What change will be caused by addition of a small amount of HCl to a solution co

Question

What change will be caused by addition of a small amount of HCl to a solution containing fluoride ions and hydrogen fluoride? how do you figure this out A) The concentration of hydronium ions will increase significantly. B) The concentration of fluoride ions will increase as will the concentration of hydronium ions. C) The concentration of hydrogen fluoride will decrease and the concentration of fluoride ions will increase. D) The concentration of fluoride ion will decrease and the concentration of hydrogen fluoride will increase. E) The fluoride ions will precipitate out of solution as its acid salt.

How many milliliters of 0.0850 M NaOH are required to titrate 25.0 mL of 0.0720 M HBr to the equivalence point?

Answer 21.2

Explanation / Answer

For the first part, the best way for you to understand this is with an example. The general reaction for the buffer is:

HF -------> F- + H+

Using the HH equation: pH = pKa + log[F]/[HF]

Let's suppose that the pH obtained was 5.45, and the buffer was prepared using 0.45 M of HF and 0.40 of the salt. Now we add about 0.1 M oh HCl to this solution, What will happen here? the reaction would be as follow:

F- + HCl --------> HF + Cl-

What this means? it means that adding the acid, promove once again the formation of the HF molecule, causing this to increase. But increasing this concentration, as this is an equilibrium, will cause the decreasing of the F- ion, because the medium is acidic.

So, the increasing of the HF will cause the pH to decrease (more acid). So the correct option would be D.

For the second part, just use the following expression:
MaVa = MbVb
Vb = MaVa/Mb
Vb = 0.0720 * 25 / 0.0850 = 21.18 mL

Hope this helps

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