Molarity question i'm not prepared to answer

[FrankP]

We're just beginning stoichiometry and molarity we have been on this chapter for 1 week now and this is a homework question. I have thought about how to start this problem several times so far and I have decided that I have no clue.

 

A chemist was asked to analyze a solution of chlordane, C₁₀H₆Cl₈,dissolved in a hydrocarbon solvent that was discovered by construction workers during demolition of an old work shed. This insecticide was banned for sale in the United States in 1988 by the EPA because of its potential for causing cancer. Reactions were carried out on a 2.017 g sample of the solution, which converted all of the chlorine to chloride ion dissolved in water. This aqueous solution required 72.61 mL of 0.2020 M AgNO₃ to precipitate all of the chloride ion as AgCl. What was the percentage of chlordane in the original solution?

The precipitation reaction was

Ag⁺(aq) + Cl^-(aq) → AgCl(s)

 

 

I know I can use the information here to get G of AGNO3 and also get a few other things but the problem is I don't know how that relates to the chemical equation provided.

I keep thinking this is a limiting reactant question in the sense that they are telling you "This aqueous solution required 72.61 mL of 0.2020 M AgNO₃ to precipitate all of the chloride ion as AgCl."

 

The only thing I can think of that 72.61 mL of 0.2020 M AgNO₃ is the limiting reactant and so using that I could transition to total potential product possible and use that conversion factor to try and solve with this second part of the question. Either that or like I said above I have no clue as I suspected. lol

[studiot]

 

 

I know I can use the information here to get G of AGNO3 and also get a few other things but the problem is I don't know how that relates to the chemical equation provided.

 

A good start might be to say

 

Let x be the number of grams of chlordane in the sample

 

Then 100(x/2.017) = required % chlordane

 

What do you think comes next?

Edited November 3, 2016 by studiot

[FrankP]

To be honest, at this point I have no clue I have tried it several times and I can't arrive at an answer at all I cant figure out how to set the problem up

[hypervalent_iodine]

You're not really interested in grams of silver nitrate, you're interested in the number of moles of Cl^- that were required to react with all of the AgNO₃. This is not a limiting reagent question, this is a volumetric / gravimetric analysis question. Limiting reagent questions are usually given where they provide the masses / volumes of two things in the equation. Here they have only given you one, not including the initial sample mass.

 

Studiot has kind of skipped to the end of the question, which doesn't help you in the initial stages. Some key points that might help you make sense of it:

 

1. The 2.017 g of solution is a solution of the chlordane, but it doesn't just contain chlordane. It might also contain water, or some other solvent. It doesn't matter what the other stuff is, so long as you can recognise that only some percentage of that 2.017 g is chlordane (that is what the question is asking about after all).
2. The question has asked you for a mass percentage of this compound in the solution, which means you have to find the mass of chlordane in the solution at some point. If you have the molecular formula, you can calculate the molar mass, but you are still missing one key number (what is it?)
3. All of the chlorine in chlordane was turned into Cl- and reacted with AgNO₃ in the equation that you correctly identified.
4. You know that the molecular formula of chlordane is C₁₀H₆Cl₈. If you know that all of the Cl^- used in the reaction came from chlordane, and you know how many atoms of Cl there are in one molecule of chlorine, then you should be able to calculate that key number I mentioned in point 1.

[Sensei]

To be honest, at this point I have no clue I have tried it several times and I can't arrive at an answer at all I cant figure out how to set the problem up

 

Then don't try to arrive to final answer immediately, but show us what you did so far,

starting from calculating moles of newly produced compound in reaction.

Not AgCl, as it's already known. AgCl precipitated. What is "left behind".. ? How many moles? What mass?

Edited November 4, 2016 by Sensei

[hypervalent_iodine]

 

Then don't try to arrive to final answer immediately, but show us what you did so far,

starting from calculating moles of newly produced compound in reaction.

Not AgCl, as it's already known. AgCl precipitated. What is "left behind".. ? How many moles? What mass?

 

 

What you've written here is a bit confusing. The compound produced in the key reaction is AgCl (which you are right to say you don't need to calculate). What do you mean by 'left behind'? You aren't worried about anything that's 'left behind' in this question.

[studiot]

 

 

I know I can use the information here to get G of AGNO3 and also get a few other things but the problem is I don't know how that relates to the chemical equation provided.

 

Have you done this?

 

If not, why not, and if so why did you not post the calculation as sensei asked?

 

 

Studiot

What do you think comes next?

FankP

To be honest, at this point I have no clue I have tried it several times and I can't arrive at an answer at all I cant figure out how to set the problem up

 

Well I would say using the start I offered, you could calculate the number of grams of chlorine in the original sample, in terms of x.

 

Can you do this?

[FrankP]

 

You're not really interested in grams of silver nitrate, you're interested in the number of moles of Cl^- that were required to react with all of the AgNO₃. This is not a limiting reagent question, this is a volumetric / gravimetric analysis question. Limiting reagent questions are usually given where they provide the masses / volumes of two things in the equation. Here they have only given you one, not including the initial sample mass.

 

Studiot has kind of skipped to the end of the question, which doesn't help you in the initial stages. Some key points that might help you make sense of it:

 

1. The 2.017 g of solution is a solution of the chlordane, but it doesn't just contain chlordane. It might also contain water, or some other solvent. It doesn't matter what the other stuff is, so long as you can recognise that only some percentage of that 2.017 g is chlordane (that is what the question is asking about after all).
2. The question has asked you for a mass percentage of this compound in the solution, which means you have to find the mass of chlordane in the solution at some point. If you have the molecular formula, you can calculate the molar mass, but you are still missing one key number (what is it?)
3. All of the chlorine in chlordane was turned into Cl- and reacted with AgNO₃ in the equation that you correctly identified.
4. You know that the molecular formula of chlordane is C₁₀H₆Cl₈. If you know that all of the Cl^- used in the reaction came from chlordane, and you know how many atoms of Cl there are in one molecule of chlorine, then you should be able to calculate that key number I mentioned in point 1.

Ok so I'm following what you are saying. However, I'm still stuck where do you start. This is the first day im doing these kinds of problems so idk if its wrong of me to assume that this problem is too hard for me but im going to assume that anyway. I know you can get conversion factors from the problem and all that but how am I supposed to get anything out of this problem. Nothing is given it's saying hydrocarbon solution which means nothing. Everything is a hydrocarbon solution.

 

To me, this problem says this. "You are at a car dealership with $200 in your pocket. There are 2000 cars at the dealership. 300 are red. You're father was born in before you were. What state do you live in?"

 

None of the prior information builds off of anything that comes before it, well at least to me. I know you guys are trying to help me and I'm following all of your logic. My problem isn't that im not smart enough to figure out what it's asking me my problem is again... I don't know how to set up the solution to the problem because I just learned this on Wednesday. I'm 25 years old and the last time I had chemistry was 2006/2007.

 

To me, the problem is this... The chemical reaction showing in the formula shows me nothing because its ionic compounds mixing together which I understand that the formula is showing you what was meant by this statement "Reactions were carried out on a 2.017 g sample of the solution, which converted all of the chlorine to chloride ion dissolved in water" but what the hell does this h ave to do with the C₁₀H₆Cl₈? Just simply the fact that 8moles of Cl will be converted into "chloride ions?"

 

Have you done this?

 

If not, why not, and if so why did you not post the calculation as sensei asked?

 

 

Well I would say using the start I offered, you could calculate the number of grams of chlorine in the original sample, in terms of x.

 

Can you do this?

 

I could, however, I can't get past the value 100 (X/2.017) what's the 100 representing why am I using that. I'm very skeptical when it comes to starting problems with arbitrary numbers.

 

Then don't try to arrive to final answer immediately, but show us what you did so far,

starting from calculating moles of newly produced compound in reaction.

Not AgCl, as it's already known. AgCl precipitated. What is "left behind".. ? How many moles? What mass?

 

One last time. I don't know how to start the problem. Therefore what I have done so far is attempt to start the problem with no avail.

Ok so I'm following what you are saying. However, I'm still stuck where do you start. This is the first day im doing these kinds of problems so idk if its wrong of me to assume that this problem is too hard for me but im going to assume that anyway. I know you can get conversion factors from the problem and all that but how am I supposed to get anything out of this problem. Nothing is given it's saying hydrocarbon solution which means nothing. Everything is a hydrocarbon solution.

 

To me, this problem says this. "You are at a car dealership with $200 in your pocket. There are 2000 cars at the dealership. 300 are red. You're father was born in before you were. What state do you live in?"

 

None of the prior information builds off of anything that comes before it, well at least to me. I know you guys are trying to help me and I'm following all of your logic. My problem isn't that im not smart enough to figure out what it's asking me my problem is again... I don't know how to set up the solution to the problem because I just learned this on Wednesday. I'm 25 years old and the last time I had chemistry was 2006/2007.

 

To me, the problem is this... The chemical reaction showing in the formula shows me nothing because its ionic compounds mixing together which I understand that the formula is showing you what was meant by this statement "Reactions were carried out on a 2.017 g sample of the solution, which converted all of the chlorine to chloride ion dissolved in water" but what the hell does this h ave to do with the C₁₀H₆Cl₈? Just simply the fact that 8moles of Cl will be converted into "chloride ions?"

 

I could, however, I can't get past the value 100 (X/2.017) what's the 100 representing why am I using that. I'm very skeptical when it comes to starting problems with arbitrary numbers.

 

One last time. I don't know how to start the problem. Therefore what I have done so far is attempt to start the problem with no avail.

[hypervalent_iodine]

C10H6Cl8 is chlordane. All you really need to take away from the first reaction is that all of the Cl- ions came from the C10H6Cl8. You can calculate the number of moles of Cl- based on the stoicheometry of the reaction between it and AgNO3 (you're given concentration and volume of this). If you have that and if you know that there is one molecule of C10H6Cl8 per 8 molecules of Cl, then you should be able calculate moles of C10H6Cl8.

 

The 100(x/2.017) is skipping ahead. It is for the last part of the question where you calculate the mass percentage of chlordane in the solution. With units it is: 100% x (mass of chlordane / 2.017 g) = mass percentage of chlordane in the solution.

[studiot]

If I told you that in a box of 50 apples, 5 were red, and asked you what percentage of apples were red how would you calculate the %?

 

You have yet to show us a single calculation.

 

What is the molecular weight of chlordane?

 

What % of this is chlorine?

 

 

Hypervalent iodine.

 

Whilst it may be more elegant to work in the direction you offer, working in the opposite direction is perhaps conceptually easier for some.

[FrankP]

How's this? Did I do my work right?

 

It won't let me attach the screenshot of my work I scanned the paper I worked the problem out on how do you attach it?

Edited November 6, 2016 by FrankP

[hypervalent_iodine]

How's this? Did I do my work right?

 

It won't let me attach the screenshot of my work I scanned the paper I worked the problem out on how do you attach it?

Just saw this. You are very close, but I am not sure where the 3.935 g comes from at the end. The question states that the solution has a mass of 2.017 g. Everything else looks correct.