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May 22, 2015

Homework Help: chem

Posted by Horacio on Tuesday, May 15, 2007 at 8:23pm.

Help with this one please?

If you place a piece of blue litmus paper in 200mL of dilute hydrochloric acid, the litmus paper turns pink. If you then add 2 drops fo phenolphthalein, the solution remains coloreless. If you add a few drops of dilute calcium hydroxide solution, the litmus returns to its blue color, but the solution remains colorless. Now if you add a few drops more of calcium hydroxide, the entire solution turns red. Does litmus or phenolphthalein indicate a higher pH? Explain.

I have no idea to the answer. Any hints please?

Litmus paper initially is red, indicating an acid. Phenolphthalein is colorless, indicating an acid. Addition of Ca(OH)2, a base, turns the litmus paper first with no change to the phenolphthalein, but MORE base turns the phenolphthalein indicator to pink, its basic color. Therefore, going from acid to base shows first on the litmus and next on the phenolphthalein. Which do you think has the higher pH. Remember pH < 7 is acid; pH > 7 = base.

Let's see. Well, we're saying that going from acid to base shows first on litmus and second on phenolphthalein. So, would that mean litmus is more acidic than pheno. and so pheno. has a higher pH?

exactly. Litmus changes in a range of about pH 4.5-8.3 and phenolphthalein changes at pH about 8-10.

thanks DrBob! :)

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