ibquestions Posted March 25, 2011 Report Share Posted March 25, 2011 (edited) I am doing a lab report on how the concentration of hydrochloric acid effects the amount of hydrogen gas that is produced {in this reaction only: Zn(s) + 2HCl(aq) --> ZnCl2(aq) + H2(g) }.I used the following concentrations of hydrochloric acid:0.100moldm-31.00moldm-33.00moldm-35.00moldm-36.00moldm-3As hypothesized, more hydrogen gas was released in higher concentrations of hydrochloric acid then lower concentrations... except no gas at all (at least none that was measurable using the method of downward displacement of water) was released for the 0.100moldm-3 concentration and I can't seem to figure out why this happened? It could be because of human/random error but I don't think that would cause no gas at all seeing as significant amounts of gas were produced for the other concentrations of HCl. Does anybody know why this has occurred?Any help is much appreciated Thank-you! Edited March 25, 2011 by ibquestions Reply Link to post Share on other sites More sharing options...
Drake Glau Posted March 25, 2011 Report Share Posted March 25, 2011 What was the volume and mass of the HCl/Zn? Reply Link to post Share on other sites More sharing options...
ibquestions Posted March 25, 2011 Author Report Share Posted March 25, 2011 What was the volume and mass of the HCl/Zn? I used 10ml of HCl and 0.5g of Zn Reply Link to post Share on other sites More sharing options...
Drake Glau Posted March 25, 2011 Report Share Posted March 25, 2011 (edited) K, so 10mL of 0.100M HCl is 0.001mol and 5g of Zn is 0.0765mol meaning your HCl became the limiting reactant when your concentration dropped so low. ^Thought process I typed out, sorry Since you need 2HCl for each Zn only 0.0005mol of H2 was formed. For gases 1mol=22.4dm3 so 0.0005*22.4=0.0112dm3 of gas or in other words only 11.2mL of gas. Your 1M concentration trial was 112mL of gas though. I think the reason you got zero was perhaps the measurement dropped so low it became almost impossible to measure in a high school. I don't know how big your uncertainties are either. It's possible that 0.0112dm3 is within your error bars for that trial in which case nothing is wrong with your data Edited March 25, 2011 by Drake Glau 1 Reply Link to post Share on other sites More sharing options...
ibquestions Posted March 25, 2011 Author Report Share Posted March 25, 2011 woah... hahah okay, you are really good with chemistry but I have a couple questions about that if you're willing to answer?first of all, I don't understand how 10ml of 0.100M HCl is 0.001mol? and how/why did you change 0.5g of Zn to 5g of Zn?and lastly, using whatever method you used there, am I able to find out how much gas should have been produced at each concentration?sorry about the questions... I am so dead today I should be able to get this Reply Link to post Share on other sites More sharing options...
Drake Glau Posted March 25, 2011 Report Share Posted March 25, 2011 (edited) woah... hahah okay, you are really good with chemistry but I have a couple questions about that if you're willing to answer?first of all, I don't understand how 10ml of 0.100M HCl is 0.001mol? and how/why did you change 0.5g of Zn to 5g of Zn?and lastly, using whatever method you used there, am I able to find out how much gas should have been produced at each concentration?sorry about the questions... I am so dead today I should be able to get this 1. c=n/v2. I'm stupid...let me rework all that real quick. Edit: Doesn't matter. Your HCl is still the limiting reactant.3. YesYou have the volume and concentration of the HClc=n/vn=cvc=0.100v=0.010n=0.001molYou have the mass of the Zn0.5g Zn65.39g/mol molar mass0.5/65.39g=0.00765mol Zn2HCl+Zn->ZnCl2+H2The least amount of moles is your HCl which means all of it will be used and react with the Zn. Since it requires 2HCl for each Zn then only 0.0005mol of the Hydrogen will be produced.For gases you know that 1mol of a gas=22.4LWell 0.0005mol of gas was made so you take the 0.0005mol*22.4 and that gives you the volume of the gas you formed. Edited March 25, 2011 by Drake Glau 2 Reply Link to post Share on other sites More sharing options...
ibquestions Posted March 25, 2011 Author Report Share Posted March 25, 2011 Hahah... you are far from stupid! Thank you very much for the help Reply Link to post Share on other sites More sharing options...
Drake Glau Posted March 25, 2011 Report Share Posted March 25, 2011 (edited) Lol no problem Understand stoichiometry and all of these problems like this become ridiculously easy. Stoichiometry will not go away either so might help to practice some if you don't know a few things. By the time you hit the HL topics it will be assumed you can do it basically in your head as concepts are explained...or at least how the process would work without having to write it all out.Have fun with topic 8 and 9 Next year for you Something to look forward too...sort of...Last Edit. Was the volume of gas released about the same for 1M-6M of HCl? If you only used 0.5g of Zn the limiting reactant with those concentration will always be the Zn resulting in a constant volume of hydrogen being release around 85mL O.o Edited March 25, 2011 by Drake Glau Reply Link to post Share on other sites More sharing options...
ibquestions Posted March 25, 2011 Author Report Share Posted March 25, 2011 (edited) Hahah yeah I'll have to practice stoichiometry some more ... no idea how I'm going to do that but I'll figure it out Haha at the moment I am not looking forward to it... chem is my hardest subject Thanks again for the help Edit: I had very slight differences in the amounts produced, but that is probably because the higher concentrations of HCl reacted faster than the lower ones and I might have ended some of the reactions too early (the reactions involving lower concentrations of HCl) because they were taking so long Edited March 25, 2011 by ibquestions Reply Link to post Share on other sites More sharing options...
ibquestions Posted April 3, 2011 Author Report Share Posted April 3, 2011 1. c=n/v2. I'm stupid...let me rework all that real quick. Edit: Doesn't matter. Your HCl is still the limiting reactant.3. YesYou have the volume and concentration of the HClc=n/vn=cvc=0.100v=0.010n=0.001molI am sorry... I feel like the stupidest person... but I just looked over this again and i have no idea how you got 0.010 for the volume of it's 10ml originally :/ Reply Link to post Share on other sites More sharing options...
Drake Glau Posted April 3, 2011 Report Share Posted April 3, 2011 (edited) 10mL is 0.010L You need to convert to dm3(Liter) when doing anything that involves concentration calculation really because molarity is moles per liter. If you didn't change your mL to L you would end up with moles per mL which isn't molarity Edited April 3, 2011 by Drake Glau 1 Reply Link to post Share on other sites More sharing options...
ibquestions Posted April 3, 2011 Author Report Share Posted April 3, 2011 Ohhhh I see, I can't believe I didn't catch that thank youuuu Okay, so I know I have been a major pain in the ass... but I have one final (and I swear to god this is the last) question... and feel free to not answer it haha, I've asked you quite a few already.Anyway... Here you've said that I should be getting 112ml of gas for my 1M concentration... and I really should have been able to measure 12ml of gas in my experiment..."Since you need 2HCl for each Zn only 0.0005mol of H2 was formed. For gases 1mol=22.4dm3 so 0.0005*22.4=0.0112dm3 of gas or in other words only 11.2mL of gas. Your 1M concentration trial was 112mL of gas though. I think the reason you got zero was perhaps the measurement dropped so low it became almost impossible to measure in a high school. I don't know how big your uncertainties are either. It's possible that 0.0112dm3 is within your error bars for that trial in which case nothing is wrong with your data."And here you've said that I should have gotten 85ml for each measurement... and you said that Zn will be the limiting reactant... but above you said HCl was... so I'm a little confused "Was the volume of gas released about the same for 1M-6M of HCl? If you only used 0.5g of Zn the limiting reactant with those concentration will always be the Zn resulting in a constant volume of hydrogen being release around 85mL O.o"Just wondering how much gas I really should have gotten, because I have figured out my theoretical yield and it's the same as one of these... but I don't know if it's correct... so just wondering Reply Link to post Share on other sites More sharing options...
Drake Glau Posted April 3, 2011 Report Share Posted April 3, 2011 at 0.5M HCl the amount of moles that went into the solution made it the limiting reactant. After you moved it up to 1M solutions the Zn became the limiting reactant because as the molarity of your HCl went up you were adding more moles.0.5M HCl in 0.010Lc=n/vn=cvn=0.5*0.010n=0.005mol1M HCl in 0.010Ln=cv0.010mol0.5g of Znn=mass/molarmass=0.5/65.39=0.0076molYour moles of HCl were lower than your Zn moles when you used the 0.5M solution, that is why it was the limiting reactant.You changed it to 1M, doing this added more moles and it was enough to be greater than the amount of Zn moles you thus making the Zn the limiting reactant now. 1 Reply Link to post Share on other sites More sharing options...
ibquestions Posted April 3, 2011 Author Report Share Posted April 3, 2011 Ah yes, I understand that, thank you But I was also wondering if I should have gotten 85ml or 112ml of gas produced for my reactions... because you said them both I'm sorry for being such a pain, you have no idea how much I appreciate you're explanations :) I am so bad when it comes to concentrations and what not... Reply Link to post Share on other sites More sharing options...
Drake Glau Posted April 3, 2011 Report Share Posted April 3, 2011 You'll get both When the HCl was limiting you should have gotten the 85mL, when the Zn was limiting it would have been 112mLThis is off memory, didn't feel like doing the calculations again.But you'll get both because when the HCl limits it would give you one value and then when the Zn limits you would then get a different value because it limits with a higher amount of moles. This sentence might not make sense, made sense in my head though. 1 Reply Link to post Share on other sites More sharing options...
ibquestions Posted April 3, 2011 Author Report Share Posted April 3, 2011 (edited) >.< oh my god my head is going to explode hahahaOkay... so, just to be clear... I should have gotten0.100moldm-3 -- 11.2ml1.00moldm-3 -- 85ml3.00moldm-3 -- 85ml5.00moldm-3 -- 85ml6.00moldm-3 -- 85ml ?didn't you say I should get 85ml from 1M-6M? but... Zn is the limiting reactant for all those... so you're saying I realy should have gotten 112ml for all of them? I don't understand at all... ;(thank you again by the way! Edited April 3, 2011 by ibquestions Reply Link to post Share on other sites More sharing options...
Drake Glau Posted April 3, 2011 Report Share Posted April 3, 2011 (edited) Yea, since I just kind of confused the crap out of you Here ya go 0.1M HCl is 0.001mol1M HCl is 0.01mol2M is 0.023M is 0.034M is 0.045M is 0.056M is 0.060.5g of zinc is 0.007646molAccording to that your limiting reactant will be Zn for everything BUT the 0.1M trialSince according to your reaction formula thingy the ratio between Zn and the H2 is 1:1 therefore the amount of gas produced for 1M-6M will ALWAYS be 0.007646mol*22.4Lmol-1 which is about 171mLFor the 0.1M trial the limiting reactant was the HCl and the HCl to H2 ratio is 1:2 (product:reactant) so you need to do 0.001*0.5*22.4 which is about 11.2mLIdk where you got that 85mL from, I might have said it actually (yea, I did, just looked back, don't ask where I got it from...), but these are fresh calculations I just did when it wasn't past 10pm Edited April 3, 2011 by Drake Glau 2 Reply Link to post Share on other sites More sharing options...
ibquestions Posted April 4, 2011 Author Report Share Posted April 4, 2011 YAY I understannnnd nooow Thank you soooo much!! Reply Link to post Share on other sites More sharing options...
Drake Glau Posted April 4, 2011 Report Share Posted April 4, 2011 Yaaaaay! Aqueous solution problems will not leave you alone, ever, so don't forget this Have fun with the rest of your lab. Reply Link to post Share on other sites More sharing options...
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