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Find Ka, % dissocation, given concentration and pH

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1. A 2.5M solution of hydrocyanic acid has a measured pH of 4.5.
Calculate the Ka for hydrocyanic acid and its % dissociation.

2. Using an equimolar mixture of formic acid (Ka=1.8 x 10^-4) and sodium formate, determine the pH that this buffer solution will have.

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Solution Summary

In this exercise, students are asked to find the Ka and the % dissociation of a weak acid, given the weak acid's concentration and the pH. In the second part, students will use the Henderson-Hasselbalch equation to find the pH of a buffer solution, given the molar ratio and the Ka of the weak acid.

Solution Preview

a 2.5M solution of hydrocyanic acid has a measured pH of 4.5
calculate the Ka for hydrocyanic acid and its % dissociation.

Solution:

HCN + H¬2O ↔ H3O+ + CN-

pH = - log [H3O+]
4.5 = - log [H3O+]
- 4.5 = log ...

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