[tg22542]

Need help with a couple chem questions..

1) Given the reaction:  Al+3Cl2->2AlCl3
It is found that chlorine is consumed at a rate of 0.2485g/s. Calculate the total mass of AlCl3 produced if this rate is maintained for 3 minutes.

2) Given the reaction: N2+3H2 -> 2NH3
If the rate of formation of NH3 is 8.0*10^-3 mol/s, calculate the rate of consumption of H2 in mol/s.

I just can't figure out these questions, not much of a chem wiz haha. Any solutions+explanations will be greatly appreciated.

Thank you in advance

[Borek]

You have to show your attempts at solving the question to receive help. This is a forum policy.

[Arkcon]

Well, lets begin with the first one, you consume 0.2485g/s of Cl₂.  How much mass is that for 3 minutes?  That mass, how many moles is it, and how much AlCl₃ will you get, in moles and then in grams?  Then just do all that in reverse to do part 2.

[tg22542]

I seriously have no idea how to do them, as I mentioned before, I absolutely suck at chemistry.
If I were to guess, I would say that I would first convert the g/s to mol/s which I don't know how to do although I've watched a number of different videos and looked at many websites. I'm so lost it's not even funny

[UG]

A mole is just a number used for counting atoms, molecules etc... its just like saying if you have a dozen atoms then you have 12 of them, similarly, if you have a mole of atoms, you have about 6.022x10²³ atoms. If you have a dozen eggs, you have 12 eggs, if you have a mole of eggs, you have about 6.022x10²³ eggs
Anyway, in chemistry we work with moles usually and not mass because mass does not give us an idea of how much stuff there is (atoms, molecules etc...) whereas moles tells us exactly how many molecules there are. In order to convert between moles and mass, you need the molar mass, which can be found on the periodic table. The molar mass of carbon is around 12 g/mol, this means if you have one mole of carbon atoms (i.e. 6.022x10²³ carbon atims) and you weighed it, the mass would be around 12 grams.
Lets look at the example you have

1) Given the reaction:  Al+3Cl₂  2AlCl₃
It is found that chlorine is consumed at a rate of 0.2485 g/s. Calculate the total mass of AlCl3 produced if this rate is maintained for 3 minutes.

You are given the rate of consumption of chlorine as 0.2485 g/s. In order to convert this into moles, you need the molar mass of chlorine. From the periodic table, the molar mass of 1 mole of chlorine atoms is 35.45 g/mol. Because in the balanced reaction, you are consuming Cl₂, the molar mass is therefore 2 x 35.45 = 70.90 grams per mole Cl₂. So if you are consuming 0.2485 g/s of Cl₂ then the number of moles of Cl₂ per second is 0.2485/70.90 
Now that we are working in moles, we can look at the balanced reaction to see how many moles of aluminium reacts. The reaction shows that 1 mole of Al reacts with 3 moles of Cl₂ (which you have worked out), its a simple ratio so you can easily work out how many moles of Al you need per second. You can then convert moles of aluminium back into mass per second. Are you able to finish this problem from here?

[tg22542]

well I know that I have to do 26.98 * 3 = 80.94g/mol Al

But honestly and I'm embarrassed to say it, but no I don't know where to go from here.. haha

[UG]

well I know that I have to do 26.98 * 3 = 80.94g/mol Al

Lets crunch the numbers again, you are given the rate of consumption of chlorine as 0.2485 g/s and we have worked out that the molar mass of Cl₂ is 2 x 35.45 = 70.90 grams per mole. The number of moles of Cl₂ that has been consumed is 0.2485/70.90 = 0.0035 mol/s. The balanced reaction shows that 1 mole of Al reacts with 3 moles of Cl₂ so because we have 0.0035 mol/s of chlorine reacting, the number of moles of aluminium we need to react with this is 0.0035/3 = 0.00175 mol/s of Al. Do you understand the calculations so far?

[Arkcon]

If you can follow what UG: did, you can reverse the process, using the formula weight of AlCl₃, to begin to answer this question.  You can even use the atomic mass of aluminum to determine how much Al is being used -- and you should do that, you may find a slightly different problem come exam time.

What you have to learn is the relationship between moles and mass for pure substances.  If I'd told there was a pound of chlorine in each quart vessel, and I gave you 3 and 3/4 quarts, you'd determine the mass with almost no effort.  This is the same.

Also, you can't let yourself get mystified by funny tricks, like the per second calculation you had to do at the beginning.  If I, again, gave you 5 quarts, you'd know what to do.  If I gave you a quart a minute, for 3 and a quarter minutes, that's just a trivial step for you to work with.

Try it out on paper, thinking about these problems the way I've explained them.

[tg22542]

I got two questions, just wondering if you guys could validate my answers/help if I went wrong somewheres. Thank you very much in advance


1) Given the reaction:  Al+3Cl2->2AlCl3
It is found that chlorine is consumed at a rate of 0.2485g/s. Calculate the total mass of AlCl3 produced if this rate is maintained for 3 minutes.

2) Given the reaction: N2+3H2 -> 2NH3
If the rate of formation of NH3 is 8.0*10^-3 mol/s, calculate the rate of consumption of H2 in mol/s

my attempts:

1) 0.2485g/s * 60s/min * 3min = 44.73mol/s ? Not sure on this
2)the ratio of consumption = 1:3, NH3 is 8.0x10^-3 thus 3(8.0x10^-3)=0.024mol/s

[tg22542]

Anyone?

[Borek]

1) 0.2485g/s * 60s/min * 3min = 44.73mol/s ? Not sure on this

You are asked for the mass of AlCl₃ produced, and mass of the substance will be never measured in mol/s. Besides, even these units are wrong:

[tex]\frac g s \times \frac s {min} \times min = g[/tex]

not mol/s.

But generally you are going the right direction, you just need to pay more attention.

[Arkcon]

I hope you don't mind, tg22542:, that I merged the new thread with your older one on the same topic.  This will help everyone understand what you should already know, and help you avoid redoing things you don't need to.  That's why we don't care for cross-posts on these forums.

[tg22542]

I don't mind at all, thank you. And okay I see my units are incorrect, but are my number values right? If not where am I going wrong, I'm lost

[tg22542]

Okay I think I finally got this;

1) 0.2485*2/3 = 0.165667
0.165667*180s=29.82

But what should my final units be? g/s^2? Not sure

2) 8.0x10^-3 * 3/2

=0.012mol/s

Look good?
Thanks

[UG]

Remember that you need to convert mass into moles, you cannot work directly with mass. The rate of consumption of chlorine is 0.2485 g/s and we have worked out above that the molar mass of Cl₂ is 2 x 35.45 = 70.90 grams per mole. The number of moles of Cl₂ that has been consumed is 0.2485/70.90 = 0.0035 mol/s

Then the number of moles of AlCl₃ is the above number multiplied by 2/3. Then to convert to g/s of AlCl₃ you need to multiply by the molar mass of AlCl₃. You can then convert this into total mass of AlCl₃ formed in 3 minutes.