Aim for FiveChemistry

Short free-response question

How soluble is magnesium hydroxide?

  • Units 7 and 8
  • 4 points
  • About 9 minutes

You can use a calculator on this question, just like on exam day.

A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.

The question

Magnesium hydroxide, Mg(OH)₂, is the active ingredient in milk of magnesia. At 25 °C, Ksp for Mg(OH)₂ is 5.6 × 10⁻¹².

Suggested time: 9 minutes

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Part (a)

1 point

Write the Ksp expression for Mg(OH)₂, and calculate the molar solubility of Mg(OH)₂ in pure water at 25 °C.

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Part (b)

1 point

Calculate the pH of a saturated solution of Mg(OH)₂ at 25 °C.

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Part (c)

1 point

Calculate the molar solubility of Mg(OH)₂ in 0.10 M MgCl₂(aq) at 25 °C. Explain why the value differs from the one in part (a).

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Part (d)

1 point

Milk of magnesia is used to neutralize excess stomach acid. Will Mg(OH)₂ be more soluble, less soluble, or equally soluble in an acidic solution compared with pure water? Justify your answer.

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