And this is always true at room temperature. So basically what this equation is, and let's call this equation A. And by the way, this is equation B, and this is equation A from equation B. If we know what the concentration of the hydrogen ion is, and we can calculate that by using this equation A, then we can easily calculate what the concentration of the hydroxide ion is by using equation B. For example, let's use this case once again. Let's suppose our PH is seven, where at room temperature.