--- title: "Weak Acids, Ka and pKa" description: "A weak acid only partly ionises, so its hydrogen ion concentration has to come from an equilibrium constant. Ka measures how far the ionisation goes, and pKa puts that on the same kind of scale as pH." canonical: https://lightmysky.com/learn/science/weak-acids-ka-and-pka-mt_q7fYb7qzMH source: https://lightmysky.com/learn/science/weak-acids-ka-and-pka-mt_q7fYb7qzMH.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Weak Acids, Ka and pKa A weak acid only partly ionises, so its hydrogen ion concentration has to come from an equilibrium constant. Ka measures how far the ionisation goes, and pKa puts that on the same kind of scale as pH. Subject: Science · Area: Chemistry · Ages 17 to 18 Page: https://lightmysky.com/learn/science/weak-acids-ka-and-pka-mt_q7fYb7qzMH ## Ready when they can - Writes the Ka expression for a named weak acid - Calculates the pH of a weak acid solution and names the two approximations the calculation makes - Converts between Ka and pKa and ranks acids by strength from either - Explains why a weak acid and a strong acid at the same concentration differ in pH yet need the same volume of alkali to neutralise ## Lesson: Weak acids, Ka and pKa Vinegar holds ethanoic acid and lemon holds citric acid. Both are weak acids that only partly split into ions. For a weak acid HA splitting into H+ and A-, the equilibrium constant is Ka = [H+][A-] over [HA]. At the same concentration and temperature, a bigger Ka means a bigger share has split, so bigger Ka means a stronger weak acid. **Example.** Take ethanoic acid, the acid in vinegar, as a 0.10 mol per cubic decimetre solution with Ka 1.7 x 10-5. The working gives [H+] near 1.3 x 10-3, so the pH is about 2.9. It leans on two quiet guesses: the water gives almost no H+ next to the acid, and 0.10 stays near 0.10 since so little splits. pKa = -log Ka puts strength on a pH-like scale: lower pKa wins, and each single unit is a factor of ten in Ka. Ranking by pKa and ranking by Ka always agree. Take equal volumes of 0.10 mol per cubic decimetre ethanoic acid and 0.10 mol per cubic decimetre hydrochloric acid. The strong acid shows a lower pH because it is fully split. Yet both need the same volume of the same alkali to neutralise. A monoprotic acid can give one proton per molecule, so both flasks hold the same total neutralisable amount, and the titre cannot tell them apart. **Recap.** Ka measures the split, pKa rescales it, approximations unlock the pH, and titre counts total neutralisable amount, not strength. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [The Equilibrium Constant Kc](https://lightmysky.com/learn/science/the-equilibrium-constant-kc-mt_3G8b6Hyz3u) - [pH, Kw and Strong Acids and Bases](https://lightmysky.com/learn/science/ph-kw-and-strong-acids-and-bases-mt_KJ4yc25Z_l) ## Opens up - [pKa as a Predictive Tool in Organic Chemistry](https://lightmysky.com/learn/science/pka-as-a-predictive-tool-in-organic-chemistry-mt_fI03mNIC_m) - [Metal Aqua Ions: Acidity, Ligand Exchange and Precipitation](https://lightmysky.com/learn/science/metal-aqua-ions-acidity-ligand-exchange-and-precipitation-mt_jrsKwNq1Oi) - [Acid-Base Balance: The Bicarbonate System and Compensation](https://lightmysky.com/learn/science/acid-base-balance-the-bicarbonate-system-and-compensation-mt_LFLgZre62C) - [Carboxylic Acids, Esters and Hydrolysis](https://lightmysky.com/learn/science/carboxylic-acids-esters-and-hydrolysis-mt_QLxUs5cOH2) - [Buffers and Titration Curves](https://lightmysky.com/learn/science/buffers-and-titration-curves-mt_vevyEjNijM)