubbers

Burning coal and refining some metals release sulfur dioxide, SO₂, which forms acid rain. Many plants remove it with a scrubber: the gas passes through a spray of limestone (CaCO₃) mixed with water. Engineers use stoichiometry to decide how much limestone to feed in: too little lets SO₂ escape; too much wastes limestone and money.

When accuracy fails: real consequences (D1.2)

• Agriculture: fertilizer rates are calculated from nitrogen and phosphorus percentages. Over-applying fertilizer wastes money and lets nutrients wash into rivers and lakes, where they feed algal blooms. Lake Erie has had large algal blooms linked largely to phosphorus running off farmland.
• Industry: a reactor fed the wrong ratio makes less product and leaves unreacted chemicals that must be recovered or treated as waste.
• Units: in July 1983, Air Canada Flight 143, flying from Montréal to Edmonton with a stop in Ottawa, was loaded with less than half the fuel it needed. The fuel load had been worked out with a conversion factor in pounds per litre (1.77) where kilograms per litre (about 0.80) was needed, during Canada's switch to metric units. Both engines stopped in flight, and the crew glided the plane to a landing at a former air force base in Gimli, Manitoba. Everyone on board survived. The plane became known as the "Gimli Glider".
• Medicine: a misplaced decimal point (0.5 mg vs 5 mg) is a tenfold error. Hospitals use double-checks, standard units and careful labelling to prevent this.

Common causes of quantitative errors: wrong or mixed units, rounding too early, using the wrong ratio, not checking purity, and not double-checking.