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SL/HL Chemistry Question (Please Help!)

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If someone could please help me answer this question, that would be great. I am really just trying to learn and understand this process.

 

1. Acetylsalicylic acid, also known as aspirin, C9H8O4, is synthesized by reacting salicylic acid, C7H6O3, with acetic anhydride, C4H6O3. 

 

C7H6O3+C4H6O3=C9H8O4+C2H4O2.

 

a) Calculate the theoretical yield, in g, of aspirin when 3.0 g of salicylic acid is reacted with 4.0 g of acetic anhydride. 

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In terms of the process, basically with these questions it's about determining how many moles of 1 substance are required to make x moles of another substance and most importantly, deciding what is the limiting factor. People struggle to understand limiting factors but it's very easy - it's a bit like a recipe. Hypothetically to explain it, if you wanted to make 3 pheasant pies and the ingredient list said - per pie, 1 pheasant, 3 carrots and 1 lump of pastry. And you had 3 pheasants, 9 carrots and one lump of pastry in your larder. Even though you have 3 pheasants and 9 carrots, you can't make 3 pheasant pies because you've only got one lump of pastry. The pastry is the limiting factor which means you'll only ever be able to make 1 pie. It's exactly the same with moles. 

 

So step one is always to balance your equations, figure out how many moles of each thing is needed so both sides worth out. How many moles of salicylic acid do you need per mole of acetic anhydride?

 

Step two is determining the limiting factor. To do this, you need to know how much of each thing you have. They've given you the weight above, but you need it in moles so you'll have to convert it. Use the weight and the atomic mass number of the element (available on the periodic table!) to figure out how many moles of salicylic acid you have, and how many moles of acetic anhydride you have.

 

Step three is pretty easy - once you know the limiting factor, you can put it into the equation. Say you need 2 salicylic acids (SA) per acetic anhydride (AA) (I'm just making this up btw, haven't calculated it) and you have 50 moles of SA and 100 moles of AA - well obviously the limiting factor is the SA. You've got loads of extra useless AA. So you put 50 moles of SA into your equation and then see how much aspirin it results in.

 

I hope that explains the method for answering these questions. They ask you all the time in Chemistry so it's worth really understanding it.

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In terms of the process, basically with these questions it's about determining how many moles of 1 substance are required to make x moles of another substance and most importantly, deciding what is the limiting factor. People struggle to understand limiting factors but it's very easy - it's a bit like a recipe. Hypothetically to explain it, if you wanted to make 3 pheasant pies and the ingredient list said - per pie, 1 pheasant, 3 carrots and 1 lump of pastry. And you had 3 pheasants, 9 carrots and one lump of pastry in your larder. Even though you have 3 pheasants and 9 carrots, you can't make 3 pheasant pies because you've only got one lump of pastry. The pastry is the limiting factor which means you'll only ever be able to make 1 pie. It's exactly the same with moles. 

 

So step one is always to balance your equations, figure out how many moles of each thing is needed so both sides worth out. How many moles of salicylic acid do you need per mole of acetic anhydride?

 

Step two is determining the limiting factor. To do this, you need to know how much of each thing you have. They've given you the weight above, but you need it in moles so you'll have to convert it. Use the weight and the atomic mass number of the element (available on the periodic table!) to figure out how many moles of salicylic acid you have, and how many moles of acetic anhydride you have.

 

Step three is pretty easy - once you know the limiting factor, you can put it into the equation. Say you need 2 salicylic acids (SA) per acetic anhydride (AA) (I'm just making this up btw, haven't calculated it) and you have 50 moles of SA and 100 moles of AA - well obviously the limiting factor is the SA. You've got loads of extra useless AA. So you put 50 moles of SA into your equation and then see how much aspirin it results in.

 

I hope that explains the method for answering these questions. They ask you all the time in Chemistry so it's worth really understanding it.

 

This has been extremely helpful!! Below I have written some of my workings. I am just getting a little confused after the point where I stopped at. Could you please guide me from there? Thank you so so much. 

 

1. The equation is already balanced.

2. I found the # of moles in SA and that was 0.021 mol. The # of moles in AA were 0.039 mol.

3. To find the limiting reagent, I used the mole ratios (1:1) and then cross multiplied. I learned that in 0.021 mol of C7H6O3, I need 0.021 mol of C4H603. We have 0.039 mol of C4H6O3 therefore it is in excess.

 

What would I do from there?

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In terms of the process, basically with these questions it's about determining how many moles of 1 substance are required to make x moles of another substance and most importantly, deciding what is the limiting factor. People struggle to understand limiting factors but it's very easy - it's a bit like a recipe. Hypothetically to explain it, if you wanted to make 3 pheasant pies and the ingredient list said - per pie, 1 pheasant, 3 carrots and 1 lump of pastry. And you had 3 pheasants, 9 carrots and one lump of pastry in your larder. Even though you have 3 pheasants and 9 carrots, you can't make 3 pheasant pies because you've only got one lump of pastry. The pastry is the limiting factor which means you'll only ever be able to make 1 pie. It's exactly the same with moles. 

 

So step one is always to balance your equations, figure out how many moles of each thing is needed so both sides worth out. How many moles of salicylic acid do you need per mole of acetic anhydride?

 

Step two is determining the limiting factor. To do this, you need to know how much of each thing you have. They've given you the weight above, but you need it in moles so you'll have to convert it. Use the weight and the atomic mass number of the element (available on the periodic table!) to figure out how many moles of salicylic acid you have, and how many moles of acetic anhydride you have.

 

Step three is pretty easy - once you know the limiting factor, you can put it into the equation. Say you need 2 salicylic acids (SA) per acetic anhydride (AA) (I'm just making this up btw, haven't calculated it) and you have 50 moles of SA and 100 moles of AA - well obviously the limiting factor is the SA. You've got loads of extra useless AA. So you put 50 moles of SA into your equation and then see how much aspirin it results in.

 

I hope that explains the method for answering these questions. They ask you all the time in Chemistry so it's worth really understanding it.

 

This has been extremely helpful!! Below I have written some of my workings. I am just getting a little confused after the point where I stopped at. Could you please guide me from there? Thank you so so much. 

 

1. The equation is already balanced.

2. I found the # of moles in SA and that was 0.021 mol. The # of moles in AA were 0.039 mol.

3. To find the limiting reagent, I used the mole ratios (1:1) and then cross multiplied. I learned that in 0.021 mol of C7H6O3, I need 0.021 mol of C4H603. We have 0.039 mol of C4H6O3 therefore it is in excess.

 

What would I do from there?

 

 

Sooo...

Assuming all of the above is correct (as I haven't checked your working) then I think you've got a bit confused by the limiting reagent bit. Cross multiplying anything is unnecessary. It's really simple.

 

If you need 1 mole SA + 1 mole AA to make 1 mole of Aspirin...

 

And you have 0.021mol SA + 0.039mol AA = ?mol Aspirin

 

Then you've logically got to have made 0.021mol Aspirin, and that's the answer. Yes you've got extra AA but because the ratio is 1:1 then the extra isn't doing anything.

 

Why? Because it's a 1:1 ratio. You have extra AA but it's useless because there's no SA to pair it off with. SA is the limiting factor, so assuming the reaction goes to 100% completion the amount of aspirin has to be equal to the amount of SA. Does that make sense?

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In terms of the process, basically with these questions it's about determining how many moles of 1 substance are required to make x moles of another substance and most importantly, deciding what is the limiting factor. People struggle to understand limiting factors but it's very easy - it's a bit like a recipe. Hypothetically to explain it, if you wanted to make 3 pheasant pies and the ingredient list said - per pie, 1 pheasant, 3 carrots and 1 lump of pastry. And you had 3 pheasants, 9 carrots and one lump of pastry in your larder. Even though you have 3 pheasants and 9 carrots, you can't make 3 pheasant pies because you've only got one lump of pastry. The pastry is the limiting factor which means you'll only ever be able to make 1 pie. It's exactly the same with moles. 

 

So step one is always to balance your equations, figure out how many moles of each thing is needed so both sides worth out. How many moles of salicylic acid do you need per mole of acetic anhydride?

 

Step two is determining the limiting factor. To do this, you need to know how much of each thing you have. They've given you the weight above, but you need it in moles so you'll have to convert it. Use the weight and the atomic mass number of the element (available on the periodic table!) to figure out how many moles of salicylic acid you have, and how many moles of acetic anhydride you have.

 

Step three is pretty easy - once you know the limiting factor, you can put it into the equation. Say you need 2 salicylic acids (SA) per acetic anhydride (AA) (I'm just making this up btw, haven't calculated it) and you have 50 moles of SA and 100 moles of AA - well obviously the limiting factor is the SA. You've got loads of extra useless AA. So you put 50 moles of SA into your equation and then see how much aspirin it results in.

 

I hope that explains the method for answering these questions. They ask you all the time in Chemistry so it's worth really understanding it.

 

This has been extremely helpful!! Below I have written some of my workings. I am just getting a little confused after the point where I stopped at. Could you please guide me from there? Thank you so so much. 

 

1. The equation is already balanced.

2. I found the # of moles in SA and that was 0.021 mol. The # of moles in AA were 0.039 mol.

3. To find the limiting reagent, I used the mole ratios (1:1) and then cross multiplied. I learned that in 0.021 mol of C7H6O3, I need 0.021 mol of C4H603. We have 0.039 mol of C4H6O3 therefore it is in excess.

 

What would I do from there?

 

 

Sooo...

Assuming all of the above is correct (as I haven't checked your working) then I think you've got a bit confused by the limiting reagent bit. Cross multiplying anything is unnecessary. It's really simple.

 

If you need 1 mole SA + 1 mole AA to make 1 mole of Aspirin...

 

And you have 0.021mol SA + 0.039mol AA = ?mol Aspirin

 

Then you've logically got to have made 0.021mol Aspirin, and that's the answer. Yes you've got extra AA but because the ratio is 1:1 then the extra isn't doing anything.

 

Why? Because it's a 1:1 ratio. You have extra AA but it's useless because there's no SA to pair it off with. SA is the limiting factor, so assuming the reaction goes to 100% completion the amount of aspirin has to be equal to the amount of SA. Does that make sense?

 

 

Thank you so much! This makes sense to me now. You've explained it really well :) 

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