Molar concentration uses the whole solution volume
A solute is dissolved in a solvent; the solution includes them together. The mole, mol, measures amount of substance rather than mass in grams. Molar concentration is solute amount divided by total solution volume, expressed in mol/L.
The example has 0.2 mol in 1 L, giving 0.2/1 = 0.2 mol/L. With no solute lost, dilution to a total of 2 L gives 0.2/2 = 0.1 mol/L. The same amount is distributed through twice the volume. Do not replace total solution volume with the volume of added water.
Acid strength and concentration use different comparisons
An acid here transfers a proton to water. The resulting ions are part of what we describe as ionization. At comparable solvent, temperature and concentration, a strong acid ionizes almost completely, whereas a weak acid ionizes only partly.
Diluting a solution reduces the amount of acid per liter; it does not by itself change the acid species from strong to weak. A weak acid’s fraction ionized can itself vary with concentration, so that fraction also requires comparison conditions. Avoid treating “strong” as another word for “concentrated.”
Calculate pH under a dilute-solution approximation
In water, hydrogen ions are associated with water molecules. Using H⁺ as shorthand, an ideal dilute aqueous calculation approximates pH as −log₁₀[H⁺], where the concentration’s numerical value is expressed in mol/L. Do not apply that concentration approximation unconditionally to concentrated solutions.
For [H⁺] = 10⁻³, the logarithm is −3, giving pH ≈ 3. A tenfold concentration, 10⁻², gives pH ≈ 2. Thus a fall of one pH unit corresponds to about ten times the concentration in this approximation. Pure water’s neutral pH of about 7 includes the condition 25°C; neutrality does not mean pH 7 at every temperature.
Extension: a buffer reacts with a small addition
A conjugate base is what remains after an acid transfers a proton. In a weak-acid/conjugate-base buffer, the base reacts with a small addition of acid, and the weak acid reacts with a small addition of base. This limits the pH change relative to an unbuffered solution.
There is only a finite amount of material available to react. Adding sufficiently much acid or base exceeds the buffer’s capacity. Nor must a buffer have pH 7. Read the reacting components and the size of the addition rather than memorizing “a liquid that never changes.” Quantitative buffer calculations are a further topic.
Separate what the calculation establishes from what it does not
Dilution from 1 L to 2 L establishes half the molar concentration when the 0.2 mol amount is preserved. Those numbers alone do not establish the acid species, its extent of ionization or its pH. A concentration question and an acid-property question can require different information.
After answering, name the quantity you judged: total solution amount, an acid property or a hydrogen-ion quantity. Check that the conclusion does not silently become “higher concentration always means a strong acid” or “a weak acid must be neutral.”