# How do you write an expression for the acid ionization constant (K_a) for HCHO_2?

Mar 16, 2017

$\text{For formic acid............?}$

#### Explanation:

We assess the equilibrium:

$H \left(O =\right) C O H \left(l\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s H \left(O =\right) C {O}^{-} + {H}_{3} {O}^{+}$

And we write the equilibrium equation in the usual way:

${K}_{a} = \text{Concentration of products"/"Concentration of reactants}$

${K}_{a} = \frac{\left[H \left(O =\right) C {O}^{-}\right] \left[{H}_{3} {O}^{+}\right]}{\left[H \left(O =\right) C O H \left(l\right)\right]}$.

This site reports that ${K}_{a}$ for formic acid $= 1.70 \times {10}^{-} 5$, so it is a weak Bronsted acid.