CHM 1220L Experiment 28: pH of Acid, Base, Salt, and Buffer Solutions

Comprehensive vocabulary flashcards covering chemical equilibria of water, strong/weak acids and bases, salt hydrolysis, dilution calculations, pH meter operation and calibration, buffer mechanism, buffer swamping, and experimental observations for CHM 1220L Lab 28.

Amphiprotic Substance

A substance that can either donate or accept a proton ($H^+$), such as water ($H_2O$).

Autoionization of Water

The equilibrium reaction in which one water molecule donates a proton to another water molecule: $H_2O(l) + H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$.

Ion-Product Constant of Water ($K_w$)

The equilibrium constant for the self-ionization of water, given by $K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$ at $25\text{ }^\text{o}\text{C}$, which leads to the relationship $pH + pOH = 14.00$.

Brønsted-Lowry Acid

A chemical species (molecule or ion) that donates a proton ($H^+$) in a reaction.

Brønsted-Lowry Base

A chemical species (molecule or ion) that accepts a proton ($H^+$) in a reaction.

Acid Ionization Constant ($K_a$)

A quantitative measure of the extent of ionization of an acid in water, defined by the equilibrium expression $K_a = \frac{[H^+][A^-]}{[HA]}$.

Strong Acid Equilibrium

An acid condition characterized by $K_a \gg 1$, where the acid is essentially $100\%$ ionized in aqueous solution, resulting in $[HA]_{eq} = 0$ and $[H_3O^+]_{eq} = [A^-]_{eq} = [HA]_{initial}$.

Weak Acid Equilibrium

An acid condition where $K_a < 1$ and the acid partially ionizes; when $K_a \ll 1$, the extent of ionization is so small that $[HA]_{eq} \approx [HA]_{initial}$.

Base Ionization Constant ($K_b$)

The equilibrium constant measuring the extent of ionization of a base reacting with water, represented as $K_b = \frac{[BH^+][OH^-]}{[B]}$.

Strong Base Equilibrium

A base condition where $K_b \gg 1$, resulting in virtually $100\%$ ionization such that $[B]_{eq} = 0$ and $[OH^-]_{eq} = [B]_{initial}$, allowing $pOH = -\log[B]_{initial}$.

Weak Base Equilibrium

A base condition where $K_b < 1$ and partial ionization occurs; when $K_b \ll 1$, it is assumed that $[B]_{eq} \approx [B]_{initial}$.

Ammonium Ion Hydrolysis

The reaction of the $NH_4^+$ cation with water acting as an acid ($K_a$): $NH_4^+(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + NH_3(aq)$, which causes salts such as $NH_4Cl$ to produce acidic aqueous solutions.

Weak Acid Anion Hydrolysis

The reaction where the conjugate base anion of a weak acid accepts a proton from water ($K_b$): $A^-(aq) + H_2O(l) \rightleftharpoons HA(aq) + OH^-(aq)$, producing a basic aqueous solution (such as with $CN^-$ in $NaCN$).

Conjugate Acid-Base Pair Relationship

The mathematical relationship linking the dissociation constants of any conjugate acid-base pair in water: $K_a K_b = K_w$.

Dilution Relationship

The formula $M_1V_1 = M_2V_2$, used to calculate the preliminary molarity ($M_2$) of a reagent after mixing solutions, which serves as the initial concentration for subsequent equilibrium calculations.

pH Meter Electrode Care

Handling the fragile glass bulb tip by keeping it submerged in a $600\,\text{mL}$ beaker of deionized (DI) water when not in use, immersing it approximately $3/4\,\text{inch}$ (to the marked horizontal line) into samples, and rinsing with DI water between readings.

pH Meter Calibration

The procedure of warming up the meter for approximately $10\,\text{minutes}$ and sequentially immersing the electrode into two standard buffer solutions ($pH = 4.00$ and $pH = 7.00$) to establish accurate readings.

Laboratory Waste Segregation

The experimental protocol requiring acidic waste to be kept strictly separate from basic waste by discarding them into separate, designated acid or base waste containers.

Buffer Solution

A solution prepared by combining a weak acid ($HA$) with a salt of its conjugate base ($NaA$), or a weak base ($B$) with a salt of its conjugate acid ($BHX$), which resists changes in pH upon addition of small amounts of acid or base.

Buffer Acidity Equation

The expression derived from the weak acid equilibrium used to determine the hydronium ion concentration in an acid/salt buffer: $[H_3O^+] = \frac{K_a [HA]}{[A^-]} = \frac{K_a [acid]}{[salt]}$.

Buffer Neutralization of Added Acid

The stoichiometric absorption of hydronium ions by the buffer's conjugate base: $H^+(aq) + A^-(aq) \rightarrow HA(aq)$, shifting concentrations to $[HA]_{new\,eq} = [HA]_{initial} + [H^+]_{added}$ and $[A^-]_{new\,eq} = [A^-]_{initial} - [H^+]_{added}$.

Buffer Neutralization of Added Base

The stoichiometric absorption of hydroxide ions by the buffer's weak acid: $OH^-(aq) + HA(aq) \rightarrow A^-(aq) + H_2O(l)$, shifting concentrations to $[HA]_{new\,eq} = [HA]_{initial} - [OH^-]_{added}$ and $[A^-]_{new\,eq} = [A^-]_{initial} + [OH^-]_{added}$.

Buffer Swamping

The destruction of a buffer's ability to resist pH change caused by adding an excess of strong acid ($[H^+]_{added} > [A^-]_{initial}$) or strong base ($[OH^-]_{added} > [HA]_{initial}$), forcing the equilibrium entirely to one side.

Potassium Chloride ($KCl$) Aqueous Behavior

A neutral salt solution in water because neither the potassium cation ($K^+$) nor the chloride anion ($Cl^-$) interacts with or hydrolyzes water molecules (net-ionic equation: NO).

Formic Acid/Sodium Formate ($HCO_2H + NaHCO_2$) System

A classic buffer solution formed by mixing equimolar amounts of the weak acid $HCO_2H$ and its conjugate base from $NaHCO_2$, effectively resisting shifts in pH.

Dilution Effect on Strong vs. Weak Acids

The principle that diluting a strong acid ($HCl$) causes a larger change in pH than diluting an equimolar weak acid ($HC_2H_3O_2$), because weak acids shift equilibrium to increase their percent ionization upon dilution, buffering against larger pH shifts.

Buffer Resistance vs. Water Response

The experimental contrast showing that adding strong acid ($HCl$) or strong base ($NaOH$) to unbuffered water produces drastic pH shifts, whereas adding identical amounts to an acetic acid/acetate buffer produces minimal pH changes.