Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: ChenistryNoob1 on October 23, 2015, 11:44:20 AM
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I have ; Calculate the volume of Hydrogen gas produced at STP when 5g of Magnesium react with Hydrochloric Acid?
I made the balance equation but I think im messsing with anohter method . Any help?
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A balanced equation is an excellent place to start. Why not write it out here and start to work through the problem. Then we can help you make progress.
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Im the same guy but from anohther pc account lol
Mg (s) + 2 HCl (aq) --> MgCl 2 (aq) + H 2
I wrote 5g under Mg and under H2 a ?
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Ok, now, you have a balanced equation. What are it's units? You can use it to relate a certain "amount" of magnesium to a certain "amount" of H2. How will you do that?
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its grams i guess
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No. It's not.
Mg (s) + 2 HCl (aq) --> MgCl2 (aq) + H2
You don't go to a shelf, and pick up a magnesium atom, and squish it together with 2 HCl molecules, so what are the units in this case.
Yes, you are given magnesium in grams, and you are asked for volume of gas, but you will have to convert from ...?
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I m not understanding
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The chemical equation has implied units. You're taking a certain "amount" of "stuff" and turning it into a different "amount" of "stuff".
It's not grams. Can you see why it's not grams? If it were grams then how much H2 would you expect to make from 1000g of Mg? Would that amount of H2 make intuitive sense?
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No it wouldnt make sense as its a big number , can you give me the correct equation then?
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The equation is correct. But you're thinking about the units wrong. Your equation says that for every unit of Mg consumed you're producing one unit of H2. It's not grams. What is that unit?
PS I just noticed the title of your post. Please read up on the forum rules here: http://www.chemicalforums.com/index.php?topic=65859.0
Specifically asking for help -- especially urgently -- goes against the forum guidelines.
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I dont know :/ , + the units is used at the end after the answer . the units come at last place .. is it 1mole of Mg = 1 mole of H2 ? I need to get this solved
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is it 1mole of Mg = 1 mole of H2 ?
Yes. It's moles. So now that you know that, how many moles of Mg do you start with? How many moles of H2 do you produce? How do you convert that to a volume?
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as I wrote 1mole of Mg = 1 mole of H2 ,,,,, its 1 becuz on the equation there isnt any numbers of the balancing. Now i write the RMM of both?
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Can you work out this one for me? As I have 12 others like this
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I've already told you to read the forum rules and you have obviously not, or else you would not be asking me to solve the question for you. Please read the forum rules; they're what keep this place nice and organized and what allow the members to help each other.
I'm being patient and trying to work through the problem with you. Perhaps the real issue is that you have not given yourself enough time to complete your assignment.
That said, you have correctly written the chemical equation and identified that it works in units of moles. But your problem has given you a starting amount of Mg in grams. So you'll have to convert grams to moles.
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Is it 24g Mg?
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You have to convert 5g Mg to moles Mg.
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You have to convert 5g Mg to moles Mg.
is it
1mole of Mg = 24g
? = 5g
5 x 1 divide 24 = 0.208
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but it says I need to calculate the volume of Hydrogen gas produced at STP , when 5g of Mg react with HCl
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1mole of Mg = 24g
? = 5g
5 x 1 divide 24 = 0.208
OK, you know number of moles of Mg that reacted. Look at the balanced reaction equation - do you know how to read it (http://www.chembuddy.com/?left=balancing-stoichiometry&right=stoichiometric-calculations)? Can you use it to calculate number of moles of hydrogen than were produced?
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1 mole of Mg = 1 mole of H2
0.208 = ?
I don't think it makes sense :/
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Please stop quoting the quote of a quote of a quote. This is a thread, we can read the last posting. You're making the thread longer for no reason.
Why do you think 0.208 moles makes no sense? What if I said it was 0.500 moles? That's a half of a mole, you could have a half of a mole of magnesium, you just said one mole of magnesium was 24 grams, you can have half a mole, and you'd know it's mass too.
That's why I told you, a balanced equation doesn't say: pick up one atom of magnesium, and smush 2 molecules of HCl into it, although new students are often told to visualize chemical reactions that way. The units of chemical equations are always moles, because that's what we work with.
Now, you've determined moles from the mass, and now you have moles of H2. They wouldn't have given you this problem if you didn't also have a formula that relates moles of hydrogen gas to mL of volume. Do you know that formula?
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Ok , so Mg is 0.208
Now we need to try and find H2
Formulas that I know are the Avogado Constant ; 6.02x1023
and volume ; 22.4dm3
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Yes, now you need to find how much H2 is produced.
As Borek stated, you can look at the balanced chemical equation and if you know how to read it (http://www.chembuddy.com/?left=balancing-stoichiometry&right=stoichiometric-calculations) you can calculate the moles of H2 produced by your 0.208 moles Mg.
Then as Arkcon stated you can relate the moles of hydrogen gas to mL of volume. How would you do that?
edit for typos 2015-10-24 11:01am PDT
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I wouldn't have used Avogadro's constant for this case, but I suppose you can derive the formula from that. Maybe you can check your class notes or text book for an easier formula to use.
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Mg (s) + 2 HCl (aq) --> MgCl 2 (aq) + H 2
0.208 ?
I don't know what to do now
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Have you read the page linked to earlier? It explains the meaning of stoichiometric coefficients.
Your approach so far is passively aggressive. You have not shown any real effort at solving the problem, instead you are whining "I don't know", ignoring hints given, and repeating the same information again and again. I am afraid it won't make you many friends, quite the opposite - people seeing your approach will soon start ignoring you.
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I think the answer should be somewhere near 0.4***
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I think I got it
0.208 division 5 = 0.0416
is it right?
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I think I got it
0.208 division 5 = 0.0416
is it right?
Why divide by 5?
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the 5 is the 5g of Mg , the answer should be somewhere near 0.4*** , idk whats wrong
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You've twice gone backwards, and twice just restated the problem. I recognize that you can become confused. I've also said, and Borek: seems to have agreed, you seem to want to lengthen this thread, because you think this is a chat room. Work on that please, and don't go backwards.
Summarizing the work so far in this thread:
You've determined 0.208 moles of magnesium are consumed. You've learned that you produce 0.208 moles of hydrogen gas.
Please look up the gas laws in your text book, class notes or online. There is a simple formula that relates pressure, volume, moles and temperature. The problem has defined some of those for you, you've determined moles, and you just need to calculate volume.
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Oh and hey, look at this thread here: http://www.chemicalforums.com/index.php?topic=82822.0 Its essentially the same problem. That poster is a little bit ahead of the game, but the math will be the same.
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This isn't a chatroom , I knw that . I searched up online the gas laws and this formula came ;
PV = nRT
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Good. Now, type it again. This time, replace the variables, P, n, R and T, with numbers that you know. R is a constant. But you have to pick the correct R for the units of P,V,T that you'll be using, I didn't see those in the original problem, so you'll have to check.
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You've learned that you produce 0.208 moles of hydrogen gas.
I am not convinced he knows that.
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P - Pressure we don't have any pressure to calculate
V - This is what we need to find
n - we have 0.208moles
T - we haven't got any temperature to calculate
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The original problem specified STP. Please look up that definition. And the answer is not a brand of motor oil, nor the name of a Grunge band.
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Standard Temperature and Pressure = STP
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Yes standard temperature and pressure. You have a formula that needs values for T and P (and n and R). What does knowing that you are working at standard temperature and pressure tell you about the values you can use for T and P in your formula?
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How do I calculate the mass of Zn required to react with HCl , to produce 3.5dm3 of Hydrogen at STP?
Zn + HCl = ZnCl2 + H2
? 3.5dm3
I got confused which formula and how to work it out
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Simplest way to deal with stoichiometry is through the moles. Can you convert 3.5 dm3 of H2 to number of moles of hydrogen?
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ChennistryNewbie: I hope you don't mind that I merged this new question, with your older one. Once you solve one, you'll be able to solve the others.
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Simplest way to deal with stoichiometry is through the moles. Can you convert 3.5 dm3 of H2 to number of moles of hydrogen?
I havent learnt how to convert that ... we usually do a statement and work out with cross divide multiplication
something like ; 1 mole of Zn = 1 mole of H2
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I havent learnt how to convert that ... we usually do a statement and work out with cross divide multiplication
I refuse to believe that with the sorts of questions you're asking that you haven't been given a way to convert a volume of gas to moles or vice versa.
Do yourself a favor and read through your lecture notes and textbook. I'm sure there is something there to help you. If somehow it isn't, read about the Ideal Gas Law (http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/idegas.html) and it should become clearer. Pay particularly close attention to the last sentence in the Ideal Gas Law section of that link.
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From that I got the formula PV = nRT
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ChennistryNewbie, I feel like you're being very defeatist through this whole process. We're hand-feeding you the process to solve these problems and you're doing very little of the work yourself. As a general guideline for school and for life, try to take a little more initiative. I understand that the concepts in chemistry can be difficult for some people, but you have to at least try.
So you know Pv = nRT.
Do you know what each of those variables mean? If you have 3.25dm3 of H2 at STP you could pretty easily calculate the number of moles, right?
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P - Pressure
V - Volume
n - number of moles
R - Gas Constant
T - Tempreture
It just because in school I didn't learn this fomula so I don't know how it works .. I learnt the 22.4dm3+ and Avogado Constant
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You keep mentioning learning 22.4dm3.
Why is this volume so special? What does it mean?
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1 mole of anything will occupy 22.4dm3+
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Yes! (Well, if by anything you mean any ideal gas at standard temperature and pressure)
Now c'mon ChennistryNewbie I know you can put it all together from here!
You have 3.25dm3 of H2. How many moles do you have? How many moles of Zn did you need to make that many moles of H2? What's the mass of those moles of Zn?
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1 mole of H2 = 1 mole of Zn
22.4dm = ?
3.25 = 65g
like that?
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The way you set up equations for solving is very strange. Maybe it's some new way that schools are teaching but I'm unfamiliar with it.
Another suggestion for you. You should actually learn the science and principles behind the equations and heuristics the teachers give you. If you understood what the equations actually meant you wouldn't be so confused as to what values to plug in where. You wouldn't even think of problems as things to plug values into.
Back to it. You've told us that you know 1 mole of anything (at STP) will occupy 22.4dm3 of volume. And you have 3.25dm3 of H2.
I asked the questions that I did because that's the order in which you should be thinking through this problem. It's a simple problem but it can be broken down into even smaller steps. So...
- How many moles of H2 do you have if you have 3.25dm3 at STP? (Hint, use your 22.4dm3/mol constant)
- How many moles of Zn did you need to make that many moles of H2? (Hint, use your balanced chemical equation Zn + 2 HCl :rarrow: ZnCl2 + H2
- What's the mass of that many moles of Zn? (Hint, the molar mass of Zn is 65.38g/mol).
I don't think it's possible for me to help you any more without actually telling you the answer. Please, please consider going back into your notes to find examples from your teacher. And you should probably reach out to your teacher for more assistance. He or she will hopefully be able to solidify some of these concepts for you.
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1 mole of Zn as there isn't any number infront of it in the equation
1 mole of Zn is the RMM in grams? 65g ?
We use rounded units not with point for the RMM
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I'm sorry but I can't provide you any more information without actually answering it for you. So here's the last post you'll get from me.
Answer the questions I asked in the order I asked them. Get a definitive answer for each one.
- Your answer should be in units of moles of H2 and you will need to use your Molar Volume of 22.4dm3.
- Your answer should be in units of moles of Zn and you will need to use your balanced equation.
- Your answer should be in units of grams of Zn and you will need to use your molar mass of Zn of ~65g/mol.
Go through the three problems methodically and intelligently. Use the correct units in your equations. Cancel units where needed to help you.
Good luck.
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The way you set up equations for solving is very strange. Maybe it's some new way that schools are teaching but I'm unfamiliar with it.
It looks like the approach described here: http://www.chembuddy.com/?left=balancing-stoichiometry&right=ratio-proportions
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Well that looks like it could be a faster way of solving these stoichiometry problems, but I feel like it also obfuscates what's going on. Students look for numbers to plug into a heuristic and as soon as some thought is involved they get confused. At least that's what seems to be going on here.
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Well that looks like it could be a faster way of solving these stoichiometry problems, but I feel like it also obfuscates what's going on.
For some it obfuscates, for some it helps ;)
I believe for those that feel confident with the proportions this approach fits like a second skin. Unfortunately, most students don't understand proportions at all.