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Consider the following indicators and their pH ranges:
Methyl orange 3.2-4.4
Methyl red 4.8-6.0
Bromothymol blue 6.0-7.6
Phenolphthalein 8.2-10.0
Alizarin yellow 10.1-12.0
Assume an indicator works best when the equivalence point of a titration comes in the middle of the indicator range. For which of the following titrations would methyl red be the best indicator?
a) 0.100M HNO3+0.100M KOH.
b) 0.100M aniline(Kb=3.8×10^-10)+0.100M HCl.
c) 0.100M NH3(Kb=1.8×10^-5)+0.100M HCl.
d) 0.100M HF(Ka=7.2×10^-4)+0.100M NaOH.
e) 0.100M acetic acid(Ka=1.8×10^-5)+0.100M NaOH.

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sebassandin

Answer:

c) 0.100M NH3(Kb=1.8×10^-5)+0.100M HCl.

Explanation:

Hello,

In this case a pH of about 5 is what we should look for, thus, we compute the pH of a 0.100M solution of ammonia as:

[tex]NH_3<-->NH_4^++OH^-\\K_b=\frac{[NH_4^+][OH^-]}{[NH_3]} \\K_b=\frac{x^2}{0.100M-x} \\0.100K_b-K_bx-x^2=0\\1.8x^{-6}-1.8x^{-5}x-x^2=0\\x=1.3x10^{-3}M\\[OH^-]=1.3x10^{-3}\\pH=14-log([OH^-])=14-log(1.3x10^{-3})=11.11[/tex]

That pH allows us to identify the methyl red to perform the titration as long as a pH of about 5, based on the equivalence point, is required.

Best regards.

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