What is the molar solubility of Fe(OH) 3 (s) in a solution that is buffered at pH 3.50 at 25°C? The K sp of Fe(OH) 3 is 6.3 x 10 –38 at 25°C.

Here the question states, what is the molar solubility of iron hydroxide in a solution that is buffered at pH 3.50 at 25 degrees Celsius? We're told further that the Ksp of iron hydroxide 6.3 times 10 to the negative 38 at 25 degrees Celsius. All right. So they're asking us to determine the molar solubility, so we need to just determine what x is for our ionic compound. It has a ksp value, so it'll exist as a solid. And we look and see how it breaks up into its ions. It breaks up into iron 3 ion plus 3 hydroxide ions. So here, we're going to say iron hydroxide is a solid so we're going to ignore it. It is in a solution with the pH of 3.50. But iron 3 hydroxide is really a base because we have a metal connected to hydroxide ions. Bases are associated with pOH, not pH. Remember, pH plus pOH equals 14. So that's 3.50 +0h equals 14. Subtract 3.50 from both sides and we'll have pOH equals 10.50. This is important because if we know pOH, we know what Oh minus concentration will be as well because Oh minus concentration equals 10 to the negative pOH. So by doing that, we'll know what the concentration of our hydroxide ion solution is. It's 3.16 times 10 to the negative 11 molar for the concentration of my hydroxide. Again, we're faced with the idea of the common ion effect because here, our ionic compound breaks up in a solution that contains hydroxide ion just as it's made up of hydroxide ion. This has no common ion initially so it's 0. This is 3.16 times 10 to the negative 11 initially. This is plus x, plus 3 x because of the 3, plus x. And then we bring down everything so that's 3.16 times 10 to the negative 11 plus 3 x. We have k s p equals products. So equals ion 3 ion times hydroxide ion cubed. Again, remember, if we have an actual value in front of our x variable, we can ignore that x variable. Because our k s p is usually such a small number, that means that x variable is so minuscule, so small that it's not going to really affect the overall number here. So we plug in what we know, so 3.16 times 10 to the negative sp is 6.3 times 10 to the negative 38 equals x times 3.16 times 10 to the negative 11 cubed. Divide both sides by 3.16 times 10 to the negative 11 cubed. Canceling those out, we'll get x at the end. X here equals 1.99 times 10 to the negative 6 molar and that represents the molar solubility of our iron 3 hydroxide compound. Now, since we've seen this version, continue onward to practice question 2. In this one, we're asked to figure out what our KSP is. We've seen a question similar to this, so go back and look at the steps taken to solve that particular question. Once you do, come back and see if your answer matches up with mine.