Solubility Product Constant: Ksp - Video Tutorials & Practice Problems
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1
concept
Solubility Product Constant (Ksp)
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Now the solubility product constant uses the variable k s p, and represents yet another type of equilibrium constant. Here, it measures solubilities of solid ionic compounds in a solvent at equilibrium. Here we're going to say that solubility is just the maximum amount of solid dissolved in a solvent, and it's usually represented as capital m, which stands for molar solubility. Now the magnitude of k s value determines the degree of solubility. Basically, we're going to say here the greater the KSP value, then the more soluble your ionic solid will be. And the lower your Ksp, then the less soluble your ionic solid will be. Here we're going to say this comparison can only be used between compounds that break up into the same number of ions. So for example, you have NaCl, which breaks up into 2 ions, and you have, let's say, lithium bromide, which also breaks up into 2 ions. Because they break up into the same number of ions, you can look at their KSPs and directly compare them. So this one has a higher KSP, so it's more soluble. If they broke up into different number of ions, then more calculations will be needed. So for example, if you add barium chloride, this breaks up into 3 ions. So you couldn't just look at its ksp value and compare it to the other 2, because it breaks up into different numbers of ions. Later on we'll see how to compare them by doing further calculations.
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example
Solubility Product Constant: Ksp Example
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Given the following ionic compounds, which have the highest will have the highest concentration of hydroxide ions concentration? Here it says, hint, which is the most soluble in water? Alright. So if we took a look at every single one of these compounds, you will see that they all break up into 3 ions. They would break up into the respective metal, and 2 hydroxide ions. Now, because they break up into the same number of ions, we can just look at their Ksp values and determine which one would produce the most o h minus. Remember, the greater your Ksp value, the more soluble your ionic solid will be, the more ions it'll produce. So if we take a look here, negative 17 to the negative 20, to the negative 13 to the negative 27. Here we'd say magnesium hydroxide is our answer. It has the highest Ksp value, therefore it is the most soluble, meaning that it will break up the most and produce the most o h minus ions as products. This in turn will create the highest hydroxide ion concentration.
3
concept
Ksp Calculations
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Now when it comes to KSP calculations, realize that solubility is an equilibrium process. Hence, calculations will require an ICE chart. So if you see KSP within a question, that means you're most likely going to have to use an ICE chart. Now here with KSP, we have a formula we need to remember. So let's say that here we have our equation. Again, KSP deals with the solubility of ionic solids, which means that you're going to start out with an ionic solid as your reactant. We look to see how it breaks up into various ions. So let's say that it breaks up into some unknown number of B ions plus some unknown number of C ions. These are ions where they'd have charges, so I'm just gonna write a random charge here for each one. Now a, b, and c are your coefficients. Ksp is an equilibrium constant, and like all equilibrium constants, it equals products over reactants. However, remember, we said Ksp deals with solids. Remember, solids and liquids are ignored. So although Ksp equals products over reactants, Like other equilibrium constants, the reactant will be a solid and therefore can be ignored. So KSP simplifies to just equal to products. Now like other equilibrium constants, we'd have our products within brackets represent concentrations, and then remember that their coefficients would become these powers. Okay? So that's how we break down the equilibrium expression for a Ksp value. Okay? So just remember, it's equal to products over reactants. The reactant is a solid, so just drop it. So Ksp equals products.
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example
Solubility Product Constant: Ksp Example
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Here we have lead 2 fluoride is a white solid and has diverse applications in pharmaceuticals, metallurgy, and technology. If the concentration of lead 2 fluoride is 4.2 molar with a Ksp of 3.6 times 10 to the negative 8, calculate the molar solubility of this solid at 25 degrees Celsius. Alright. So when they say solubility, whether they say calculate solubility or calculate molar solubility, they're just asking you to find x. And when they're asking for the molar solubility of the ionic solid, that's just equal to x. Here, we're going to follow the steps in order to solve for this particular question. So step 1, we set up an ICE chart with solid as the only reactant, because we're looking at how it breaks up into ions. Cross out the reactant side, because remember, in an ICE chart we ignore solids and liquids. This is an ICE chart, so this is initial, change, equilibrium. Using the initial row, set products equal to 0. Now remember, we lose reactants in order to make products. So using the change row, place a plus x for the products. We also have to take into account coefficients. So here this would be plus x, there's a 2 here so this would be plus 2 x. Using the equilibrium row, we're gonna set up the equilibrium constant expression with k s p and solve for x. So now this is plus x and this is plus 2 x. Now, here we're going to say that the variable x in the ICE chart represents the molar solubility of my ionic solid. Alright. So here, we're gonna say Ksp equals products over reactants, but again, the reactant is a solid, so we ignore it. So it just equals p b 2 plus or plus 2 times f minus. Remember the coefficient also pops up here too as a power. Alright. So now we're going to, plug in values that we have. So we're gonna say here KSP is 3.6 times 10 to the negative 8. At equilibrium, lead 2 is x. Fluoride is 2x, and that's still squared. So we're gonna say this is x times, 2 squared is 4, x squared is x squared. So 4x squared times x comes out to 4x cubed, and that's still equal to my Ksp. So all we have to do now is solve for x. So divide both sides by 4, and we'll get x cubed equals 9.0 times 10 to the negative 9. Take the cube root of both sides, When we do that, we get x equals 2.08 times 10 to the minus 3, and that'll be molarity. So this represents the molar solubility of my ionic solid. So again, if anytime they're asking for the molar solubility or solubility of your ionic compound, it's just equal to x. Now if they were asking for 1 of the ions though, it wouldn't just simply be x. What you would need to do is you need to look at the equilibrium row, and then look to see what does that ion represent at equilibrium. Here, lead 2 is x, so it would still be this number, but Fluoride Ion is 2 x, which would mean that you take this x answer and plug it into this 2 x. So it'd be 2 times x and that would be the true molar solubility of this fluoride ion. So again, be careful with when it comes to the ions. For the ions, you have to check the equilibrium row and see what their equation is. For the solid, when you solve for x, that is your final answer.
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Problem
Problem
Solubility of Sn(OH)2 was found to be 1.11 x 10-9 M; calculate Ksp of this compound.
A
5.47 × 10−27
B
4.44 × 10−9
C
1.23 × 10−18
D
1.37 × 10−27
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Problem
Problem
If a saturated solution of Ag2CO3 contains 2.56 × 10−4 M of Ag+ ions, determine its solubility product constant.
A
4.30 × 10−15
B
8.39 × 10−12
C
1.68 × 10−11
D
6.55 × 10−8
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Problem
Problem
What is the solubility of CN− ions in a solution of 5.5 M Hg2(CN)2, with a Ksp of 5.0 × 10−40?
A
4.5× 10−20 M
B
5.0 × 10−14 M
C
1.1 × 10−20 M
D
1.0 × 10−13 M
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