Question

Consider two acids: (ch3)3cco2h (pivalic acid, pka = 5.0) and hco2h (formic acid, pka = 3.8). Which acid is the stronger acid? Select the single best answer.

Answer 1

Answer:

            Formic acid is the stronger acid.

Explanation:

                   The strength of an acid is determined by the power of it to dissociate and produce H⁺ ions. Greater the dissociation of an acid, the stronger that acid is. Also, extent of dissociation for particular acid is given by its dissociation constant i.e. Ka.

The greater the Ka value, the stronger an acid is and vice versa.

Calculating Ka value of Pivalic acid:

As,

                                                 pKa  =  -log [Ka]

So,

                                                 Ka  =  10^-pKa

Putting value of pKa,

                                                 Ka  =  10⁻⁵

                                                Ka  =  1.0 × 10⁻⁵

Calculating Ka value of Formic acid:

As,

                                                 pKa  =  -log [Ka]

So,

                                                 Ka  =  10^-pKa

Putting value of pKa,

                                                 Ka  =  10⁻³'⁸

                                                 Ka  =  1.58 × 10⁻⁴

Conclusion:

                  As the Ka value of Formic acid is greater than Pivalic acid therefore, it is stronger acid than Pivalic acid.

TIP:

     When comparing strength of acids, the acid with the lowest pKa value is considered the strongest acid and vice versa.