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Common Strong & Weak Acids/Bases cheat sheet - grade 9-12

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Chemistry Grade 9-12

Common Strong & Weak Acids/Bases Cheat Sheet

A printable reference covering strong acids, weak acids, strong bases, weak bases, dissociation, conjugates, and $K_a$ and $K_b$ for grades 9-12.

TopicsAcids and BasesChemical ReactionsMoleculesChemistryStudy Guide
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This cheat sheet covers the common strong and weak acids and bases students need to recognize in high school chemistry. It helps students predict whether a substance fully dissociates, partially ionizes, or establishes an equilibrium in water. Knowing these categories makes acid-base reactions, pH\mathrm{pH} calculations, titrations, and net ionic equations much easier to solve.

The main goal is to quickly identify strong species from memory and treat weak species using equilibrium ideas.

Strong acids and strong bases are written with one-way dissociation because they essentially ionize completely in water. Weak acids and weak bases are written with equilibrium arrows because only some particles react with water at a time. Acid strength is compared using KaK_a, base strength is compared using KbK_b, and larger values mean stronger ionization.

Conjugate acid-base pairs differ by one proton, written as H+\mathrm{H^+}.

Key Facts

  • The common strong acids are HCl\mathrm{HCl}, HBr\mathrm{HBr}, HI\mathrm{HI}, HNO3\mathrm{HNO_3}, HClO4\mathrm{HClO_4}, HClO3\mathrm{HClO_3}, and H2SO4\mathrm{H_2SO_4} for its first ionization.
  • A strong acid dissociates essentially completely in water, such as HCl(aq)→H+(aq)+Cl−(aq)\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}.
  • Common weak acids include HF\mathrm{HF}, CH3COOH\mathrm{CH_3COOH}, H2CO3\mathrm{H_2CO_3}, H3PO4\mathrm{H_3PO_4}, HCN\mathrm{HCN}, and H2S\mathrm{H_2S}.
  • A weak acid partially ionizes in water, such as CH3COOH(aq)⇌H+(aq)+CH3COO−(aq)\mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)}.
  • Strong bases include Group 11 hydroxides such as NaOH\mathrm{NaOH} and KOH\mathrm{KOH}, plus soluble Group 22 hydroxides such as Ca(OH)2\mathrm{Ca(OH)_2}, Sr(OH)2\mathrm{Sr(OH)_2}, and Ba(OH)2\mathrm{Ba(OH)_2}.
  • A strong base dissociates completely to produce hydroxide ions, such as Ba(OH)2(aq)→Ba2+(aq)+2OH−(aq)\mathrm{Ba(OH)_2(aq) \rightarrow Ba^{2+}(aq) + 2OH^-(aq)}.
  • Common weak bases include NH3\mathrm{NH_3}, amines such as CH3NH2\mathrm{CH_3NH_2}, and many anions that accept H+\mathrm{H^+} from water.
  • For a weak acid HA\mathrm{HA}, Ka=[H+][A−][HA]K_a = \frac{[\mathrm{H^+}][\mathrm{A^-}]}{[\mathrm{HA}]}, and for a weak base B\mathrm{B}, Kb=[BH+][OH−][B]K_b = \frac{[\mathrm{BH^+}][\mathrm{OH^-}]}{[\mathrm{B}]}.

Vocabulary

Strong acid
A strong acid is an acid that essentially completely ionizes in water to produce H+\mathrm{H^+} or H3O+\mathrm{H_3O^+}.
Weak acid
A weak acid is an acid that only partially ionizes in water and forms an equilibrium mixture of acid and ions.
Strong base
A strong base is a base that dissociates completely in water to produce OH−\mathrm{OH^-} ions.
Weak base
A weak base is a base that partially reacts with water to form OH−\mathrm{OH^-} and its conjugate acid.
Conjugate acid-base pair
A conjugate acid-base pair consists of two species that differ by exactly one proton, H+\mathrm{H^+}.
Ionization constant
An ionization constant such as KaK_a or KbK_b measures how strongly an acid or base forms ions at equilibrium.

Common Mistakes to Avoid

  • Calling every acid with hydrogen strong is wrong because many hydrogen-containing acids, such as HF\mathrm{HF} and CH3COOH\mathrm{CH_3COOH}, ionize only partially.
  • Treating NH3\mathrm{NH_3} as a strong base is wrong because it does not contain OH−\mathrm{OH^-} and only partially reacts with water to form NH4+\mathrm{NH_4^+} and OH−\mathrm{OH^-}.
  • Forgetting coefficients in strong base dissociation is wrong because Ca(OH)2\mathrm{Ca(OH)_2} produces 22 hydroxide ions per formula unit, so [OH−]=2[Ca(OH)2][\mathrm{OH^-}] = 2[\mathrm{Ca(OH)_2}] for complete dissociation.
  • Using a one-way arrow for a weak acid or weak base is wrong because weak species establish equilibrium and should be written with ⇌\rightleftharpoons.
  • Assuming a larger KaK_a means a weaker acid is wrong because a larger KaK_a means more products form and the acid ionizes more strongly.

Practice Questions

  1. 1 Classify each as a strong acid, weak acid, strong base, or weak base: HNO3\mathrm{HNO_3}, HF\mathrm{HF}, KOH\mathrm{KOH}, and NH3\mathrm{NH_3}.
  2. 2 What is [OH−][\mathrm{OH^-}] in a 0.150 M0.150\ \mathrm{M} solution of Ba(OH)2\mathrm{Ba(OH)_2} if it dissociates completely?
  3. 3 A 0.200 M0.200\ \mathrm{M} weak acid HA\mathrm{HA} has equilibrium concentrations [H+]=1.0×10−3 M[\mathrm{H^+}] = 1.0 \times 10^{-3}\ \mathrm{M}, [A−]=1.0×10−3 M[\mathrm{A^-}] = 1.0 \times 10^{-3}\ \mathrm{M}, and [HA]=0.199 M[\mathrm{HA}] = 0.199\ \mathrm{M}. Calculate Ka=[H+][A−][HA]K_a = \frac{[\mathrm{H^+}][\mathrm{A^-}]}{[\mathrm{HA}]}.
  4. 4 Explain why HCl\mathrm{HCl} is written with →\rightarrow in water but CH3COOH\mathrm{CH_3COOH} is written with ⇌\rightleftharpoons.

Understanding Common Strong & Weak Acids/Bases

Acid strength is not the same as acid concentration. Strength describes how readily an acid transfers protons to water. Concentration describes how much acid was dissolved in a certain volume.

A dilute solution of a strong acid can have a higher pH than a concentrated solution of a weak acid. This distinction prevents many calculation errors. The same idea applies to bases.

A concentrated ammonia solution may feel strongly irritating, even though ammonia is classified as a weak base. Its particles react only partly with water, but a large starting amount can still produce many hydroxide ions.

Water is an active part of most acid-base reactions. An acid does not simply release a free proton into water. A water molecule accepts the proton and forms hydronium.

When a base such as ammonia reacts, water gives up a proton, leaving hydroxide behind. This explains why acids and bases come in conjugate pairs. After an acid loses a proton, the remaining particle can accept one again.

After a base gains a proton, the new particle can donate one. Strong acids form very weak conjugate bases.

For example, chloride has almost no tendency to take a proton from water. Weak acids form conjugate bases with a more noticeable ability to react.

Some substances can donate more than one proton, but each step has its own strength. Sulfuric acid loses its first proton very readily. Its second proton is much less easily removed.

Phosphoric acid has three possible proton losses, and each later loss is weaker than the one before it. The remaining negative charge makes it harder to remove another positive proton. This matters in multi-step pH problems and in titrations.

Do not assume that every acidic hydrogen in a formula behaves the same way. A substance can be strong in its first step yet weak in a later step.

You meet these ideas in household products, food, pools, soil, medicines, and cleaning. Vinegar contains a weak acid, while many drain cleaners contain strong hydroxide bases that can damage skin and pipes if used carelessly. Carbonic acid forms when carbon dioxide dissolves in water.

It helps control the acidity of rain, blood, and carbonated drinks. In laboratory titrations, the shape of the pH change depends on whether the acid and base are strong or weak. Learn the short list of common strong species first.

Then practice writing particles that are actually present in water. For weak substances, keep the unreacted molecule in the picture because it is usually the largest amount at equilibrium.