The table below lists measured values at for three fluorinated carboxylic acids

CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases

Problem

The table below lists measured KaK_a values at 25∘C25^{\circ}\text{C} for three fluorinated carboxylic acids.
AcidCondensed formulaKaK_a
MonofluoroaceticFCH2COOH4.0×10−34.0\times10^{-3}
DifluoroaceticF2CHCOOH1.6×10−21.6\times10^{-2}
TrifluoroaceticF3CCOOH1.2×10−11.2\times10^{-1}
A researcher wants to predict the acid strength of hypothetical tetrafluoroacetic acid (F4CCOOH), in which a fourth fluorine replaces the remaining hydrogen on the alpha carbon. (a) Convert each KaK_a value in the table to its corresponding pKapK_a value to two decimal places. (b) Using the pKapK_a data, determine the average change in pKapK_a per additional fluorine atom. State any assumptions you make. (c) Extrapolate your result from part (b) to predict the pKapK_a of tetrafluoroacetic acid. Show your calculation. (d) Discuss one reason why your extrapolated value might deviate from an experimentally measured value, even if the trend you assumed is generally valid.

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