Chemistry Practice Test: Acids, Bases and Solutions

An Arrhenius acid increases the concentration of H⁺ (hydronium, H₃O⁺) in water, as HCl does. Producing OH⁻ is what an Arrhenius base does.

A Brønsted-Lowry acid is a proton (H⁺) donor. Water gives a proton to ammonia and becomes OH⁻, so water is the acid. Ammonia accepts the proton, which makes it the base.

pH = −log[H⁺] = −log(1 × 10⁻³) = 3, an acidic solution. The value 11 is the pOH of this solution (14 − 3), not its pH.

Each pH unit is a factor of 10, and the solutions differ by 2 units, so the difference is 10 × 10 = 100. A lower pH means more H⁺, so the pH 4 solution has 100 times more, not less.

At 25 °C, pH + pOH = 14, so pOH = 14 − 9 = 5 and [OH⁻] = 1 × 10⁻⁵ M. The value 1 × 10⁻⁹ M is the hydrogen ion concentration of this solution.

An acid and a base neutralize each other to form a salt and water: HCl + NaOH → NaCl + H₂O. The H⁺ from the acid and the OH⁻ from the base combine to make water.

Moles of NaOH = 0.20 M × 0.0250 L = 0.0050 mol, so the acid also contains 0.0050 mol HCl. 0.0050 mol ÷ 0.0200 L = 0.25 M. Swapping the two volumes gives 0.16 M.

Molarity = moles of solute ÷ liters of solution = 0.50 mol ÷ 0.250 L = 2.0 M. Multiplying moles by volume instead of dividing gives 0.125 (with liters) or 125 (with milliliters).

M₁V₁ = M₂V₂: 6.0 M × 50.0 mL = 0.50 M × V₂, so V₂ = 300 ÷ 0.50 = 600 mL. The value 550 mL is the volume of water added, not the final volume.

Moles needed = 0.20 mol/L × 0.500 L = 0.10 mol, and 0.10 mol × 58.5 g/mol = 5.85 g. The answer 11.7 g is the mass for a full liter of solution.

Only the baking soda solution is basic (pH above 7), so only there does phenolphthalein turn pink. Phenolphthalein starts to change at about pH 8, so pure water at pH 7 stays colorless.

At 40 °C, 100 g of water can hold up to 64 g of KNO₃. With only 50 g dissolved, more could still dissolve, so the solution is unsaturated.

At 20 °C only 32 g stays dissolved in 100 g of water, so 50 − 32 = 18 g crystallizes. The answer 14 g is the extra amount that could still have dissolved at 40 °C (64 − 50), not what comes out on cooling.

Unlike most solid solutes, gases become less soluble as temperature increases, because faster-moving gas molecules escape from the liquid more easily. So warm soda loses its CO₂ more quickly.

HCl is a strong acid: nearly every molecule gives up its H⁺, so there are many ions and a low pH. Acetic acid is a weak acid; it dissolves well but only a small fraction of its molecules ionize. HCl actually has fewer hydrogen atoms per molecule than acetic acid, which has four.