How much grams of calcium hydroxide is needed to make 1500mL of solution of calcium hydroxide with a pH of 12.50?

Question

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Answer 1

calcium hydroxide = Ca(OH)2

In water, Ca(OH)2 ==> Ca²⁺(aq) + 2OH^-(aq)

From the given pH we can find the [OH^-] and from that we can find the [Ca(OH)₂]. Once we know that, we can find grams of Ca(OH)₂

pH = 12.50

pOH = 14 - pH = 1.5

[OH-] = 1x10^-1.5 = 3.16x10^-2 M

1500 ml = 1.5 L

moles OH^- = 3.16x10^-2 mol/L x 1.5 L = 4.74x10^-2 moles OH^-

Since each mole Ca(OH)₂ provides 2 moles OH^-, the moles of Ca(OH)₂ = 4.74x10^-2 mol/2 = 2.37x10^-2 moles

Mass Ca(OH)₂ = 2.37x10^-2 moles x 74.1 g/moll = 1.76 g Ca(OH)₂ needed

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Done a slightly different way:

pH = 12.5

[H+] = 1x10^-12.5 = 3.16x10^-13 M

[OH-] = 1x10^-14 / 3.16x10^-13 = 3.16x10^-2 M

Remainder of calculations same as above.