Final answer:
Knowing the temperature of the water is important because it affects the concentration of water and the extent of ionization. Higher temperature water produces more ions. The ionization constant of water is usually given at 25 °C.
Step-by-step explanation:
Knowing the temperature of the water is important when dealing with ions in an aqueous solution because temperature affects the concentration of water and the extent of ionization. The autoionization of water, which is the process of water molecules ionizing to form ions, is endothermic, meaning it increases with temperature. This means that higher temperature of water will produce more ions than lower temperature water. The ionization constant of water, Kw, is usually given at 25 °C, so knowing the temperature is important for accurate calculations and understanding the behavior of ions in the solution.