Chemistry homework help! : Othor Games

» Thu Dec 08, 2011 7:36 pm

Okay, so I'm studying for an upcoming test... and well, I seem to have forgotten COMPLETELY how to do the simplest calculation equation possible. I know everything else, but looking at this question has made my mind go blank .___.

Question: "What mass of hydrogen gas would be evolved if 6.075g of magnesium reacted completely with dilute sulphuric acid?"

I have the equation "Mg + H2SO4 --> MgSO4 + H2" but besides that I am stumped.

Please forgive my noobiness

» Thu Dec 08, 2011 7:14 pm

table of elements

» Thu Dec 08, 2011 8:45 am

Convert the grams of Mg into moles by diving the grams of Mg with Mg's molecular weight.
Seeing has how there isn't a reported mass, volume, or moles of sulfuric acid, I'll assume it's in excess, and therefore Mg is the limiting reagent.
The reaction of Mg to H2 is a 1:1 reaction, so 1 mole of Mg will give 1 mole of H2
Convert the moles of H2 into grams, and there's your answer.

Do all of this, and you'll get your answer. Remember to round at the very end.

Also, why "dilute sulfuric acid"? It's going to completely dissociate anyway (for all intents and purposes anyway), and it mentions this goes to completion. It could have been pure H2SO4 for all it matters.

» Thu Dec 08, 2011 9:25 pm

Oh wow, I haven't done this kind of chemistry in a while... I'm working on it now, but while I'm doing that just check to see if your equation is balanced.

Edit: Okay, after a bunch of mucking about with atomic masses and what not I finally came upon the answer of either:

Spoiler

removed

I'm really really not sure at all though.

» Thu Dec 08, 2011 5:09 pm

table of elements

Uh.

Convert the grams of Mg into moles by diving the grams of Mg with Mg's molecular weight.
Seeing has how there isn't a reported mass, volume, or moles of sulfuric acid, I'll assume it's in excess, and therefore Mg is the limiting reagent.
The reaction of Mg to H2 is a 1:1 reaction, so 1 mole of Mg will give 1 mole of H2
Convert the moles of H2 into grams, and there's your answer.

Also, why "dilute sulfuric acid"? It's going to completely dissociate anyway (for all intents and purposes anyway), and it mentions this goes to completion. It could have been pure H2SO4 for all it matters.

Thank you! and, I dunno :shrug:

» Thu Dec 08, 2011 6:48 am

It is balanced, Holy Assassin. And don't give him the answer, then he won't learn.

And Katana Master, that's just me ranting. Fluffy language like that is only to trip up people, and some teachers/professors do it on purpose. But I assume you're in highschool chemistry, so I see even less point in writing a bunch of fluff. Saying "In a solution of sulfuric acid in excess" would have been nicer.

» Thu Dec 08, 2011 3:35 pm

It is balanced, Holy Assassin. And don't give him the answer, then he won't learn.

I got 0.5g of Hydrogen. Correct?

» Thu Dec 08, 2011 3:44 pm

It is balanced, Holy Assassin. And don't give him the answer, then he won't learn.

Yup first thing you always look for is to see if the equation is balance.

» Thu Dec 08, 2011 4:40 pm

I haven't done anything, just explained what to do. One sec, I'll double check with you.

EDIT: Close, but unless one of your numbers had only 1 significant figures, the answer should be 0.5039g, as the original mass of Mg had the lowest Sig. Figs. when I calculated it.

» Thu Dec 08, 2011 3:15 pm

It is balanced, Holy Assassin. And don't give him the answer, then he won't learn.

Alright, I removed it.

» Thu Dec 08, 2011 5:20 am

Well, not until he finds the answer, which he seems to have done. Sorry, I should have been more clear.

» Thu Dec 08, 2011 8:22 am

It is balanced, Holy Assassin. And don't give him the answer, then he won't learn.

And Katana Master, that's just me ranting. Fluffy language like that is only to trip up people, and some teachers/professors do it on purpose. But I assume you're in highschool chemistry, so I see even less point in writing a bunch of fluff. Saying "In a solution of sulfuric acid in excess" would have been nicer.

It's cool

I was just lifting the question from this booklet I have.

I haven't done anything, just explained what to do. One sec, I'll double check with you.

EDIT: Close, but unless one of your numbers had only 1 significant figures, the answer should be 0.5039g, as the original mass of Mg had the lowest Sig. Figs. when I calculated it.

My steps:
no. of moles = 6.075/24.3 (24.3 being the gram formula mass of magnesium, I got the relative atomic mass from my data booklet) which equalled 0.25. Then using the 1:1 ratio I found the mass = 0.25 x 2 (2 being the gfm of hydrogen) equalling 0.5g

» Thu Dec 08, 2011 8:13 am

Does that still fit in with sig figs?
I've forgotten that stuff.

» Thu Dec 08, 2011 12:34 pm

that number is horrifically...short. Sorry, used to seeing MWs with 4 decimals at least. Also, you really should use the accurate weight of H2 gas, as Hydrogen is not 1g/mole, but 1.0079. Then again, it's likely a HS course, so I doubt this level of accuracy is required.

» Thu Dec 08, 2011 4:59 pm

that number is horrifically...short. Sorry, used to seeing MWs with 4 decimals at least. Also, you really should use the accurate weight of H2 gas, as Hydrogen is not 1g/mole, but 1.0079. Then again, it's likely a HS course, so I doubt this level of accuracy is required.

Yeah, I should have said it's just wee numbers. Just HS (technically Academy here) stuff :laugh: Thank you for the help.

» Thu Dec 08, 2011 5:06 pm

EDIT: Does your book say Hydrogen is 1.01, or just 1.0? If it's the former, 3 sig figs are needed, and the moles should be in the form of 2.50x10^-1 moles if so. Otherwise, yeah just 0.5

» Thu Dec 08, 2011 5:02 pm

EDIT: Does your book say Hydrogen is 1.01, or just 1.0? If it's the former, 3 sig figs are needed, and the moles should be in the form of 2.50x10^-1 moles if so. Otherwise, yeah just 0.5

Just 1.0 :laugh:

» Thu Dec 08, 2011 10:03 am

Also, why "dilute sulfuric acid"? It's going to completely dissociate anyway (for all intents and purposes anyway), and it mentions this goes to completion. It could have been pure H2SO4 for all it matters.

Probably because the reaction of concentrated sulfuric acid with magnesium can be rather... energetic. Basically if the experiment were to actually be performed you'd want to use dilute sulfuric acid because otherwise you'd be collecting the broken remnants of your glassware off the floor.

» Thu Dec 08, 2011 5:56 pm

True....but still.

» Thu Dec 08, 2011 11:50 am

Sodium
Water

» Thu Dec 08, 2011 1:26 pm

Just 1.0 :laugh:

Pfft. Back in my day we used 1.00794 amu for Hydrogen.