Calculate the pH of Sr(OH)2 0.25M.
The correct answer and explanation is:
Correct Answer: pH = 13.70
Explanation:
Strontium hydroxide, written chemically as Sr(OH)₂, is a strong base. This means it completely dissociates in water. When Sr(OH)₂ dissolves in water, it separates into its constituent ions:
Sr(OH)₂ → Sr²⁺ + 2OH⁻
From the equation, each mole of Sr(OH)₂ yields 2 moles of OH⁻ ions. Given that the concentration of the Sr(OH)₂ solution is 0.25 M, this means the concentration of OH⁻ ions is:
[OH⁻] = 0.25 M × 2 = 0.50 M
To find the pH, first calculate the pOH using the formula:
pOH = -log[OH⁻]
Substitute the concentration:
pOH = -log(0.50) ≈ 0.30
Now, to find the pH, use the relationship between pH and pOH:
pH + pOH = 14
So:
pH = 14 – pOH = 14 – 0.30 = 13.70
This result shows the solution is highly basic, which aligns with expectations for a strong base like Sr(OH)₂ at this concentration. High pH values indicate low hydrogen ion concentration and high hydroxide ion concentration. Since Sr(OH)₂ contributes two hydroxide ions per formula unit, its effect on pH is greater than that of a base producing only one OH⁻ per molecule. Strong bases like this are fully dissociated, which simplifies pH calculation because the OH⁻ concentration directly comes from the stoichiometry of the dissociation reaction. In conclusion, for a 0.25 M Sr(OH)₂ solution, the pH is 13.70.