Chemistry guides
Short, practical lessons that explain how to approach common chemistry problems.
How to Calculate Molar Mass
Add the atomic masses of every atom in a chemical formula, accounting for subscripts and groups. For H₂O: 2 × 1.008 + 15.999 = 18.015 g/mol. Hydrates must include their waters of crystallization.
How to Calculate Molarity
Divide the amount of dissolved solute in moles by the final volume of solution in liters. For 0.50 mol in 2.0 L, M = 0.25 mol/L. Do not use the volume of solvent alone.
How to Balance Chemical Equations
Balance equations by changing coefficients in front of formulas, never subscripts within a formula. Check that each element has the same total atom count on both sides.
How to Calculate pH
Use pH = −log₁₀[H⁺] with concentration in mol/L. For [H⁺] = 1.0 × 10⁻³ mol/L, pH = 3. This relationship uses hydrogen ion activity approximately equal to concentration in dilute solutions.
How to Solve Stoichiometry Problems
Start with a balanced equation, convert the known quantity to moles, apply the coefficient ratio, then convert to the desired unit. A balanced reaction is essential.
How to Calculate Percent Yield
Divide the actual measured yield by the theoretical maximum yield, then multiply by 100%. Keep both yields in the same units. Values above 100% can indicate impurities or measurement error.
How to Use the Ideal Gas Law
Use PV = nRT with consistent units: P in atm, V in liters, n in moles and T in kelvin when R = 0.082057 L·atm·mol⁻¹·K⁻¹. Gas behavior departs from ideality at high pressure and low temperature.