Auto-Ionization - Video Tutorials & Practice Problems
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1
concept
Auto-Ionization and Kw
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Autoionization occurs when water molecules react with one another in an aqueous solution. So here recall that water is amphoteric, meaning it can act as both an acid or a base. Here one of them is going to act as a base, the other one's going to act as an acid. The acid will donate an H+ to the base. The base basic water molecule that accepts the H+ becomes H3O plus the water that donated the H+ becomes Oh-. Here we're going to say that associated with this reaction is kw. Kw represents our ionization constant of water. It is an equilibrium constant and like other equilibrium constants it's a ratio of products over reactants. And remember it does not take into account liquids and solids. It only pays attention to aqueous and gaseous compounds. Looking at this equation that we have, we're going to say that the liquids will be ignored, so the reactants on the bottom will be ignored. So kw just equals H3O plus times Oh kw is equal to 1.0 times 10 to the negative 14 at a temperature of 25 degrees Celsius. This fact is what connects us to the formula of pH+pOH equals 14. Now this whole idea of H3O plus and Oh minus, remember they are kind of like counterbalancing one another. If one goes up the other one goes down. This is a way of maintaining the acidity or basicity of any aqueous solution. And realize that if we're dealing with pure water, that's when their concentrations are equal to one another. And that's when we can talk about aqueous solution being neutral. So keep this in mind, auto ionization is the key to understanding the relationship between H3O plus your hydronium ion concentration with Oh minus your hydroxide ion concentration. Together, they help us to create this ionization constant expression for water, which then leads us into pH+pOHequaling14.
2
concept
Kw and Temperature
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Now recall that at 25 degrees Celsius, k w equals 1.0 times 10 to the negative 14. This is a value you'll have to remember on your own. You're not going to be expected to mem, to be given a formula sheet with this value present. But remember kw is an equilibrium constant and like the other equilibrium constants it is temperature dependent. If I play around with my temperature where it strays away from 25 degrees Celsius, then the value itself will change. We're going to say the general trend is as the temperature increases our k w increases. If we take a look here we have temperatures ranging from 0 degrees Celsius all the way up to a 100 degrees Celsius. And if you look, you can see that as our temperature starts to increase going from 0 to a 100, we can see that the general trend is that my k w value is increasing. Again, at 25 degrees Celsius, kW is equal to this value. This is what you're expected to remember on your own. If the temperature changes from 25 degrees Celsius, you'll be given that new value for k w because it could really be any number, So it's hard for you to memorize an entire list of k w at all these different temperatures. Okay. And remember the general trend is as the temperature increases, our k w generally increases as well.
3
example
Auto-Ionization Example
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A particular aqueous solution at 50 degrees Celsius contains 3.7 times 10 to the minus 4 hydronium ions. It says to calculate the hydroxide ion concentration and identify the solution as either being acidic, basic, or neutral. Alright, so hydronium ion is H3O+ and they want us to find Oh minus. The equation that connects them together is Kw equals the hydronium ion concentration times the hydroxide ion concentration. Our temperature is at 50 degrees Celsius which means our k w value changes. If you look up above you'll see that at 50 degrees Celsius kw equals 5.476 times 10 to the minus 14. Plug in our number for the hydronium ion concentration, so 3.7 times 10 to the negative 4, and then we just have to solve for the hydroxide ion concentration. Divide both sides by 3.7 times 10 to the minus 4, When you do that you're gonna get your hydroxide ion concentration being equal to 1.48 times 10 to the negative 10 molar. Now how do we determine if it's acidic, basic, or neutral solution? Well, you can see that your hydronium ion concentrations is to the negative 4, but your hydroxide is to the negative 10. Since hydronium ion concentration is greater than hydroxide ion concentration, that means that we are dealing with an acidic solution. Right? So we have both the concentration of hydroxide ion, and the fact that our solution is acidic.
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Problem
Problem
Chemistry student prepared an aqueous solution at 30ºC. If the solutions contains 7.42 × 10−9 M of hydroxide ions, calculate the pH.
A
5.703
B
8.130
C
8.300
D
5.980
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Problem
Problem
Calculate the Kw of pure water given the pH = 6.34.