The acid illustrated is hydrochloric acid. As the germicidal effects of HOCl is much higher than that of OCl-, chlorination at a lower pH is preferred. Since the concentration of acid is 2.0x10-7 M, HCl will produce a [H +] of 2.0x10-7. Hydrochloric acid, HCl, is a strong acid, so right from the start you should expect it to ionize completely in aqueous solution. This is because it dissociates completely in water (the very definition of a strong acid/base) into H+ and Cl-. However, if the [H+] produced by the strong acid is less than 1.0x10-6, we need to redo the calculation taking water into account. Calculate the pH of a strong acid solution, taking the dissociation of water into consideration. I understand that dissociation in water can result in H+(aq). The water molecules surround the negative chloride ions and positive sodium ions and pull them away into the solution. Below a pH of 6.5, no dissociation of HOCl occurs, while above a pH of 8.5, complete dissociation to OCl- occurs. HCl → H+ + Cl- HCl is a strong acid with total dissociation. [H+]2 > 1.0x10-12 table: H2O For weak acids, we consider an equation derived during the full mathematical treatment of weak acid solutions in which water dissociation is considered: Again, if [H+]2 >> Kw, this equation simplifies to. Therefore are aqueous H+ ions the same as H30. Hydrogen chloride is a room temperature gas. Even in the absence of water, hydrogen chloride can still act as an acid. According to the Arrhenius definition, acids like HCl are compounds that increase the hydrogen ion concentration solution. The details of the calculation are different for strong and weak acids. This means that we must discard the assumption that water makes a negligible contribution to the pH in order to accurately calculate the pH of the solution. Since nearly all of it is dissociated in water, it is called a strong acid. As we have seen, a lot more calculation is needed when we consider the dissociation of water in calculating pH than when we ignore it. The answer is "yes; HCl will dissociate in water". The entire reaction is all about dissociation and combination of ions. Well, heat evolves as the water hydrates the hydrogen chloride molecule, and your addition is safe. Ca = ( [H+]2 - Kw ) / [H+], Now, we see that if [H+]2 >> Kw, the equation simplifies to. The acid dissociation or ionization constant, Ka, is large, which means HCl dissociates or ionizes practically completely in water. Unlike strong acids, where water dissociation is important only when a very dilute acid is involved, water dissociation can contribute to the pH of a weak acid solution if the acid is very dilute, very weak, or dilute and weak. HCl is an ionic compound that dissociates itself upon its addition to the solvent to form a solution. However in HCl + H2O, my text book gives the answer as H+(aq) + Cl-(aq). [H+] > 1.0x10-6 M. So we can conclude that if the [H+] produced by the acid without considering the dissociation of water is greater than 1.0x10-6 M, the answer is accurate enough. We consider HCl to dissociate first, with H2O then responding. We consider HCl to dissociate first, with H 2 O then responding. Determine whether or not water needs to be considered in order to perform an accurate calculation of the pH of a strong acid solution. Sodium Hydroxide For those who are interested, the complete calculation is presented here. Since the concentration of acid is 2.0x10-7 M, HCl will produce a [H+] of 2.0x10-7. In the case of sodium chloride ($$\text{NaCl}$$) for example, the positive sodium ions ($$\text{Na}^{+}$$) are attracted to the negative pole of the water molecule, while the negative chloride ions ($$\text{Cl}^{-}$$) are attracted to the positive pole of the water molecule. So, we can ignore the dissociation of water if [H+]2 >> Kw. Hwill leave its own electron toCl ion and H gets attached to water to form H3O+, i.e., hydronium ion. Using this as the initial [H+] in the equilibrium dissociation of water, we can find the equilibrium [H+] with an I.C.E. In other words, every molecule of hydrochloric acid that is added to water will donate its proton, H+, to water molecule to form a hydronium cation, H3O+. Here, [H+] = Ca just like in a strong acid solution in which water is ignored. It is the polar nature of water that allows ionic compounds to dissolve in it. The pH of the solution is controlled by two reactions: Since HCl is a strong acid, it dissociates completely, with no equilibrium to adjust to the presence of additional H+ ions. It is important to be able to write dissociation equations. Dissociation is the separation of ions that occurs when a solid ionic compound dissolves. 0 = x2 + (2.0x10-7 * x) - 1.0x10-14. Using this as the initial [H +] in the equilibrium dissociation of water, we can find the equilibrium [H +] with an I.C.E. The confusion arises when the internet tells me the equation is also HCl + H2O ----> [H3O+]+[Cl-]. So, just like strong acids, we can ignore the dissociation of water if [H+]2 >> Kw. (We will see later how to tell whether or not the [H+] is low enough for water to contribute to the pH.). Dissociation of Sodium Chloride in Water. (1)HCl(aq)⟶H+(aq)+Cl−(aq) After successful dissociation of HClin water, the following reaction takes place: HCl+H2O⟶H3O++Cl−. An analogous situation exists for weak acids in which the [H+] due to acid dissociation is comparable to the [H+] due to water dissociation (1.0x10-7 M). Consider a dilute acid solution. Until now, we have assumed that the pH of an aqueous solution of acid or base is determined only by the dissociation of the acid or base itself. However, for this tutorial, students need only know how to determine whether or not water will contribute significantly to the pH of a weak acid solution, which is dealt with below. (Why?). 1.0x10-14 = x2 + (2.0x10-7 * x) The actual calculation of the pH of a weak acid solution in which water dissociation is considered is much more complicated than the one for a strong acid solution, and is above the level of this tutorial. If so why is aqueous Cl- not represented as H20Cl- or some such thing. In aqueous solution, phosphoric acid behaves as a triprotic acid, having three ionizable hydrogen atoms. At 25oC, pure water dissociates to give a [H+] of 1.0x10-7M. Simply undo the crisscross method that you learned when writing chemical formulas of ionic compounds. Using this as the initial [H+] in the equilibrium dissociation of water, we can find the equilibrium [H+] with an I.C.E. This process is called dissociation.