$$ \ce{HF(aq) + H2O(l) <--> H3O+(aq) + F-(aq)} $$

The dissociation of the weak acid \(\ce{HF}\) in water is represented by the equation above. Adding a 1.0 mL sample of which of the following would increase the percent ionization of HF(aq) in 10 mL of a solution of 1.0 \(M  \ce{HF}\) ?