In most of its ionic compounds, cobalt is either Co(II) or Co(III). One such compound, containin

In most of its ionic compounds, cobalt is either Co(II) or...

Key Concepts

Hydrated Ionic Compounds

Hydrated ionic compounds contain water molecules integrated into their crystal structure, known as waters of hydration. Understanding the role of these water molecules is important when determining the empirical formula and percent composition of a compound, as they contribute to the total mass and influence the stoichiometry of the compound.

Precipitation Reactions

Precipitation reactions involve the formation of an insoluble solid (a precipitate) when two soluble substances react. In analytical chemistry, these reactions are often used to isolate and quantify ions by converting them into a product with low solubility, which can then be separated, dried, and weighed for further analysis.

Gravimetric Analysis

Gravimetric analysis is an analytical technique that involves the isolation and weighing of a compound to determine the amount of a specific element or ion within a sample. By converting the analyte into a precipitate of known composition, its mass can be measured accurately and used to back-calculate the concentration or percentage of the target component in the initial compound.

Percent Composition Calculation

Percent composition calculation involves determining the percentage by mass of each element in a compound. This calculation is essential in analytical chemistry to understand the makeup of a compound, verify its purity, and deduce its empirical formula by comparing the mass contributions of its constituent elements relative to the whole sample.

Chemical Stoichiometry

Chemical stoichiometry refers to the quantitative relationships between reactants and products in a chemical reaction. It uses the mole concept to relate masses to numbers of atoms or molecules and ensures that calculations are consistent with the conservation of mass. Stoichiometry is key in converting measured masses of reaction products into the amount of reactant present in the original sample.

Transcript

00:01 This question says that in most of its ionic compounds, cobalt is either cobalt 2 or cobalt 3.

00:07 One such compound containing chloride ions and waters of hydration was analyzed and the following results were obtained.

00:15 A 0 .256 gram sample of the compound was dissolved in water and excess silver nitrate was added.

00:21 The silver chloride was filtered, dried, weighed, and the mass was 0 .308 grams.

00:26 Then the second sample, 0 .416 grams of the compound was dissolved in water, excess sodium hydroxide was added.

00:33 Hydroxide salt was filtered, heated in a flame, forming cobalt 3 oxide, and the mass of cobalt 3 oxide was 0 .145 grams.

00:43 And now for the questions, it asks us what the percent composition by mass of the compound is, what the formula of a compound is, and the balanced equations for those three reactions we just read.

00:58 So let's start with a.

01:00 What is the present composition by mass of the compound? so we need to figure out what percentage of this compound is cobalt, chloride, hydrogen, and oxygen.

01:09 So for the first reaction where you're adding silver nitrate, i've written kind of a general equation where we don't know a few of the term.

01:17 We don't know what this compound is.

01:20 If it's cobalt two, this would be a two.

01:22 If it's cobalt three, this x would be a three.

01:24 And we also don't know how many water molecules are coordinated to it.

01:27 So i have that with y here.

01:29 And it's unbattle.

01:30 But it does tell us that we start with 0 .256 grams of the compound and when we have excess silver nitrate we form 0 .308 grams of the precipitate silver chloride from this we can figure out how much chloride there is in the original compound and so tell we start with that that mass point three zero eight we divide by the molar mass of silver chloride and those cancel we then convert between moles of silver chloride and moles of chloride there's only one chlorine per silver chloride so it's a one -to -one ratio those moles cancel and then we can simply multiply by the molar mass of chlorine these cancel and we're left with grams of chlorine that's 0 .076 so there are 0 .076 grams of chlorine in this product that means there must have been 0 .076 grams of chlorine in the original so if that's the amount of chlorine and we know the amount of the original compound, the total amount, we just simply divide those two numbers and we can see that we have 29 .7 % chlorine by mass in this unknown compound.

02:47 So that's one piece of information.

02:50 In those second and third reactions, we figure out how much cobalt there is.

02:57 So again, general equation for mixing with sodium hydroxide and then this product is heated on the carbon.

03:03 A flame or combined with oxygen as being the same thing to form cobalt three oxide and tells us we form 0 .145 grams of that precipitate and so we can do the same calculation we can say that um put this one in but if we were to take the mass that we got from the product and we were to divide it by the molar rass which is 165 .86 grams of cobalt.

03:40 Volt three oxide, those grams cancel.

03:48 And then if we go between moles of cobalt three oxide and moles of cobalt, there are two cobalts per molecule of cobalt three oxide.

03:56 So it's a two to one ratio.

03:59 These will cancel.

04:00 And then finally multiply by the molar mass of cobalt to be left with grams of cobalt is 0 .103 grams.

04:08 And we do the same calculations before, but we had a different amount of starting material this time.

04:12 So we divide how much cobalt we had, because all of it had to have come from this original unknown compound.

04:17 We divide it by how much of that compound we had.

04:20 We can see that we have 24 .8 % cobalt by mass.

04:24 So there's two pieces of information.

04:28 And that's what i've written here, 29 .7 % chlorine, 24 .8 % cobalt.

04:33 But we don't know anything about this water, so we can kind of intuitively figure this out.

04:39 We know that the total percent mass has to add up to 100, and we have 54 .5 percent between cobalt and chlorine, so that means the other 45 .5 percent has to be coming from the water, the hydrogens, and the oxygens.

04:52 That's one thing we know.

04:54 We also know that every water has the same composition, has two hydrogens and one oxygen.

05:00 If you divide the mass of those hydrogens by the total mass of water, you get 11 .2 percent hydrogen, and then if you divide the mass of oxygen by the mass of water, we get 88 .8 % oxygen.

05:12 So we know that this is the proportion of each of those atoms to a water molecule.

05:16 We can just simply take 11 .2 % of this remaining percent mass to see that there is 5 .09 % hydrogen in this unknown, and there's 40 .4 % oxygen in the unknown.

05:29 And you can double -check your work and add these up, and they should come out really close to 100%.

05:34 So now we have the percent masses of each of the atoms that make up this unknown compound.

05:40 That's part a solved.

05:42 Part b says that assuming the compound contains one cobalt ion per formula unit, what is the formula? it's a little bit more tricky.

05:53 So what i did was, let's pretend, based on these percent masses, that we had 100 grams of the compound.

06:02 100 is just a nice round number...

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