Auto-Ionization - Video Tutorials & Practice Problems
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1
concept
Auto-Ionization and Kw
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Auto ionization occurs when water molecules react with one another in an aqueous solution. So here we call that water is amphoteric, meaning can act as both an acid or a base. Here, one of them is gonna act as a base. The other one's gonna act as an acid. The acid will donate an H plus to the base. The base basic water molecule that accept the H plus becomes H +30 plus. The water that donated the H plus becomes oh minus. Here, we're going to say that associated with this reaction is KWKW 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 H 30 plus times oh minus KW is equal to 1.0 times 10 to negative 14 at a temperature of 25 °C. This fact is what connects us to the formula of ph plus POH equals 14. Now, this whole idea of H 30 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 vicinity 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 uh equi solution being neutral. So keep this in mind, auto ionization is the key to understanding the relationship between ATRIO plus your hydro 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 plus POH equaling 14.
2
concept
Kw and Temperature
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Now recall that at 25 °C KW 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 me um 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 °C, then the value itself will change. We're gonna say the general trend is as the temperature increases. RKW increases. If we take a look here, we have temperatures ranging from zero °C all the way up to 100 °C. And if you look, you can see that as our temperature starts to increase, going from 0 to 100 we can see that the general trend is that my KW value is increasing again at 25 °C. KW is equal to this value. This is what you're expected to remember on your own. If the temperature changes from 25 °C, you'll be given that new value for KW because it could really be any number. So it's hard for you to memorize an entire list of KW at all these different temperatures. Ok. And remember the general trend is as the temperature increases, our KW generally increases as well.
3
example
Auto-Ionization Example
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A particular aqueous solution at 50 °C contains 3.7 times 10 to the minus four hydro ions. It says to calculate the hydroxide ion concentration and identify the solution as either being acidic, basic or neutral. All right. So hydro ion is H 30 plus and they want us to find out H minus the equation that connects them together is KW equals the hydro ion concentration times the hydroxide ion concentration. Our temperature is at 50 °C, which means our KW value changes. If you look up above, you'll see that at 50 °C. KW equals 5.476 times 10 to the minus 14 plug in our number for the hydro ion concentration. So 3.7 times 10 to the negative four. And then we just have to solve for the hydroxide ion concentration divide both sides by 3.7 times 10 to the minus four. 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 hydro ion concentrations is to the negative four. But your hydroxide is to the negative 10. Since hydro 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 a sick.
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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.