Question

What is the fractional dissociation of 0.05 F ( not M) formic acid at pH 4? Ka= 1.80 E-14 and pH 5. Please help. Thanks.

Answer 1

Here the unit F in 0.05 F of formic acid means the Formality of Formic acid. Formality is also the same as Molarity (M) of a compound and is used to indicate the concentration of a compound.

There is, however, a subtle difference between molarity and formality. Molarity is the concentration of a particular chemical species. Formality, on the other hand, is a substance’s total concentration without regard to its specific chemical form. There is no difference between a compound’s molarity and formality if it dissolves without dissociating into ions. The formal concentration of a solution of glucose,

for example, is the same as its molarity. (A solution that is 0.0259 M in glucose is 0.0259 F in glucose as well.)

Here the formality of the Formic acid indicates the Molarity of Formric acid in its undissociated state. Thus for calculation, we can take the molarity of the acid = the formality of the acid = 0.05 M

Agaian the Fractional dissociation means the fraction of substance dissociated from the orginal substance. Since Formic acid is a weak acid, it cant dissociate completey. Only a fraction of it get dissociated. Thus-