Chemistry Practice Test 2: Acids and Bases

A conjugate base is what remains after an acid gives away one H⁺, so H₂PO₄⁻ becomes HPO₄²⁻. H₃PO₄ is its conjugate acid, formed by adding H⁺.

NaOH is a strong base, so [OH⁻] = 0.010 M and pOH = 2. Then pH = 14 − 2 = 12; the answer 2 is the pOH, not the pH.

pH = −log(2.0 × 10⁻³) ≈ 2.70. The concentration is a little more than 1 × 10⁻³ M, so the pH must be a little less than 3, which rules out 3.3.

HCO₃⁻ can give away its H⁺ to form CO₃²⁻ or accept one to form H₂CO₃, so it is amphoteric. NH₄⁺ can only donate a proton, and Cl⁻ has no proton to donate.

A buffer needs a weak acid and its conjugate base, which can absorb added base or acid. HCl and HNO₃ are strong acids, so their salts cannot take up added H⁺.

An acid and a hydroxide base form a salt and water: HNO₃ + KOH → KNO₃ + H₂O. Hydrogen gas forms when an acid reacts with a metal such as zinc, not with a base.

Acetic acid is a weak acid: at equilibrium only a small fraction of its molecules donate H⁺. The two solutions have the same concentration, so the difference is in how much each ionizes.

A Lewis base donates a pair of electrons to form a bond, as NH₃ does when it bonds to BF₃. Accepting an electron pair is what a Lewis acid does.

The acid contains 0.0250 L × 0.10 M = 0.0025 mol, which needs 2 × 0.0025 = 0.0050 mol of NaOH, or 0.0050 ÷ 0.20 = 0.0250 L. 12.5 mL ignores that sulfuric acid gives two H⁺ ions.

Diluting 100 times lowers [H⁺] from 10⁻² M to 10⁻⁴ M, so the pH rises to 4. A pH of 1 would mean the solution became more acidic, which dilution cannot do.