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

In most of its ionic compounds, cobalt is either $\mathrm{Co}(\mathrm{II})$ or $\mathrm{Co}(\mathrm{III})$. One such compound, containing chloride ion and waters of hydration, was analyzed, and the following results were obtained. A $0.256-\mathrm{g}$ sample of the compound was dissolved in water, and excess silver nitrate was added. The silver chloride was filtered, dried, and weighed, and it had a mass of $0.308 \mathrm{~g}$. A second sample of $0.416 \mathrm{~g}$ of the compound was dissolved in water, and an excess of sodium hydroxide was added. The hydroxide salt was filtered and heated in a flame, forming cobalt(III) oxide. The mass of cobalt(III) oxide formed was $0.145 \mathrm{~g}$. a. What is the percent composition, by mass, of the compound? b. Assuming the compound contains one cobalt atom per formula unit, what is the molecular formula? c. Write balanced equations for the three reactions described.

Transcript

00:01 Okay, we're told that we have a compound with cobalt in it that contains chloride and waters of hydration.

00:11 We analyzed it and got the following results.

00:18 0 .256 grams of compound.

00:26 It's dissolved in water.

00:36 Ag, n -03 added.

00:44 Ag -c -l has as dried in water.

00:50 Filtered and it has a mass of 0 .308 grams.

00:57 Then a second sample, second sample, and that has a mass of 0 .416 grams and we get cobalt 3 oxide at 0 .145 grams.

01:28 And then we're asked to find the percent composition by mass and the molecular formula and write balanced equations for the three reactions described.

01:52 So i've got one where we add silver nitrate to get silver chloride and then we add silver hydroxide, get hydroxide salt, and then the hydroxide forms cobalt oxide.

02:07 Okay, let's see if i can get this.

02:19 Okay.

02:34 So we're going to have to figure out a lot of different things here.

02:38 First, i'm going to go look up something super quick here.

03:07 Okay, so we've got, i'm thinking on the fly here.

03:21 So i've got cobalt chloride.

03:50 I'm thinking what i've got here.

03:52 So first of all, let's figure out our chloride.

03:59 I've got 0 .308 grams of silver chloride.

04:06 And we're going to convert this to chlorine.

04:17 Molar mass, ag, c, l.

04:24 143 .32.

04:51 And then for chlorine, we'll use 35 .45.

05:07 35 .45 times 0 .308 divided, 143 .32.

05:17 And i get 0 .0 .32.

05:23 6 .2 grams of chloride.

05:33 0 .762 grams of chloride and 0 .076.

05:46 And then i'm gonna use 183 grams of chloride divided by 0 .256 times 100 equals.

06:08 29 .76 % cl.

06:28 Having trouble finding a solution here.

06:52 Okay, then my next one i have, so now let us figure out from our second one, i have 0 .0145 grams of cobalt.

07:21 So let's figure out our cobalt next.

07:41 And that has a molar mass of 165 .9 grams per mole.

07:55 I have a 2 to 1 mole ratio of cobalt to ball 3, i mean, and 58 .93 grams of cobalt per mole.

08:22 Oh my goodness.

08:24 And that's going to be 0 .103.

08:28 Grams of cobalt, then i'm going to take 0 .103 grams of cobalt divided by 0 .416 grams times 100 will equal 24 .8 percent cobalt.

08:58 So let's take a look.

09:00 So now i know that since i've got cobalt chloride and i don't know what this is, this is x times x.

09:12 H2o.

09:15 My cobalt is my water is going to be 100 -100 minus 24 .8 minus 29 .8 will be 45 .5 % water.

09:57 So let me see what i've got here.

10:01 What is the percent composition by mass of the compound? so i get 24 .8 % 0 .8 % cobalt and i get 45 .5 % h2o.

10:23 And if you wanted me to split that up, i could figure that up by my h and o by saying 45 .5 and water is 2 divided by 18 .02, 2 .02, 2 .2.

11:04 This times 0 .1121 equals i'm just going to go 0 .112 and that would equal 5 times 45 .5 equals 5 .10 and this would be 45 .5 times 15 .5 times 16 divided by 18 .02 times 0 .88 equals 45 .5 times point 888 equals 40 .4...

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