Although there are extensive tables available for the pK a of weak acids, you might be dealing

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Although there are extensive tables available for the pKa of weak acids, you might be dealing with an unknown acid or a known acid at an unlisted temperature. You could then use a procedure like this to determine the Ka and pKa. The pH of a 0.010 m aqueous solution of a certain carboxylic acid is 2.95. What are its Ka and pKa?

ANTICIPATE All carboxylic acids are weak acids; therefore, expect Ka ≪ 1.

PLAN Calculate the hydronium ion concentration from the pH and then calculate the value of Ka from the equilibrium concentration of the acid and the hydronium ion concentration.

What should you assume? As in Example 6D.1, because pH


Example 6D.1

Acetic acid is a weak acid commonly found in both laboratory and household, but to what extent have its molecules actually been deprotonated? Calculate the pH and percentage deprotonation of CH3COOH molecules in 0.080 m CH3COOH(aq), given that Ka for acetic acid is 1.8 * 10–5.

ANTICIPATE Because the solution is that of an acid, expect pH

PLAN Following the procedure in Toolbox 6D.1, write the proton transfer equilibrium and construct the equilibrium table with concentrations in moles per liter.

What should you assume? You can make two assumptions, but they need to be verified at the end of the calculation.

(1) Deprotonation is so slight that the equilibrium concentration of the acid is approximately the same as its initial concentration.

(2) The autoprotolysis of water does not contribute significantly to the pH.Toolbox 6D.1 HOW TO CALCULATE THE PH OF A SOLUTION OF A WEAK ACID CONCEPTUAL BASIS Because the proton

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Chemical Principles The Quest For Insight

ISBN: 9781464183959

7th Edition

Authors: Peter Atkins, Loretta Jones, Leroy Laverman

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