Question

To 1.0 L of a 0.32 M solution of HClO2 is added 0.14 mol of NaF. Calculate the [HClO2] at equilibrium.

I calculate the answer twice, which are 0.18M, 0.19M, but they are incorrect.

Answer 1

The k_eq for an acid-base reaction can be obtained from the individual pka values:

Where k_a1 is the value pf the acid dissociation constant of the species that will act as an acid (HClO₂) and k_a2 of the species that will act as a base (NaF, and we use the value for HF, the conjugated acid):

so:

So:

	HClO2	+ NaF	----> NaClO2	+ HF
initial	0.32mol	0	0	0
added	-	0.14mol	-	-
reacts	x	x	x	x
equilibrium	0.32-x	0.14-x	x	x

and solving:

(non acceptable solution, produces a negative concentration)

So, the number of moles of HClO₂ at equilibrium is:

as the volume is 1L, the number of moles equal the concentration: