A solution of Ca(OH)2 has a measured pH of 11.60 . What is the molar concentration of the Ca(OH)

step 1 In this question, a solution of calcium hydroxide has a pH of 11 .60. We have to find the molar

A solution of Ca(OH)2 has a measured pH of 11.60 . What is...

A solution of $\mathrm{Ca}(\mathrm{OH})_{2}$ has a measured $\mathrm{pH}$ of 11.60 . What is the molar concentration of the $\mathrm{Ca}(\mathrm{OH})_{2}$ in the solution? What is the molar concentration of $\mathrm{Ca}(\mathrm{OH})_{2}$ if the solution is diluted so that the $\mathrm{pH}$ is $10.60 ?$

Key Concepts

Strong Base Dissociation

Ca(OH)2 is classified as a strong base, which means it dissociates completely in water into its constituent ions. This complete dissociation allows one to assume that the measured hydroxide ion concentration directly results from the base, with no significant contribution from other equilibria. Therefore, every mole of Ca(OH)2 produces two moles of hydroxide ions.

pH and pOH Relationship

pH is a measure of the hydrogen ion concentration in a solution, and pOH is a measure of the hydroxide ion concentration. In water at 25°C, these two are related by the equation pH + pOH = 14, allowing one to determine the hydroxide concentration from the measured pH value. This concept is essential when converting between pH and the concentration of basic species in solution.

Molar Concentration and Stoichiometry

When calculating the molar concentration of a compound like Ca(OH)2, it is important to consider the stoichiometry of its dissociation. Since each formula unit yields two hydroxide ions, the concentration of Ca(OH)2 is obtained by dividing the hydroxide ion concentration (determined from the pOH value) by two. This stoichiometric relationship is key to connecting the measured pH (via pOH) to the original salt concentration.

Dilution Effects

Dilution involves adding more solvent to a solution, which lowers the concentration of the dissolved species and consequently affects the pH. The relationship between the volume of the solution and the molarity of the solute is inversely proportional. When a solution is diluted, the concentration of hydroxide ions decreases, leading to a higher pOH and lower pH, illustrating the quantitative impact of dilution on the solution's basicity.

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