Use the Ka values in Table 15.6 to calculate the pH of the following solutions. (a) 0.33 M HNO2

step 1 For this problem, we are asked to find the pH of various solutions. The K -A and KB values are g

Use the Ka values in Table 15.6 to calculate the pH of the...

Use the $K_{\mathrm{a}}$ values in Table 15.6 to calculate the $\mathrm{pH}$ of the following solutions.
(a) $0.33 \mathrm{M} \mathrm{HNO}_{2}$
(b) $0.016 M$ phenol, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}$
(c) $0.25 \mathrm{M} \mathrm{HF}$
(d) $0.010 \mathrm{M} \mathrm{HCOOH}$

Key Concepts

Acid Dissociation Constant (Ka)

Ka is a measure of the strength of an acid in solution, representing the equilibrium constant for the dissociation of a proton from the acid. It quantifies the tendency of the acid to donate its proton to water, forming hydronium ions. A higher Ka indicates a stronger acid, while a lower Ka is characteristic of a weaker acid.

pH and Its Calculation

pH is a logarithmic scale used to specify the acidity or basicity of an aqueous solution, defined as the negative logarithm of the hydronium ion concentration. Calculating the pH of a solution requires determining the concentration of H+ ions present at equilibrium, which is influenced by the acid's dissociation constant and its initial concentration.

Equilibrium Calculations for Weak Acids

In the context of weak acids, the acid does not completely dissociate in solution. Instead, an equilibrium is established between the undissociated acid and its dissociation products. To calculate the pH, one usually sets up an equilibrium expression using the Ka value along with an ICE (Initial, Change, Equilibrium) table to solve for the concentration of hydronium ions.

Approximations in Weak Acid Equilibria

When dealing with weak acid equilibria, it is common to use approximations to simplify calculations. If the degree of dissociation is very small compared to the initial concentration of the acid, the change in concentration can be considered negligible. This 'small x' approximation greatly simplifies solving the equilibrium expressions, enabling one to approximate the hydronium ion concentration without solving complex quadratic equations.

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