The concentrations of various cations in seawater, in moles per liter, are lon Na^+ Mg

The concentrations of various cations in seawater, in moles...

Key Concepts

Common Ion Effect

The common ion effect refers to the shift in equilibrium that occurs when an ion common to the dissolution equilibrium of a sparingly soluble salt is added to the solution. This additional presence of the common ion decreases the solubility of the salt, causing precipitation to occur at lower concentrations of the precipitating agent. This concept is critical for controlling precipitation processes and for understanding how the simultaneous presence of multiple ions influences solubility.

Selective Precipitation

Selective precipitation is a technique used to separate ions in solution by exploiting differences in their solubility products (Ksp values). By carefully adjusting the concentration of a precipitating agent, one can preferentially precipitate one ion while keeping others in solution. This method is essential in analytical chemistry for isolating specific ions from a mixture based on the differing thresholds for precipitation.

Reaction Stoichiometry in Precipitation

Reaction stoichiometry in precipitation involves using the balanced chemical equation to relate the amount of ions in solution to the mass of the precipitate formed. This relationship enables quantitative analysis, allowing chemists to calculate how much solid will form from given concentrations of reactants, as well as to determine the extent of precipitation of different ions when a known quantity of precipitating agent is added.

Solubility Product (Ksp)

The solubility product is a constant that expresses the equilibrium between a solid ionic compound and its dissolved ions in a saturated solution. For a sparingly soluble salt, the product of the molar concentrations of its constituent ions, each raised to the power of its coefficient in the dissolution equation, must not exceed this constant. When the ionic product reaches or exceeds the Ksp, precipitation occurs, as the solution can no longer sustain the dissolved species in equilibrium.

Precipitation Reactions

Precipitation reactions involve the formation of a solid from the reaction of two aqueous solutions. In these reactions, when the product of the ion concentrations in solution exceeds the solubility product, an insoluble precipitate forms. This concept is fundamental in determining the conditions under which a particular salt will start to precipitate and is used to predict and control the formation of solids from ions in solution.

Transcript

00:01 Okay, so we're going to decide when our magnesium hydroxide precipitates.

00:05 Okay, so ksp for magnesium hydroxide is going to be a concentration of the magnesium ion times the concentration of our hydroxide squared.

00:19 So ksp for magnesium hydroxide is 6 times 10 to the negative 12.

00:26 So that's going to be the concentration of our magnesium, which is 0 .056.

00:32 Times concentration of oh minus squared.

00:36 So we should have precipitation when our oh minus is 1 times 10 of the minus 5 molar.

00:51 So then we're going to try to figure out at that concentration of hydroxide, would any of these other ions precipitate? well, for the sodium ion, sodium ion is never going to precipitate, right? it's highly soluble, so we wouldn't even have to, find a ksp for that.

01:11 So solids with sodium ion in them are always soluble.

01:15 Okay, so let's move on to for calcium ion.

01:22 What we're going to do is we're going to solve for q.

01:25 So q is our calcium ion concentration times hydroxide squared.

01:33 So the calcium is given as 0 .01, and we know that hydroxide is 1 times 10 and minus 5 and we'll square that.

01:43 So q comes out to be 1 times 10 of the negative 12.

01:48 This is going to be smaller than 1 times 10 to negative 10, which is the ksp.

01:56 So no precipitate here.

02:00 Okay, so we won't see a precipitate of calcium hydroxide.

02:06 How about for aluminum? so q is a concentration of aluminum ion times hydroxide, and that'll be cubed.

02:23 So that's going to be equal to the concentration of the aluminum ion, which is given as 4 times 10 in the negative 7.

02:31 And then 1 times 10 of the minus 5 cubed.

02:36 Okay, so our q comes out to be 4 times 10 of the minus 22.

02:43 And this is going to be larger than 2 times 10 of the negative 31.

02:48 Which is the ksp.

02:51 So we will see a precipitate of aluminum hydroxide.

03:04 And then for the iron, for the fe3 plus, so q is a concentration of your fe3 plus times the oh minus cubed, and that's going to be equal to 2 times 10 of the minus 7 is given for the iron 3 ion.

03:28 And again, 1 times 10 of the minus 5 cubed.

03:33 So q is going to be 2 times 10 to the negative 22, and that is going to be larger than 3 times 10 of the negative 39, which is our ksp.

03:48 So we will see a precipitate of the iron 3 hydroxide.

04:01 Okay, so if 50 % of our magnesium ion is precipitating, then the concentration of our magnesium ion is going to cut in half.

04:14 So it was 0 .056.

04:17 So we'll divide that by 2, and it will be 0 .028 molar.

04:25 So again, we'll plug in our ksp expression now for magnesium hydroxide to find the hydroxide ion concentration.

04:36 So 6 times 10 to the negative 12 is 0 .0 .0 .0 .1.

04:44 028 times a concentration of oh minus squared.

04:48 So the concentration of oh minus comes out to be 1 times 10 of the negative 5.

04:55 Well, that's what it was in part a.

04:58 The problem is that we only have one sig -fig.

05:01 All right, so all that really means is that we're still just going to see only, the only thing we're going to see precipitates for are aluminum hydroxide and fe o .h .h.

05:16 H3, because our hydroxide concentration isn't changing, so only these two things will precipitate.

05:24 So what we'll do is we can see, we want to figure out now how much of them are precipitating.

05:30 So let's take a look at our aluminum.

05:38 So ksp is a concentration of aluminum times hydroxide's cubed.

05:48 So 2 times 10 and negative 31.

05:53 Looking for our concentration of aluminum ion here...

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