Final answer:
To find Kw when [H+] and [OH-] are known, use the formula Kw = [H+][OH-]. At 25°C, Kw is always 1.0 × 10^-14. Knowing one ion concentration allows you to calculate the other by dividing Kw by the known concentration.
Step-by-step explanation:
To find Kw if you have [H+] (hydrogen ion concentration) and [OH−] (hydroxide ion concentration), you simply use the formula Kw = [H+][OH−]. This formula represents the ionic product of water which, at 25°C, is always equal to 1.0 × 10−14. Therefore, if you are given the concentration of either the hydrogen ions or hydroxide ions in a solution, you can calculate the concentration of the other by rearranging the equation to solve for the unknown concentration. For example, if [H+] is known, you can find [OH−] by dividing Kw by [H+]. Conversely, if [OH−] is known, [H+] can be determined by dividing Kw by [OH−].
In summary, the product of the two ion concentrations will always equal Kw, and knowing one ion concentration allows for the calculation of the other. This relationship is also demonstrated by the pH and pOH scales, where pH = −log[H+] and pOH = −log[OH−]. Additionally, these quantities are related such that pH + pOH = 14, reflecting the constant nature of Kw in aqueous solutions.