Ion concentration in solutions

Calculating pH using a strong base solution as an example

Let us examine the algorithm for calculating the pH of a potassium hydroxide ($KOH$) solution. Since this is a strong base, we assume that the hydroxide ion concentration is entirely equal to the initial concentration of the base itself.

Suppose the initial concentration $c(KOH)$ is $0.0020$ mol/L. Consequently, $c(OH^-)$ is also $0.0020$ mol/L.

To find the hydrogen ion concentration $c(H^+)$, it is necessary to use the ion-product constant of water ($K_w$), which is equal to $1 \cdot 10^{-14}$.

Step-by-step calculation:

1. We find $c(H^+)$ by dividing $K_w$ by $c(OH^⁻)$:

$\frac{1 \cdot 10^{-14}}{0.0020} = 5.0 \cdot 10^{-12}$ mol/L.

1. We calculate the pH value using the negative base-10 logarithm:

$pH = -\lg(5,0 \cdot 10^{-12}) = -(0,7 + (-12)) = 11,3$.

Assessment of the dilution effect If the resulting solution is diluted with an equal volume of pure water, the potassium hydroxide concentration will decrease by exactly half, to $0.0010$ mol/L.

Let us recalculate the parameters for the new conditions:

• The new $c(H^+) = \frac{1 \cdot 10^{-14}}{0.0010} = 1 \cdot 10^{-11}$ mol/L.
• The new value of $pH = -\lg(1 \cdot 10^{-11}) = -(-11) = 11$.

From this follows a crucial conclusion: when an alkali solution is diluted with water, the hydrogen ion concentration increases. This occurs because the concentration of hydroxide ions decreases.

Mechanism of ionic reactions

Chemical processes in solutions occur due to direct particle collisions. Since electrolytes form free ions upon dissolution, it is these ions that interact with other ions and molecular structures.

> A reaction is classified as ionic if ions take part in its elementary transformations.

A conventional molecular equation is merely a formal way of writing and does not reflect the real picture of what is happening, as it conceals the participation of charged particles. In reality, the formulas of soluble strong electrolytes always conceal ions due to their complete dissociation (for example, a sodium chloride solution is actually a mixture of $Na^+$ and $Cl^-$, and barium hydroxide consists of $Ba^{2+}$ and $OH^-$ ions).

A clear example is the interaction of metallic zinc with various acids, such as hydrochloric ($HCl$) or hydrobromic ($HBr$). The molecular equations of these processes look completely different. However, the true essence of the reaction is the same: zinc atoms interact with hydrogen cations $H^+$ (or hydronium $H_3O^+$). To see this true picture, we transition from molecular equations to ionic ones.

Algorithm for writing ionic equations

To correctly represent chemical processes, a strict sequence of four rules must be followed:

1. Choice of substance notation form. Only strong electrolytes that are in a dissolved state are permitted to be written as separate ions. All other participants—non-electrolytes, weak electrolytes, and strong electrolytes if they are in a crystalline (solid precipitate) or gaseous state—must be written strictly in molecular form.
2. Cancellation of inactive particles. To construct a net ionic equation, those ions that remained unchanged and did not actually participate in the chemical event must be excluded from the left and right sides.
3. Checking charge balance. This is a crucial self-check step: the algebraic sum of the electrical charges of all particles on the left side of the equation must strictly equal the total charge on the right side.
4. Using reference materials. When writing equations, one must constantly cross-reference lists of strong and weak electrolytes, as well as the solubility table of salts, acids, and bases.

Briefly: answers to topical questions

What else to study on this topic

• Solubility table of salts, acids, and bases
• Classification of strong and weak electrolytes
• The role of hydronium ions in aqueous solutions
• Calculation of the ion product of water at various temperatures
• Behavior of strong electrolytes in the crystal lattice

Other topics in "Chapter 6. Patterns of Chemical Reactions"

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