Chemistry Formulas
A copy-able sheet — moles, gas laws, solutions, thermochemistry, equilibrium, acids, kinetics, and cells.
Showing 49 of 49. Click any formula to copy it.
Every temperature in these equations is absolute: convert °C to kelvin by adding 273.15 before substituting. The gas constant takes the value matching your pressure units — 0.08206 L·atm/(mol·K) for PV = nRT in atmospheres, 8.314 J/(mol·K) for energy work.
Almost every chemistry calculation passes through the mole. n = m ÷ M converts a mass to moles, the balanced equation converts moles of one substance to another, and n = m ÷ M runs backwards to give the answer as a mass. The sheet above groups the rest by topic.
The mole sits in the middle of everything
Balanced equations are written in moles, but nothing in a laboratory is measured in them — you weigh grams, pipette millilitres, read a pressure. So the shape of a stoichiometry problem is always the same three steps: convert what you measured into moles, use the coefficients of the balanced equation to cross to the substance you care about, then convert back into whatever unit the question wants.
Which conversion you use depends only on what you were given. A mass goes through n = m ÷ M. A gas volume goes through the ideal gas law. A solution goes through molarity, moles = M × V. All three arrive at the same place, and once you are in moles the rest of the problem is the ratio from the equation.
Enthalpy, entropy, and what actually decides a reaction
ΔH tells you whether a reaction releases or absorbs heat, but not whether it happens. ΔG = ΔH − TΔS is what decides: a reaction is spontaneous when ΔG is negative. Because temperature multiplies the entropy term, a reaction that is not spontaneous when cold can become so when hot — which is why the T in that formula must be in kelvin.
Choosing the conversion into moles
What you were given decides the route; the balanced equation then carries you to the substance you want.
| You are given | Use | To get |
|---|---|---|
| A mass in grams | n = m ÷ M | Moles of that substance |
| A gas volume, pressure, temperature | n = PV ÷ RT | Moles of gas |
| A solution volume and molarity | n = M × V | Moles of solute |
| A particle count | n = N ÷ Nₐ | Moles of particles |
| Moles of one reactant | coefficient ratio | Moles of any other species |
What the sign of ΔG means
ΔG = ΔH − TΔS, with T in kelvin. Spontaneous does not mean fast — that is kinetics, not thermodynamics.
| ΔH | ΔS | Spontaneous when |
|---|---|---|
| Negative (exothermic) | Positive | Always, at every temperature |
| Negative (exothermic) | Negative | At low temperature |
| Positive (endothermic) | Positive | At high temperature |
| Positive (endothermic) | Negative | Never, at any temperature |
The errors that ruin an otherwise correct answer
Temperature is the big one. Every gas law, the Gibbs equation, and the Arrhenius equation need absolute temperature, so °C must become kelvin by adding 273.15 before substituting. A gas law worked in Celsius does not fail loudly — it just returns a confidently wrong number.
The gas constant is the second. R takes different values in different unit systems: 0.08206 L·atm/(mol·K) for PV = nRT with pressure in atmospheres, and 8.314 J/(mol·K) when the answer is an energy, as in the Arrhenius and Nernst equations. Pick the one matching your units. Third, a limiting-reagent problem must be tested reagent by reagent — converting only the reactant named first in the question is the most common way to lose the whole calculation.