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Magnesium hydroxide (milk of magnesia)

Ks, common ion effect, Q vs Ks, dissolving in acid

Milk of magnesia is a suspension of Mg(OH)X2\ce{Mg(OH)2} used as an antacid. Ks(Mg(OH)X2)=5.60×10−12K_s(\ce{Mg(OH)2}) = 5.60\times10^{-12} at 25 °C. M(Mg(OH)X2)=58.3M(\ce{Mg(OH)2}) = 58.3 g mol−1^{-1}.
Facts you may need
  • Ks expressions
  • common ion effect
  • Q vs K
  • pH ↔ [OH⁻]

(a)

Write the KsK_s expression and calculate the solubility of Mg(OH)X2\ce{Mg(OH)2} in water (mol L−1^{-1} and g L−1^{-1}). Calculate the pH of the saturated solution.

(b)

Calculate the solubility of Mg(OH)X2\ce{Mg(OH)2} in 0.0500 mol L−1^{-1} NaOH and explain the difference from (a).

(c)

50.0 mL of 0.00200 mol L−1^{-1} MgClX2\ce{MgCl2} is mixed with 50.0 mL of a buffer of pH 10.00. Determine by calculation whether a precipitate forms.

(d)

Explain, with equations, why Mg(OH)X2\ce{Mg(OH)2} dissolves in stomach acid (HCl) and also dissolves in concentrated NHX4Cl\ce{NH4Cl} solution.