What is a concentrated acid?

• A. A solution that contains a large number of H+ ions
• B. A solution with a high pH
• C. A solution with a low density
• D. A solution with many water molecules

Answer: (A)

Concentrated acid means there is a large amount of acid particles in a given volume of solution. In an acid solution, the H+ ions carried by the acid are what determine its concentration. So a solution with a large number of H+ ions per unit volume has a high acid concentration, which is exactly what this option describes. The pH is related—stronger or more concentrated acids tend to have lower pH—but pH itself is a measure of how acidic or basic the solution is, not a direct measure of how much acid is present. A high pH would indicate a basic solution, not a concentrated acid. Density and the amount of water molecules aren’t what define concentration here; adding water dilutes the acid and lowers its concentration. So the best description is a solution that contains many H+ ions.