Ionic Equilibrium MCQ

Explanation: The problem asks to calculate the buffer capacity of a solution when a known amount of strong Base is added and a small pH change occurs. Buffers resist changes in pH, and their capacity depends on the concentrations of the Acid and conjugate Base. Buffer capacity is quantitatively defined as the amount of strong Acid or Base that changes the pH by one unit. To solve, first determine the change in pH and the amount of added Base. Then, relate these to the buffer capacity formula, which involves dividing the moles of Base added by the change in pH. This approach allows an understanding of how much the solution can neutralize added Acid or Base without significant pH shift. For analogy, think of a buffer as a shock absorber for pH; just like a car’s shock absorbs bumps to maintain smooth motion, the buffer absorbs added OH⁻ or H⁺ ions to maintain pH. This method ensures a clear calculation of buffer efficiency.

Explanation: The question asks to identify the Lewis Base among the given compounds. A Lewis Base is defined as an electron pair donor. To answer, examine the Molecular structures and electronic configurations of SnCl₄, AlCl₃, PF₃, and CO₂. Look for lone pairs that can be donated to an electron-deficient species. Compounds with available nonbonding electrons on central atoms typically act as Lewis Bases, whereas electron-deficient compounds often behave as Lewis Acids. By analyzing which species can provide a lone pair, one can determine the Lewis Base. An analogy: a Lewis base is like a person offering a gift (electron pair) to someone in need (electron-deficient species). This approach highlights the donor–acceptor concept central to Lewis Acid-base theory.

Explanation: This question focuses on calculating the pH of a solution of a Salt of a weak Acid. Sodium acetate is the conjugate base of acetic Acid and will hydrolyze in water to produce OH⁻ ions, increasing the pH. The pH can be estimated using the relationship between Ka of the Acid, concentration of the Salt, and the hydrolysis equilibrium. The hydrolysis constant Kh is derived from Kw and Ka. Using this, the hydroxide ion concentration is calculated, and from that, the pH is determined. For analogy, consider sodium acetate as a sponge that absorbs H⁺ ions from water, thereby making the solution basic. This allows prediction of the pH based on the Acid–base equilibrium.

Explanation: This problem examines the reaction of a highly reactive metal with water. Sodium reacts vigorously with water to produce sodium hydroxide and hydrogen gas. The hydroxide ions released increase the pH, making the solution basic. First, determine moles of sodium reacted by dividing Mass by molar Mass. Then, calculate the resulting OH⁻ concentration in the solution volume. Finally, use the pOH to estimate the pH. Analogy: adding sodium to water is like releasing a large amount of base into a neutral solution, shifting the pH significantly. This shows how small amounts of reactive Metals can dramatically change solution properties.

Explanation: The question concerns a basic buffer solution and asks for the ratio of base to Salt concentrations. In a buffer, the relationship between pH, pKa (or pKb for Bases), and concentrations is given by the Henderson–Hasselbalch equation. For basic buffers, pOH and pKb can be used similarly. The sum of pH and pKb provides a relationship that allows the ratio of base to its conjugate Acid (Salt) to be calculated using logarithms. Analogy: the buffer ratio is like a balance; knowing the total “weight” of acidity and basicity lets you determine how much of each component is present to maintain the desired pH.

Explanation: This question examines how the ratio of a weak acid to its conjugate base affects pH. Using the Henderson–Hasselbalch equation, pH depends on pKa and the logarithm of the base-to-acid ratio. A high acid-to-Salt ratio lowers the pH because the acid predominates, while a higher base-to-acid ratio raises it. Stepwise, take the log of the ratio and adjust the pH relative to the acid’s pKa. Analogy: the solution’s pH is like a seesaw, tilted toward acidity or basicity depending on the proportion of acid and conjugate base.

Explanation: The question asks to identify the most acidic solution. pH depends on hydrogen ion concentration; strong Acids like HCl fully dissociate, giving higher H⁺ concentration, while weak Acids or dilute solutions produce fewer H⁺ ions. Compare the molarities and strengths of each option: strong Acids at higher concentration have the lowest pH, whereas strong Bases or dilute Acids have higher pH. Analogy: pH is like a thermometer for acidity; the more H⁺ ions present, the “hotter” (lower) the pH reading.

Explanation: The problem asks for hydrogen ion concentration given the molarity and percent ionization of a weak acid. First, calculate the moles of H⁺ produced by multiplying initial concentration by ionization fraction. Since it’s monoprotic, each acid Molecule produces one H⁺ ion. Then, divide by solution volume if needed to obtain the concentration. Analogy: this is like taking a batch of molecules and seeing what fraction “breaks up” to release protons into solution, determining its acidity.