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Find the approximate $\mathrm{pH}$ range suitable for separating $\mathrm{Mg}^{2+}$ and $\mathrm{Zn}^{2+}$ by the precipitation of $\mathrm{Zn}(\mathrm{OH})_{2}$ from a solution that is initially $0.010 \mathrm{M}$ in $\mathrm{Mg}^{2+}$ and $\mathrm{Zn}^{2+}$

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Thus, read $p H$$8.11<p H<9.54$

Chemistry 102

Chapter 16

Acid-Base Equilibria and Solubility Equilibria

Acid-Base Equilibria

Aqueous Equilibria

University of Maryland - University College

University of Toronto

Lectures

00:41

In chemistry, an ion is an atom or molecule that has a non-zero net electric charge. The name was coined by John Dalton for ions in 1808, and later expanded to include molecules in 1834.

24:14

In chemistry, a buffer is a solution that resists changes in pH. Buffers are used to maintain a stable pH in a solution. Buffers are solutions of a weak acid and its conjugate base or a weak base and its conjugate acid, usually in the form of a salt of the conjugate base or acid. Buffers have the property that a small change in the amount of strong acid or strong base added to them results in a much larger change in pH. The resistance of a buffer solution to pH change is due to the fact that the process of adding acid or base to the solution is slow compared to the rate at which the pH changes. In addition to this buffering action, the inclusion of the conjugate base or acid also slows the process of pH change by the mechanism of the Henderson–Hasselbalch equation. Buffers are most commonly found in aqueous solutions.

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Hello Today we're going to be looking at a solution where we have 0.1 molar ity off magnesium and zinc ions, and this is going to be in basic solution. And zinc and magnesium can both form as in there, a slightly soluble ionic substance that can precipitate out of solution sink hydroxide, magnesium hydroxide. So let's take a look at their case peas to find out which one will precipitate first. Lucy Zinc has a Caspi of 1.8 times 10 to negative 14 and magnesium hydroxide has a kiss p of 1.2 times 10 to the negative 11. So zinc has a smaller kiss p. So it will be the 1st 1 to drop out of solution because it's less soluble. And so we're gonna try toe look at what come what p. H do we need for the zinc hydroxide to precipitate but the magnesium hydroxide to not precipitate. So first, let's take a look at what concentration of hydroxide we need for zinc to start to precipitate. So let's write out the the sink hydroxide Solid isn't equilibrium with the stink eye on and to hydroxide ions, so the zinc has given would be 0.10 malaria T and then the hydroxide will be some concentration acts and the que The reaction quotient has to be slightly above the Caspi and is when it's slightly above the K SP. Then it'll start to precipitate out, so the Q is the same way as the So let's find out when the Q is equal to the Casspi. So the Casspi, the expression is Casspi is equal to the zinc concentration times. The hydroxide concentration two squared because there's a two coefficient in front of the hydroxide, So the Caspi for zinc is 1.8 times 10 to the negative. 14. The sink is 0.1 in the hydroxide concentration squared, so the hydroxide concentration squared would be quote a 1.8 times 10 to the negative 12. And if we take a square root well, we will see that the hydroxide concentration when it is saturated, there's 1.3 times 10 to the negative six. So if we have so we need ah hydroxide any hydroxide concentration above 1.3 times 10 to the negative six mil arat e will cause zinc to precipitate. So let's calculate the pH. So let's count calculate the P O. H, which is the negative log of the age concentration, so we see the peel H would be five 0.9. Now remember, the pH is can be found from the P. O. H 14 minus two peel age and give the pH so it would be eight 0.1 is the P H, where it has to be so basically a pH that is greater than 8.1, because that is a basic and that means there's hydroxide. There's enough hydroxide to cause Inc to precipitate, but we don't want magnesium to precipitate. So let's see how saturated magnesium convey be without precipitating. So zinc is the same way. I mean, magnesium is the same way. A zinc and equilibrium with magnesium and to hide rock sites and magnesium also starts off a 0.1 and some hydroxide concentration X and our cast P for magnesium would be one 0.2 times 10 to the negative 11 end. It's equal to the magnesium concentration 0.10 and then the hydroxide concentrations squared, so that would be hydroxide concentration squared is equal to 1.2 times 10 to the negative nine. So let's take a look at this take of the square root of both sides, and we will see that acts, which is the hydroxide concentration, is equal to 3.5 times 10 to the negative five. So now let's take the P O. H. Let's take the negative log. We will find that the P. O. H is 4.5, so the pH, which is 14 minus the peel age, would be 9.5. So if the pH of we're looking for has to be greater than 8.1 and it has to be less than or equal to 9.5, so that is the range in which we will precipitate the zinc, but not in the magnesium.

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