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Compare the molar solubility of $\mathrm{Mg}(\mathrm{OH})_{2}$ in water and in a solution buffered at a pH of 9.0

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in water: $s=1.44 \cdot 10^{-4} \mathrm{M}$in a solution buffered at pH $9.00: \mathrm{s}=0.12 \mathrm{M}$

Chemistry 102

Chapter 16

Acid-Base Equilibria and Solubility Equilibria

Acid-Base Equilibria

Aqueous Equilibria

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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. So today we're going to be looking at magnesium hydroxide and looking at its Moeller Salyer bility in different situations. So first, let's talk about the soluble ity of among magnesium hydroxide in, ah, just water. So just normal circumstances which is taking magnesium hydroxide, were dissolving it in water. So let's draw the equilibrium that's taking place. Magnesium hydroxide is dissociating partially to magnesium hydroxide and to hydro's magnesium and to hydroxide ions. So initially we have zero concentrations of both. Then we're going to to solve a certain amount of moles of magnesium hydroxide, which will create one US. They'll be on 1 to 1 ratio. So if we don't solve one mole of magnesium hydroxide, then we will get one mole of magnesium ions per volume, so fear dissolving the maximum that we can. That would just be the Mueller Souljah bility, but then notice for everyone mold magnesium hydroxide that's dissolved. We get to hydroxide ion, so that means we're gonna have to do it will increase by twice the Mueller Celje ability. And so now it's drier. Case writer kiss p expression. So the case P is equal to the concentration magnesium times, the concentration of hydroxide squared because it has a two coefficient in front. So our case p for magnesium hydroxide is one 0.2 times 10 to the negative 11. Now we plug in our Moeller. So you ability brother concentrations. We'll get four. A skewed will be equal toe 1.2 times 10 to the negative 11. So let's divide by four. So it will be as cubed is equal to three times 10 to the negative 12. And so let's take the Cuba crude and we will get one point for times 10. Teoh negative for for the Mueller Celje bility. So now let's talk. Take a look at this different situation. We're solving magnesium hydroxide, but the but it's buffered at Ph of nine. So basically that means when we add magnesium hydroxide well, what happens when we dissolved magnesium hydroxide hydroxide hands were produced, but because the solution is buffered, the concentration of the hydroxide will stay constant at Ph. Nine. So first off, let's find out the concentration of hydroxide. If the peaches nine So the Ph nine, the P. O. H. Would be 14 minus the pH. So it's five. So that means that the concentration of hydroxide is fixed at 10 to the negative fifth similarity. So what does that mean? So I equilibrium. That means the concentration of magnesium. We'll be the Moeller Celje bility, while the concentration of hydroxide will be fixed at 10 to the negative five. Similarity. So now let's take a look at our case P expression again. So our kiss p as much engine before it's the concentration of magnesium two plus the constitution of hydroxide squared. So our case p is 1.2 times 10 to the negative 11. The concentration of magnesium is air Moeller Saul, you bility and our concentration of hydroxide is fixed at to tend to the negative fists. So that would be 10 to the negative temp. So what happens when we divide 1.2 times 10 to the negative, 11th by 10 to the negative fifth, We will get that. The Mueller Celje bility is 0.12 polarity

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