What is the theoretical phenolphthalein alkalinity of a saturated solution of Ca(OH)2?
Answer (2)
The theoretical phenolphthalein alkalinity of a saturated solution of Ca(OH)2 is determined by its hydroxide ion concentration, which is approximately 0.2 M. This results in a high pH of about 13.30, indicating strong alkalinity. Therefore, the solution is considered highly alkaline due to its hydroxide concentration.
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To determine the theoretical phenolphthalein alkalinity of a saturated solution of calcium hydroxide, Ca(OH)_2, we need to understand a few concepts related to solubility and alkalinity assessment.
1. Understanding Saturated Solutions
A saturated solution of Ca(OH)_2 means that the solution contains the maximum amount of dissolved calcium hydroxide that can exist in equilibrium with undissolved Ca(OH)_2.
The solubility product constant (K_sp) for calcium hydroxide dictates this equilibrium:
K s p = [ C a 2 + ] [ O H − ] 2
2. Phenolphthalein Alkalinity
Phenolphthalein alkalinity measures the amount of hydroxide ions, [OH^-], that are present in the solution, capable of turning phenolphthalein from colorless to pink (which happens at a pH around 8.2 to 10).
In a solution of Ca(OH)_2:
Calcium hydroxide dissociates as: C a ( O H ) 2 ( s ) ⇌ C a 2 + ( a q ) + 2 O H − ( a q )
3. Calculating the Alkalinity
Assuming the solubility of Ca(OH)_2 in water at 25°C is roughly 0.165 g/L:
Convert the solubility from g/L to moles/L (mol/L):
Molecular weight of Ca(OH)_2 is approximately 74.1 g/mol.
Solubility in mol/L = 74.1 g/mol 0.165 g/L ≈ 2.23 × 1 0 − 3 mol/L.
Since each molecule of Ca(OH)_2 produces two OH^- ions:
[OH^-] = 2 × 2.23 \times 10^{-3} \text{ mol/L} \approx 4.46 \times 10^{-3} \text{ mol/L}.
Phenolphthalein alkalinity, for hydroxide ions:
Alkalinity = [ O H − ] = 4.46 × 1 0 − 3 mol/L .
Thus, the theoretical phenolphthalein alkalinity of a saturated solution of Ca(OH)_2 is approximately 4.46 \times 10^{-3} mol/L of OH^- ions, indicating its basic character in terms of [OH^-] concentration.
