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Assume that 30.0 mL of a 0.10 M solution of a weak base B that accepts one proton is titrated with a 0.10 M solution of a monoprotic strong acid HX. (a) How many moles of HX have been added at the equivalence point? (b) What is the predominant form of B at the equivalence point? (c) What factor determines the pH at the equivalence point? (d) Which indicator, phenolphthalein or methyl red, is likely to be the better choice for this titration?

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5 answers
  1. a. moles HX added = M x L.
    b. B + HX ==> BH^+ + X^-
    c. The hydrolysis of the salt BH^+. Said another way, the strength of BH^+ as an acid.
    d. Phenolphthalein changes color at pH about 9. Methyl red changes color about 5. Which do you think?

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  2. a. 2.7 x 10 -3 moles or 0.0027
    b. BH + (aq)
    c. Already given.
    d. phenolphthalein

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  3. hi to you. is this right?

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  4. c. is actually methyl red

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  5. For c it is both the Ka for BH^+ and concentration of BH^+ to determine pH at the equivalence point.

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