Solution:
Calculate the molarity of a solution:
Molarity = Moles of solute / L of solution
Moles of solute = Mass of solute / Molar mass of solute
or
Molarity = Mass of solute / (Molar mass of solute × L of solution)
The molar mass of Sr(OH)2 is 121.63 g/mol.
Hence,
Molarity of a solution = (2.00 g) / (121.63 g/mol × 0.25 L) = 0.06577 mol/L = 0.06577 M
We assume Sr(OH)2 is a strong base, such that
Sr(OH)2(aq) → Sr2+(aq) + 2OH−(aq)
and thus, it gives rise to 0.06577 × 2 = 0.13154 M OH−.
As a result,
pOH = −log[OH−] = −log(0.13154) = 0.88
At any temperature, pH + pOH = pKw,
and at 25∘C, pKw = 14.
Hence,
pH = 14 − 0.88 = 13.12
pH = 13.12
Answer: pH = 13.12 is expected.
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