In chemistry, the arithmetic is never what goes wrong.

Almost every lost mark is one of five setups

General chemistry looks like dozens of problem types. In practice the same handful of setups repeat with different nouns, and the errors repeat with them.

• The equation was never balanced — so the mole ratio is wrong, and every number after it is wrong in exactly the same proportion. Nothing downstream can warn you.
• Grams used as moles. Coefficients are ratios of moles, never of mass. Convert first, ratio second, convert back last.
• Limiting reagent assumed rather than tested. The smaller mass is not the limiting reagent. Divide each reactant's moles by its coefficient and compare those.
• Molarity treated as an amount. mol/L is a rate. It becomes an amount only after multiplying by the volume in litres — and the volume is usually given in mL.
• Sig figs applied to the wrong quantity. Counted numbers and coefficients are exact. Logarithmic quantities like pH count only the decimal places.

Work in units, not in numbers

The habit that makes chemistry problems self-checking is the same one nursing programmes drill for dosage: write every quantity as a fraction carrying its unit, arrange so the unwanted units cancel, and confirm the surviving unit is the one the question asked for.

If grams survive to the end of a question asking for litres of gas, the setup is invalid — and you know that before evaluating anything. A wrong setup still produces a confident number, which is why checking the answer alone catches so little.

Grams → moles → mole ratio → moles → grams. Nearly every stoichiometry question is that path with one or two extra hops. When a problem feels unfamiliar, find which hop it is asking you to add.

Where each topic actually trips people

Topic	The usual trap
Stoichiometry	Unbalanced equation; mass-to-mass without passing through moles
Limiting reagent	Comparing masses instead of moles-over-coefficient
Percent yield	Theoretical yield taken from the excess reactant
Molarity & dilution	mL not converted to L; M₁V₁ = M₂V₂ applied to a reaction rather than a dilution
Gas laws	Celsius not converted to kelvin; R chosen in units that do not match P and V
Thermochemistry	Sign convention on ΔH; mass of solution vs mass of solute in q = mcΔT
Equilibrium (ICE)	Change row not scaled by the coefficients; the −x approximation used where it is not valid
Acids & bases	pH of a weak acid computed as if it were strong; Kw forgotten when going from pOH to pH
Redox	Half-reactions balanced for atoms but not for charge

How to use a checker without skipping the learning

1. Do the whole problem on paper first, with units on every line. The line you would skip mentally is the line the error hides in.
2. Check the units cancel before evaluating.
3. Sanity-check the magnitude. A percent yield over 100%, a negative concentration, or 400 L of gas from a gram of solid means the setup is wrong, not the calculator.
4. Then photograph the working and get the first invalid line named. "Wrong" teaches nothing; "the mole ratio on line three came from an unbalanced equation" fixes a category.
5. Log the category, not the question. Your misses cluster into two or three setups. That cluster is your revision plan.

Being straight with you

This is a homework and exam-practice tool. It is not a laboratory reference, it is not a safety resource, and nothing it outputs should be used to plan, scale or carry out a real reaction. Check anything that touches a bench against your lab manual, your SDS sheets and your instructor.

Related: dosage calculations covers the same unit-cancelling habit where the stakes are clinical, and checking answers without an answer key covers the estimation habits that generalise past chemistry.