Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: labec on October 10, 2008, 07:53:38 AM
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The question says:
From the information below, determine the mass of substance C that will be formed if 45.0 grams of substance A reacts with 23.0 grams of substance B. (Assume that the reaction between A and B goes to completion.)
a.) Substance A is a gray solid that consists of an alkaline earth metal and carbon (37.5% by mass). It reacts with substance B to produce substances C and D. Forty million trillion formula units of A have a mass of 4.26 milligrams.
b.) 47.9 grams of substance B contains 5.36 grams of hydrogen and 42.5 grams of oxygen.
c.) When 10.0 grams of C is burned in excess oxygen, 33.8 grams of carbon dioxide and 6.92 grams of water aree produced. A mass spectrum of subbstance C shows a parent molecular ion with a mass-to-charge ratio of 26.
d.)Substance D is a hydroxide of the metal in substance A.
I seriously need help. I can't even begin the problem:/ If you know how to do this and feel like trying please list your steps accordingly. Thanks!
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Well, lets try and see what you do know. Can you break this question apart, into balanced chemical reactions, using a placeholder, like X, for the unknown element?
Also, can you express this part --
The question says:
*SNIP*Forty million trillion formula units of A *SNIP*
in exponential numbers?
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Sadly, I haven't learned how to do that yet.. so as for the exponential function 4.0 x 10?
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I would assume, given the use of both million and trillion in the same sentence that this means 1024, and that trillion is being used in the long scale sense of 1018, but this seems to make a very small relative molecular weight....?
BTW Arkcon, good to see you back, how's things?
S
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No in this question a trillion is 1012 so 40 million trillion is 4x1019 molecules.
Overall it looks complicated but it is actual quite a simple question.
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Well, yes, if it is short scale trillion things are a lot easier, but why use million and trillion in the same sentence, why not just say quintillion?
Anyway 4x1019 formula units have mass 4.26mg, what does this tell you about the solid...
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Ok from the formula I found I got the answer to be 16.64 grams???? Is this right?
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Yes 16.6g acteylene. Note the number of signifcant figures in the question and my answer.
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OK thank you!:)
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From the information below, determine the mass of substance C that will be formed if 45.0 grams of substance A reacts with 23.0 grams of substance B. (Assume that the reaction between A and B goes to completion.)
a.) Substance A is a gray solid that consists of an alkaline earth metal and carbon (37.5% by mass). It reacts with substance B to produce substances C and D. Forty million trillion formula units of A have a mass of 4.26 milligrams.
b.) 47.9 grams of substance B contains 5.36 grams of hydrogen and 42.5 grams of oxygen.
c.) When 10.0 grams of C is burned in excess oxygen, 33.8 grams of carbon dioxide and 6.92 grams of water aree produced. A mass spectrum of subbstance C shows a parent molecular ion with a mass-to-charge ratio of 26.
d.)Substance D is a hydroxide of the metal in substance A.
so for step A i assumed that that Carbon had a mass of 3/8 of the total meaning the total amu is 32, which leaves 20 amu for the metal. the only metal that fits this is Be.
then for step B H20 was really the only substance hydrogen and oxygen that fit these ratios:
2/18 = .1 and 5.36/47.9 = 1
likewise, 16/18 = .89 and 42.5/47.9 = .89
so now i have Be2C + H20 -> C + D
for step C i wrote out a whole stoich chart and when i solved it i got C2H2 or HC2H for substance C
here's my stoich chart:
Balanced Equation 2C2H2 + 502 -> 4CO2 + 2H20
Molar Ratio 2 5 4 2
Molar Mass 26 32 44 18
Grams 10.0 30.72 23.8 6.92
Moles .38 .96 .77 .38
also (2/18) = (x/6.92) yields .8 grams of Hydrogen meaning out of the water 6.12 grams are oxygen
and (16/18) = (x/33.8) yields 9.2 grams of Carbon meaning out of the carbon dioxide 24.6 grams are oxygen
6.12 plus 24.6 gives you 30.72 hence the 30.72 grams of oxygen in the chart
everything else was either given or solved from there
then for step d it says that substance d is the hydroxide of the metal in substance A so Be(OH)2
putting that all together gives you the equation
Be2C + H2O -> C2H2 + Be(OH)2
but i can't balance that equation
I know Be2C doesn't have the exact percentage of carbon that I need but I don't know what substance A could possibly be.
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so for step A i assumed that that Carbon had a mass of 3/8 of the total meaning the total amu is 32, which leaves 20 amu for the metal. the only metal that fits this is Be.
Think again, nowhere does it say there can only be one carbon substance A.
then for step B H20 was really the only substance hydrogen and oxygen that fit these ratios:
Yes you got that bit right, B is water.
so now i have Be2C + H20 -> C + D
for step C i wrote out a whole stoich chart and when i solved it i got C2H2 or HC2H for substance C
here's my stoich chart:
Balanced Equation 2C2H2 + 502 -> 4CO2 + 2H20
Molar Ratio 2 5 4 2
Molar Mass 26 32 44 18
Grams 10.0 30.72 23.8 6.92
Moles .38 .96 .77 .38
also (2/18) = (x/6.92) yields .8 grams of Hydrogen meaning out of the water 6.12 grams are oxygen
and (16/18) = (x/33.8) yields 9.2 grams of Carbon meaning out of the carbon dioxide 24.6 grams are oxygen
6.12 plus 24.6 gives you 30.72 hence the 30.72 grams of oxygen in the chart
everything else was either given or solved from there
then for step d it says that substance d is the hydroxide of the metal in substance A so Be(OH)2
putting that all together gives you the equation
Be2C + H2O -> C2H2 + Be(OH)2
but i can't balance that equation
You've got C correct but because you've got A wrong then so is D, that means you can't balance the equation.
I know Be2C doesn't have the exact percentage of carbon that I need but I don't know what substance A could possibly be.
Look at the other alkaline earths one of them forms a compound that is 37.3% carbon and reacts with water to give C2H2 plus the corresponding hydroxide.
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i really have no idea what it could be. is the formula that i'm looking for M2C (where M is the metal) because if that's the formula then none of the metals fit. If you could please point me in the right direction, i'd greatly appreciate it. Thanks for all the help.
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You need to look for MC2.
(Edited twice because of extremely terminal stupidity. It's a miracle I remember how to breathe.)
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that doesn't exist. Carbon has a charge of 4- and alkaline earth metals have a 2- charge, right? or can carbon sometimes have a 1- charge because i know that alkaline earth metals always have a 2- charge.
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Edit: looks like I shouldn't be posting at this moment of the night, but you're looking for MC2. The carbons are covalently linked, which explains the apparent charge difference.
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is it CaC2? Because that would be a total of 64 amu. C2 has 24 amu and 24/64 is 37.5%.
How would you name CaC2? Calcium what?
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Is the answer 18.2 grams of C2H2?
This is the final equation I got:
CaC2 + 2H2O -> C2H2 + Ca(OH)2
is that correct?
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How would you name CaC2? Calcium what?
Carbide or acetylide.
Is the answer 18.2 grams of C2H2?
No, correct answer was already listed earlier in the thread.
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As Borek said CaC2 is calcium carbide and there are 16.6g acetylene.
Go look up the derivation of the word limelight and "being in the limelight"
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ok, well if it's 16.6 then it can't be CaC2.
But the formula is MC2, yes?
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1826, popular name for Drummond light, a brilliant light created by the incandescence of lime (1), adopted for lighthouses and later for the Victorian stage, where it illuminated the principal actors, hence the figurative sense of "on stage, at the center of attention" (1877).
how does that help?
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CaO is burnt lime but that only gives you 23% carbon.
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ok, well if it's 16.6 then it can't be CaC2.
But the formula is MC2, yes?
Then your calculations are wrong. All correct answers have already been posted.
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ok, well if it's 16.6 then it can't be CaC2.
23 grams of B.
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i'm not looking for just the answer. i need to show all my work of how i got to the answer.
so, is it or is it not CaC2? because, i don't know what else substance A could be.
also, how does the forty million trillion formula units have a mass of 4.26 milligrams help at all?
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also, how does the forty million trillion formula units have a mass of 4.26 milligrams help at all?
Allows calculation of molar mass.
Yers, it can be CaC2. 45 grams were mixed with 23 grams of water. THINK.
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is it a limited reaction problem?
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ok, I reworked it and this is what i got.
C is 3/8 of the mass and C is covalently bonded so it's MC2 which means the metal is CaC2 (molar mass of 64)
Here is the final stoich chart I made
BE CaC2 + 2H2O -> C2H2 + Ca(OH)2
MR 1 2 1 1
MM 64 18 26 74
G 45 23
Mol .70 1.3
so, there's an excess of the CaC2, it's only .65 moles, .05 moles excess (.65 x 2 = 1.3)
so, for the C2H2 we have: .65 moles = g/26 so, g=16.9
but earlier it said the answer was 16.6, so now i'm confused again.
where am i going wrong in my calculations?
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If you go from 1.3 to 1.28 (one more significant number) the result becomes 16.6.
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i figured it out. thanks for all your *delete me* i just rounded earlier on so i rounded 1.278 to 1.3 which when you divide by 2 gives you .65 and when you multiply that by 26 you get 16.9 instead of 16.64. thanks again!
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Well done with getting there in the end. Sorry about the limelight bit I thought, wrongly it seems, that the first ones used oxy-acteylene formed from calcium carbide to heat the calcium oxide.