In the autoionization of water, one water molecule transfers a proton to another, forming H3O+ and OH-.
Correct Answer
True
True. Water is amphiprotic, so one molecule can act as an acid and another as a base. The equilibrium lies far to the left, producing only 1.0 × 10^-7 M of each ion in pure water at 25 °C.
Category: Bronsted-Lowry Acids and Bases
Question 2
A solution with a pH of 4 has a hydronium ion concentration ten times greater than a solution with a pH of 3.
Correct Answer
False
False. Each unit of pH is a factor of ten in [H3O+], and a higher pH means less hydronium, so a pH 4 solution has one tenth the hydronium ion concentration of a pH 3 solution.
Category: pH and pOH
Question 3
Because Kw increases with temperature, the pH of pure water at 50 °C is below 7.
Correct Answer
True
True. The autoionization of water is endothermic, so Kw grows as temperature rises and both [H3O+] and [OH-] increase. Pure water is still neutral, since the two concentrations remain equal, but its pH is below 7.
Category: pH and pOH
Question 4
In a basic solution at 25 °C, the pOH is greater than 7.
Correct Answer
False
False. A basic solution has [OH-] greater than 1.0 × 10^-7 M, so its pOH is less than 7 and its pH is greater than 7.
Category: pH and pOH
Question 5
The stronger an acid is, the weaker its conjugate base is.
Correct Answer
True
True. Because Ka × Kb = Kw for a conjugate pair, a larger Ka means a smaller Kb. Strong acids such as HCl have conjugate bases, such as Cl-, that are too weak to affect pH.
Category: Acid and Base Strength
Question 6
A weak acid is an acid that is present at a low concentration in solution.
Correct Answer
False
False. Weak describes an acid that ionizes only partially in water, regardless of its concentration. A low concentration is described as dilute; a dilute solution of a strong acid is still fully ionized.
Category: Acid and Base Strength
Question 7
Diluting a solution of a weak acid generally increases its percent ionization.
Correct Answer
True
True. Ka stays constant, but at lower concentration a larger fraction of the acid molecules must ionize to satisfy the equilibrium expression, so percent ionization rises as the solution is diluted.
Category: Acid and Base Strength
Question 8
Hydrofluoric acid, HF, ionizes completely in water like the other hydrogen halides HCl, HBr and HI.
Correct Answer
False
False. HF is a weak acid that ionizes only partially in water. HCl, HBr and HI are strong acids.
Category: Acid and Base Strength
Question 9
In water, all strong acids appear equally strong because each one is converted completely to hydronium ion.
Correct Answer
True
True. This is the leveling effect: H3O+ is the strongest acid that can exist in water, so the differences among strong acids show up only in a less basic solvent.
Category: Acid and Base Strength
Question 10
Ammonia is a strong base because it reacts completely with water to form NH4+ and OH-.
Correct Answer
False
False. Ammonia is a weak base; only a small fraction of NH3 molecules accept a proton from water, so its Kb is small. Strong bases include soluble metal hydroxides such as NaOH and KOH.
Category: Acid and Base Strength
Question 11
A solution of ammonium chloride in water is acidic.
Correct Answer
True
True. NH4+ is the conjugate acid of the weak base ammonia and donates protons to water, forming H3O+. The chloride ion, the conjugate base of a strong acid, has no effect on pH.
Category: Hydrolysis of Salts
Question 12
A solution of potassium chloride in water is basic because the chloride ion accepts protons from water.
Correct Answer
False
False. Cl- is the conjugate base of the strong acid HCl and is too weak a base to accept protons from water, and K+ does not affect pH either, so the solution is neutral.
Category: Hydrolysis of Salts
Question 13
Sulfuric acid is a diprotic acid whose first ionization is strong.
Correct Answer
True
True. H2SO4 ionizes completely to H3O+ and HSO4-, and the HSO4- ion then ionizes only partially, with a much smaller ionization constant.
Category: Polyprotic Acids
Question 14
Phosphoric acid, H3PO4, is a diprotic acid.
Correct Answer
False
False. H3PO4 has three ionizable hydrogens and loses them in three steps, so it is a triprotic acid.
Category: Polyprotic Acids
Question 15
A buffer resists changes in pH when small amounts of strong acid or strong base are added to it.
Correct Answer
True
True. The weak acid in a buffer neutralizes added base, and its conjugate base neutralizes added acid, so the pH changes only slightly.
Category: Buffers
Question 16
The capacity of a buffer does not depend on the concentrations of its weak acid and conjugate base.
Correct Answer
False
False. Buffer capacity grows with the amounts of weak acid and conjugate base present; a more concentrated buffer can neutralize more added acid or base before its pH changes significantly.
Category: Buffers
Question 17
At the equivalence point of a strong acid-strong base titration at 25 °C, the pH is 7.00.
Correct Answer
True
True. The products are water and a salt whose ions do not react with water, such as NaCl, so the solution at the equivalence point is neutral.
Category: Titrations
Question 18
Phenolphthalein is pink in acidic solutions and colorless in basic solutions.
Correct Answer
False
False. Phenolphthalein is colorless in acidic and neutral solutions and turns pink in basic solutions, above about pH 8.
Category: Titrations
Question 19
An acid-base indicator is a weak acid or weak base whose conjugate forms have different colors.
Correct Answer
True
True. The ratio of the two forms depends on pH, so the color of the solution changes over a range of roughly 2 pH units centered near the indicator's pKa.
Category: Titrations
Question 20
A buffer is most effective when the pH is far from the pKa of its weak acid.
Correct Answer
False
False. A buffer works best when the pH is close to the pKa, where the weak acid and its conjugate base are present in similar amounts and both can neutralize additions.
Category: Buffers