Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: peterpan1372 on September 08, 2017, 03:52:03 PM

Can someone help me out with this one?
I have 105cm^3(2*100+10)/2)

Per forum rules you have to show your attempts at solving the problem.

I already have?

I already have?
Ok then. Why does:
Can someone help me out with this one?
I have 105cm^3(2*100+10)/2)
give the correct answer, or why doesn't it? You add a bunch of numbers, 2 100 10 slash 2  where did those come from? Why are they there, instead of not there, or somewhere else?
Are you guessing? What good will that do the next person who uses this thread?
Or do you know what you're doing, and just looking for us to check, to insure you get the bestesterest possible grade number?
Don't do those things on our forum, that's not what where here for.

All I know is that 1mole of any gas occupies 24dm^3. We would have then 48dm^3 + 24dm^3 ... however, from here on i do not know further...

All I know is that 1mole of any gas occupies 24dm^3.
Remember that 1 mole of gas doesn't always occupy 24 L of space, however, as long as we keep this assumption consistent throughout our calculations (as temperature and pressure don't change) we should be fine.
We would have then 48dm^3 + 24dm^3
I don't understand your logic here. The question specifically says that we initially have 100 mL of X and 10 mL of y... You also know that 1 mole of gas occupies 24 L of space (from your assumption)...
EDIT

well, obviously Y is the limiting factor since it is less available than X, however i do not know how to calculate the rest..

I seriously do not know why they state in the solutions that 100cm^cm would be the right answer...

Do you know Avogadro's hypothesis?
With what volume of X will 10 mL of Y react?
What will be the volume of Z produced?
By how much will the volume of the mixture change?

well, 10ml of y react with a volume of 10ml of x or not?. but i do not know how much they produce.. i first thought of adding them up, but 110ml is obviously wrong... since the answer is 100ml, i thought of multiplying them, but do not know the sense behind it since you have to add them up...

well, 10ml of y react with a volume of 10ml of x or not?
No, you are guessing, it won't work.
Do you know Avogadro's hypothesis?

we have 100ml of x reacting with 10ml of y, giving 110ml of z... thats for me the only logical way, since we have to add them up...

well, ive looked up what it says, but do not know how to adapt it into our context...
Can you state what it says?
Do you remember that things react in molar ratio?
we have 100ml of x reacting with 10ml of y, giving 110ml of z... thats for me the only logical way, since we have to add them up...
This logic is wrong, as you have to follow stoichiometry.

equal volumes of gases at the same temperature and pressure contain equal numbers of molecules...
the ratio is 2:1...

and no, I do not know how to transfer it into our context...

equal volumes of gases at the same temperature and pressure contain equal numbers of molecules...
And what is the relationship between moles and number molecules?
If you have identical numbers of molecules of two substances, does it mean there are identical numbers of moles?

we CANT know the number of moles since they are unknown...
I never asked you whether we know number of moles. I asked:
And what is the relationship between moles and number molecules?
If you have identical numbers of molecules of two substances, does it mean there are identical numbers of moles?
Your problems stem from the fact you don't know basic definitions and you don't know how to apply them. Even if we give you answers you will be still not able to solve problems, because you will not understand what you are doing. I am trying to guide through the correct line of thinking, but  sorry to say that  you are not even trying to understand questions I am asking.

I think that I got I now!
correct me if I am wrong: since any gas has (under sam temp. and pres.) the same amount of molecules, and x and z have both 2 molecules, z must be 100, too, since x is 100 as well, or?

correct me if I am wrong: since any gas has (under sam temp. and pres.) the same amount of molecules
Same number of molecules in SAME volume. Please  be precise in what you write, you are leaving out details as if they were not important and in effect your statements become incorrect or ambiguous. That in turn means they need to be corrected as we have no way of knowing whether you just left them by mistake or you still have no idea what you are writing about  that's one of the reasons why we don't move forward.
and x and z have both 2 molecules, z must be 100, too, since x is 100 as well, or?
You are confusing things.
X and Z don't have "two molecules". As you said somewhere earlier, quite correctly, we don't know number of molecules of x nor z. What we do know is that:
a. one molecule of Y reacts with 2 molecules of X and produce 2 molecules of Z (that in turn means n Y molecules reacts with 2n X molecules producing 2n Z molecules)
b. per Avogadro's hypothesis under same conditions identical volumes of gas contain identical numbers of molecules.
What conclusion about volumes of reacting gases can you draw from these two facts? Can you calculate what volume of X will react with 10 mL of Y, and what volume of Z will be produced?

yes, since 10 ml of y reacts with 100ml of 2* x, one x would equal 10 ml, too... is that right?

but why wouldn't my initial argument be right, too?
Avogrado states: equal volumes of all gases, at the same temperature and pressure, have the same number of molecules.
One can also say (because it is the same effect):same number of molecules, at the same temperature and pressure, have the equal number of volumes...
Since the type of gas doesnt matter => x=z
since we have also 2x and 2z, we also have the same volumes: 2x=100ml and 2z=100ml..

yes, since 10 ml of y reacts with 100ml of 2* x, one x would equal 10 ml, too... is that right?
10 mL of Y doesn't react with 100 mL of X, 10 mL of Y is mixed with 100 mL of X. Most of X won't react  do you understand why? There is a grain of truth in what you said about "one X". However, try to answer question I asked: what volume of X reacts and what volume of Z is produced.
One can also say (because it is the same effect):same number of molecules, at the same temperature and pressure, have the equal number of volumes...
No such thing as "equal number of volumes". I suppose you mean "same volumes".
since we have also 2x and 2z, we also have the same volumes: 2x=100ml and 2z=100ml.
If I understand correctly what you mean, you are confusing gas present with gas reacting. So no, we don't have 100 mL of X and 100 mL of Z.
Final volume is a sum of Z produced and whatever gas is left.

ok, so we have 2 molecules of x reacting with one molecule of y,
then 20ml of x react with 10ml of y, right?

ok, so we have 2 molecules of x reacting with one molecule of y,
then 20ml of x react with 10ml of y, right?
Exactly.
And what volume of Z is produced?
Knowing what volume of X reacted with what volume of Y (that is: they were consumed) and what volume of Z was produced, can you calculate by how much has the volume changed?

There are 30ml of z produced... but normally we must have 110ml of z, or?

There are 30ml of z produced...
No. Look at the stoichiometry, as given by the balanced reaction equation. Recheck what you read at the page I linked to earlier in the other thread (page about reading reaction equations (http://www.chembuddy.com/?left=balancingstoichiometry&right=stoichiometriccalculations)).
but normally we must have 110ml of z, or?
No. It is not about adding volumes. You won't be able to solve the problem as long as you will not follow the stoichiometry given by the stoichiometric coefficients.

since we have two molecules of z, we must have 60ml of z then right?

since we have two molecules of z, we must have 60ml of z then right?
No. Stop guessing. Where did you got 30 from? Sum of volumes of X and Y? That's not how it works, you are again and again ignoring the reaction stoichiometry.
Look at the reaction equation. Write it as "... moles of X reacting with ... mole of Y producing ... moles of Z" (fill the numbers and show them here!).
Then, knowing that ratio of volumes will be identical with the ratio of numbers of moles, write the same thing but with volumes: "... mL of X reacting with 10 mL of Y producing ... mL of Z".

2moles of x reacting with 1mole of y = 2moles of z
reacting: 20ml x reacting with 10ml of y = 20ml of z, right?

2moles of x reacting with 1mole of y = 2moles of z
reacting: 20ml x reacting with 10ml of y = 20ml of z, right?
Yes, that's exactly what is happening.
Now, how many mL reacted and how many mL were produced? By how many mL did the volume change? What was the initial volume? What is the final volume?

the initial volume was 100, it decreased by 80ml, or?

Answer my questions one by one.
And no, initial volume was not 100 mL.

2moles of x reacting with 1mole of y = 2moles of z
reacting: 20ml x reacting with 10ml of y = 20ml of z, right?
Yes, that's exactly what is happening.
Now, how many mL reacted and how many mL were produced? By how many mL did the volume change? What was the initial volume? What is the final volume?
well, 20ml of x reacted, producing 20ml, too. I don't know to what you're referring to with the rest of your questions...

well, 20ml of x reacted, producing 20ml, too.
No. How many mL of X reacted? How many mL of Y reacted? What was the total volume of these two reacting gases before the reaction?
(For now we are ignoring nonreacting excess of X, as it doesn't change).

ok, hold on... we have 20ml of x reacting with 10ml of y giving 20ml of z, right?

ok, hold on... we have 20ml of x reacting with 10ml of y giving 20ml of z, right?
Yes. So, what was the initial volume of X and Y that reacted, and what was the final volume of produced Z? What was the volume change?

of x we had 100, of these 20ml reacted. And 10ml of y was used up...

of x we had 100, of these 20ml reacted. And 10ml of y was used up...
All true so far. But I am not going to repeat other questions for the umpth time, up to you to proceed.

my question now is: of what the volume change?? 20ml of z is the initial volume...

Can someone help me out with this one?
I have 105cm^3(2*100+10)/2)
So you know that equal volume of gases have equal volume at the same temperature and pressure.
It means 2V of X combines with 1 V of Y to give 2V of Z
One of the gases is the limiting reactant .Find out the limiting reactant and calculate volume of Z formed from limiting reactant and volume of gas left as excess reagent.
1V of Y combines with 2V of X
so.... 10mL of Y will combine with how much volume of X?
10mL of Y will give how much volume of Z?
How much volume of X left unreacted ?

y=limiting reactant
x> there are 80ml left...

my question now is: of what the volume change?? 20ml of z is the initial volume...
Initial volume consist of 100 mL of X and 10 mL of Y. Z is not in the initial volume, as at the start of the reaction, or before reaction, it doesn't exist at all.
Final volume consists of produced Z and unreacted X that was left after the reaction went to completion.
Unreacted X doesn't change  it just sits there.

However, this isn't still 100ml...

However, this isn't still 100ml...
And how am I going to guess what 100 mL you mean?

from the solution... it says that z must be 100ml...

Don't bother with the answer key till you finish solving the problem.
What gases and what volumes of them will be present after the reaction?

z will be present after the reaction?
and 20ml of it?

z will be present after the reaction?
and 20ml of it?
Yes, there will be 20 mL of Z after the reaction.
Will it be the only gas present?

Well, I dont know, but that is irrelevant or not? youre asked to look for z, not for anything else...

You are asked about the final volume of the gas in the syringe, that means all gases present combined.

ok, now that seems logical...