 Okay. So I think this is a good problem to try. It's more of a simpler pH problem or concentration problem. So calculate the concentration of hydronium ion in an aqueous solution where the concentration of hydroxide ions is 1.0 x 10 to the negative 9th mole. So you need to know this expression. The kW equals the OH concentration times the H3O plus concentration. kW itself equals 1.0 x 10 to the negative 14th. So we've got to isolate the variable H3O plus. So in order to do that we divide both sides by the concentration of OH minus, hence the amount. So we get this to be equal, hydronium ion equals kW over OH minus. So we've got a kW, 1.0 x 10 to the negative 14th. And we've got hydroxide ion concentration, 1.0 x 10 to the negative 9th. That's going to equal 1.0 x 10 to the negative 5th molar. You can actually find the pH of this now, because remember the pH of the...