Acid-Base Chemistry and Reaction Calculations, Worked Through

From finding an unknown concentration to predicting a buffer's pH, these five calculations cover the core of introductory reaction chemistry.

Titration: finding an unknown concentration

Titration uses Ma × Va = Mb × Vb. If 25 mL of a 0.1 M titrant neutralizes 20 mL of an unknown base, the unknown concentration is (0.1 × 25) / 20 = 0.125 M — assuming a 1:1 reaction stoichiometry.

Henderson-Hasselbalch: predicting buffer pH

For an acetic acid buffer with pKa 4.76 and a conjugate base-to-acid ratio of 3 (0.3 M base to 0.1 M acid), pH = pKa + log₁₀(base/acid) = 4.76 + log₁₀(3) ≈ 5.237. When base and acid concentrations are equal, the log term is zero and buffer pH exactly equals pKa — the point of maximum buffering capacity.

Reaction rate: how fast concentration changes

Average reaction rate is the change in concentration divided by elapsed time. A reactant dropping from 0.5 M to 0.2 M over 10 seconds gives a rate of (0.2 − 0.5) / 10 = −0.03 M/s — negative because the reactant is being consumed.

Gas density: from molar mass to density at STP

Gas density at standard conditions is molar mass divided by 22.4 L/mol. CO₂, with a molar mass of 44 g/mol, has a density of 44 / 22.4 ≈ 1.964 g/L at STP.

Isotope-weighted atomic mass: why the periodic table shows decimals

Boron's atomic mass (10.81) comes from averaging its two isotopes weighted by natural abundance: (10.013 × 0.199) + (11.009 × 0.801) ≈ 10.801. Every non-integer atomic mass on the periodic table reflects this same weighted-average calculation across an element's naturally occurring isotopes.

Working through your own reaction chemistry

These five calculations show up constantly in general and analytical chemistry coursework. Check your work with the titration calculator, Henderson-Hasselbalch calculator, reaction rate calculator, gas density calculator, and average atomic mass calculator.