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Chapter 18: Problem 51

Hydrogen phosphate \(\left(\mathrm{HPO}_{4}^{2-}\right)\) can be removed in water treatment by the addition of slaked lime, $\mathrm{Ca}(\mathrm{OH})_{2} .$ Write a balanced chemical equation for the reaction (using ions as reactant), in which \(\mathrm{Ca}_{5}(\mathrm{OH})\left(\mathrm{PO}_{4}\right)_{3}\) forms as a precipitate.

Short Answer

Expert verified
The balanced chemical equation for the reaction between hydrogen phosphate ions (HPO4^2-) and slaked lime, Ca(OH)2, to form the precipitate Ca5(OH)(PO4)3 is: \(Ca(OH)_{2} + 3HPO_{4}^{2-} \rightarrow 5Ca^{2+} + OH^- + Ca_{5}(OH)(PO_{4})_{3}\)

Step by step solution

01

Write the ionic equation for hydrogen phosphate (HPO4^2-)

Since hydrogen phosphate is in the form of an ion, its ionic equation will simply be: \(HPO_{4}^{2-}\)
02

Write the ionic equation for slaked lime (Ca(OH)2)

Slaked lime, Ca(OH)2, ionizes in water to form calcium ions (Ca^2+) and hydroxide ions (OH^-). The ionic equation for this process is: \(Ca(OH)_{2} \rightarrow Ca^{2+} + 2OH^-\)
03

Write the ionic equation for the formation of the precipitate (Ca5(OH)(PO4)3)

The precipitate is made up of five calcium ions (Ca^2+), one hydroxide ion (OH^-), and three hydrogen phosphate ions (HPO4^2-). The ionic equation for the formation of the precipitate will be: \(5Ca^{2+} + OH^- + 3HPO_{4}^{2-} \rightarrow Ca_{5}(OH)(PO_{4})_{3}\)
04

Combine the individual ionic equations into the overall balanced chemical equation

Now we just need to combine the ionic equations from steps 1, 2, and 3 into the overall balanced chemical equation: \(Ca(OH)_{2} + 3HPO_{4}^{2-} \rightarrow 5Ca^{2+} + 2OH^- + Ca_{5}(OH)(PO_{4})_{3}\) Since 2OH^- ions are formed when slaked lime ionizes, and only 1OH^- ion is needed in the formation of the precipitate, 1OH^- ion will remain in solution at the end of the reaction: \(Ca(OH)_{2} + 3HPO_{4}^{2-} \rightarrow 5Ca^{2+} + OH^- + Ca_{5}(OH)(PO_{4})_{3}\) This equation is balanced, as there are equal numbers of each type of ion on both sides of the equation.

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