A weak acid is an acid that only partially dissociates in water, meaning that it does not completely break into its ions when dissolved in water. Instead, it establishes an equilibrium between the undissociated acid and its ions. The dissociation of phosphoric acid can be represented by the following chemical equations:

$$H_3PO_4 \rightleftharpoons H^+ + H_2PO_4^-$$

The equilibrium constant for this reaction is denoted as $K_a1$ and is equal to:

$$K_{a1} = \frac{[H^+][H_2PO_4^-]}{[H_3PO_4]}$$

The value of $K_{a1}$ is 7.5 * 10^-3 at 25°C. This indicates that phosphoric acid is a weak acid, as its $K_{a}$ value is less than 1.

Phosphoric acid can also undergo further dissociation steps to release the remaining hydrogen ions:

$$H_2PO_4^- \rightleftharpoons H^+ + HPO_4^{2-}$$

The equilibrium constant for this reaction is denoted as $K_{a2}$ and is equal to:

$$K_{a2} = \frac{[H^+][HPO_4^{2-}]}{[H_2PO_4^-]}$$

The value of $K_{a2}$ is 6.2 * 10^-8 at 25°C.

$$HPO_4^{2-} \rightleftharpoons H^+ + PO_4^{3-}$$

The equilibrium constant for this reaction is denoted as $K_{a3}$ and is equal to:

$$K_{a3} = \frac{[H^+][PO_4^{3-}]}{[HPO_4^{2-}]}$$

The value of $K_{a3}$ is 4.8 * 10^-13 at 25°C.

These values indicate that phosphoric acid is a stronger acid than many other weak acids, such as acetic acid, but weaker than strong acids, such as hydrochloric acid, which completely dissociate in water.