A student wishes to know the concentration of an unlabeled bottle of calcium hydroxide solution, Ca(OH)2 (aq). This solution is the
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. They fill a 100.0 mL buret with the Ca(OH)2 solution and they fill another buret in the same stand with 100.0 mL of 0.250 M HCl (aq). The HCl is the
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. They then dispense 10.0 mL of Ca(OH)2 into an Erlenmeyer flask containing two drops of phenolphthalein, a dye that is pink in basic solution and clear in acidic solution. The phenolphthalein acts as the
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.
They carefully add HCl to the Erlenmeyer flask until it turns clear, and then they add Ca(OH)2 dropwise until it turns pink again. Then, adding one drop of HCl turns it clear, and adding one drop of Ca(OH)2 reverts it to pink. At this time they know the solution is at the equivalence point.