Ranking acidity practice problems, with ARIO and pKa

The one idea behind every ranking problem

For similar acids, stabilizing the conjugate base relative to its acid makes the acid stronger.

An acid gives up H⁺ and keeps the electrons from that bond. If the leftover base is comfortable holding those electrons, the proton leaves easily. If it isn't, the proton stays put.

pKa puts a number on this. Lower pKa means a stronger acid, and each 1 unit lower means about 10 times stronger. So acetic acid (4.8) is about 160 billion times stronger than ethanol (16), meaning its Ka is that much larger under comparable conditions. This is not a ratio of pH values.

ARIO: four questions, in order

Start with the conjugate base. Check all four factors before deciding: a strong later effect can overturn an earlier one. These trends work best when the other features and solvent are similar.

• AAtom

  Which atom holds the charge? Across a row, more electronegative wins: F > O > N > C. Down a group, bigger wins: I > Br > Cl > F, and S beats O.

• RResonance

  Can the charge spread out? Resonance can stabilize the conjugate base. Contributors that keep negative charge on oxygen usually matter more than those that put it on carbon; the number of drawings alone does not decide acidity.

• IInduction

  Is something pulling on the charge through the bonds? Electronegative atoms like F or Cl help. More of them help more, closer helps more, and more electronegative helps more.

• OOrbital

  What orbital holds the charge? More s character keeps electrons closer to the nucleus: sp > sp² > sp³.

How to rank acidity in 4 steps

1. Find the proton. If a molecule has more than one kind of H, the one that matters is the one that leaves the best conjugate base. Start with O–H, S–H, and halogen–H, then N–H, then C–H.
2. Draw the conjugate bases. Remove H⁺ and leave the electrons behind. Put the charge on the atom that lost the proton.
3. Run ARIO on each pair. Go letter by letter, then check whether a later factor overturns your first guess. The exceptions below show why this matters.
4. Sanity-check with pKa. Compare your order with the benchmarks in the table below. If an alkyne came out ahead of a carboxylic acid, go back and look again.

Worked examples, one for each letter

When the ARIO order gets flipped

ARIO is a priority list, not a law. These four pairs show why you should check more than the first letter. A big enough effect further down the list can beat a small one higher up.

• CH₃COOH vs CH₃SH

  First guess: S is bigger, so CH₃SH should win.

  What really happens: CH₃COOH wins, 4.8 vs 10.3. Sharing charge over two oxygens beats a big atom.

• HC≡CH vs NH₃

  First guess: N beats C, so NH₃ should win.

  What really happens: HC≡CH wins, 25 vs 35. An sp orbital holds charge close to the nucleus.

• CH₃COCH₂COCH₃ vs CH₃CH₂OH

  First guess: O beats C, so ethanol should win.

  What really happens: The diketone wins, about 9 vs 16. Two C=O groups spread the charge onto two oxygens.

• CH₃CH₂OH vs (CH₃)₃COH

  First guess: Same atom and no resonance, so it should be a tie.

  What really happens: Ethanol wins, 16 vs 18. Bulky alkoxides are harder for solvent to surround. ARIO doesn't cover this one.

pKa table: the benchmarks to know

Use these as rough textbook benchmarks, not universal constants. pKa depends on solvent, temperature, and the convention used. Values for very weak acids can differ by several units across sources. For a quantitative comparison, use values from the same solvent and scale; use your class's table on exams.

Values for very strong and very weak acids are estimates. Water is shown as 15.7, the number OpenStax Organic Chemistry uses, but see the FAQ for why you may also see 14.

Mistakes that cost points

• Comparing the acids instead of the bases. Compare the conjugate base relative to its acid. A neutral acid gives an anion; a positively charged acid can give a neutral base.
• Using electronegativity down a group. Going down a group, size wins. HI beats HF.
• Reading pKa backwards. Lower is stronger. A pKa of −7 is a very strong acid, not a weak one.
• Counting resonance drawings. What counts is where the charge lands, not how many pictures you can draw.
• Mixing up acid and base rankings. The strongest acid has the weakest conjugate base. If the question asks for bases, flip the order.

Ranking practice: 9 sets

Move each acid with its up and down buttons until the strongest is at the top, then check your order. If it's wrong, you'll get a hint about one acid to rethink. The pKa values appear when you get it right.

Which way does it go?

Ranking pays off here. Acid-base reactions favor the side with the weaker acid, because H⁺ goes from the stronger acid to the stronger base. Compare the pKa of the acid on each side. The equilibrium constant is about 10 raised to the difference: pKa of the product acid minus pKa of the starting acid.

This tool estimates the formal proton-transfer equilibrium from the displayed textbook numbers. It does not predict reaction speed, yield, or competing reactions such as organolithium addition to a carbonyl. Strong bases such as NaNH₂ and n-BuLi require suitable nonaqueous conditions. For a precise K, use a consistent solvent and pKa scale.

Example: acetic acid (4.8) reacts with bicarbonate to give acetate, CO₂, and water. The familiar 6.4 value is the apparent pKa of dissolved CO₂ plus H₂CO₃, not pure H₂CO₃, whose pKa is about 3.5. Including CO₂ formation gives an apparent K of 10^(6.4 − 4.8), about 40; escaping CO₂ drives the reaction further. Ordinary phenol (9.9) has K about 0.0003 with bicarbonate, so it does not give the same bubbling test. More acidic substituted phenols can behave differently.

More questions: 8 quick checks

Frequently asked questions

What does ARIO stand for in organic chemistry?

ARIO stands for Atom, Resonance, Induction, and Orbital. They are four useful checks for conjugate-base stability, not an absolute priority order. Solvation and the stability of the starting acid also matter.

Is ARIO the same as CARDIO?

They are closely related. CARDIO adds Charge and spells out induction as Dipole Induction: Charge, Atom, Resonance, Dipole Induction, Orbitals. For otherwise similar structures, a positively charged acid is usually more acidic than its neutral counterpart. Use your class's mnemonic and check all the factors.

Is a lower pKa a stronger acid?

Yes. A lower pKa means a stronger acid, and each 1 unit drop means about 10 times stronger. An acid with pKa 5 is about 100,000 times stronger than one with pKa 10.

How do you rank acidity without a pKa table?

Pull the proton off each acid, draw the conjugate bases, and compare them with ARIO. Find the atom holding the negative charge first, then look for resonance, then nearby electronegative groups, then the hybridization of the atom holding the charge.

How do you find the most acidic proton in a molecule?

Go through each kind of hydrogen, remove it on paper, and ask which negative charge would be happiest. O–H, S–H, and halogen–H protons usually beat N–H, and N–H usually beats C–H, but strong resonance or an sp carbon can overturn that first guess. A CH₂ between two C=O groups is a common exception, with a pKa near 9.

Why does my orgo book say water has a pKa of 15.7 when my gen chem book says 14?

Older organic textbooks list 15.7 because of how the concentration of water was handled in the math. Measured values and newer tables use 14.0. This guide uses 15.7 because OpenStax Organic Chemistry and many orgo classes still do. This can change close rankings, such as water versus methanol (15.5). Use one consistent convention and your class's table; do not mix scales in an equilibrium calculation.

Do I need to memorize a pKa table for organic chemistry?

A short set of rough benchmarks is useful, such as 5 for carboxylic acids, 10 for phenols, 16 for alcohols, 25 for terminal alkynes, and 50 for alkanes. Check what your professor provides, but benchmarks make ranking and predicting reactions much faster.