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### Course: Chemistry library>Unit 12

Lesson 3: Solubility equilibria

# Worked example: Calculating solubility from Kₛₚ

A compound's molar solubility in water can be calculated from its Kₛₚ value at 25°C. To do so, first prepare an ICE (Initial, Change, and Equilibrium) table showing the equilibrium concentrations of the ions in terms of x, the molar solubility of the compound. Then, plug these expressions into the solubility-product expression for the compound and solve for the value of x. Created by Jay.

## Want to join the conversation?

• At why do you raise the concentration of the reactants to the power of the coefficient? In what video is that explained?
• how did we get x times 4x squared, and then 4x cubed?
• Jay misspoke, he should have said x times 2x squared which results in 4x cubed. Small math error on his part.

Hope that clears it up.
• What would you do if you were asked to find the ppm of the cu2+ ion or the OH- ion?
• Ppm means: "how many in a million?" as in, "How many grams of Cu²⁺ in a million grams of solution"?
We know that [Cu²⁺] = 1.8 × 10⁻⁵ g/L.
We also know that 1 L of solution = 1000 mL, and the density of water
is 1 g/mL, so we have 1000 g of solution.
∴ [Cu²⁺] = 1.8 × 10⁻⁵ g/1 L = 1.8 × 10⁻⁵ g/10³ g
We want to know how many grams of Cu²⁺ in 10⁶ g of solution, so we multiply the denominator by 1000 to get 10⁶ and the numerator by 1000 to keep the fraction constant.
[Cu²⁺] = (1.8 × 10⁻⁵ g/10³ g) × (10³/10³) = 1.8 × 10⁻² g/10⁶ g = 1.8 × 10⁻² ppm or 0.18 ppm or 180 ppb.
• At around , why do you times the 2x by 2 to get 4x? Thanks
• You aren't multiplying, you're squaring. 2 times 2 is 4 and x times x is x^2, so 2x times 2x equals 4x^2. Then, multiplying that by x equals 4x^3.
• At , is there some reason not to type (5.5*10^-21)^(1/3) instead of scrolling through the catalog?
• He is using a calculator simulator, so it might be a bit different from a normal graphing calculator
• At while he was solving the problem, he mentioned that we should multiply the solubility "X" by 2 "mole ratio" to get the solubility right. I wonder why we don't do so while calculating the equilibrium expressions generally, and the Ksp specifically, using the molarity "concentration".
• You actually would use the coefficients when solving for equilibrium expressions. Looking back over my notes that I took over the Khanacademy MCAT prep videos I don't see any examples with this, but doing just a little research you can confirm that the coefficients are incorporated when determining any equilibrium expression (even if it is just 1).
• What is the difference between the solubility and the solubility product constant?
• How would you take the cube root of something WITHOUT a calculator? Is there a trick to it?
(1 vote)
• Not really. I hate plugging a non-Khan video but i`ve looked for it and I can`t find it. Start here, she does a very good job of math without a calculator. Good luck. https://www.youtube.com/watch?v=4ApPRHD1ahQ&feature=youtu.be PS. I would explain it myself if I thought I could do a better job than her.
• How do you know what values to put into an ICE table?
(1 vote)
• Ice table stands for:
Initial concentrations (M)
Change
Equilibrium concentrations (M)
so whatever values you have that you can put in that table you can.