Anybody that is good with Chemistry want to help me out?

[akushuki]

The question is:
Calculate the weight in grams of the primary standard solid oxalic acid needed to prepare 250 ml of a 0.200 N solution(answer in significant figures)

Knowing Normality = grams equivalent/ liters, I tried .2/1 = x/.25 and got x=0.05 and then 5.00e-2 in significant figures.
This apparently is wrong. So maybe grams equivalent doesnt actually equal grams. Or I am using the wrong formula.

Any help/hints/solutions would be greatly appreciated.

[akushuki]

1. Help?

Anybody at all?


Hello?

echos:
hello
hello
hello

[u4ic]

2. Sorry, can't help you

I like fencing, though. :-)

[akushuki]

3. What weapon are you?

I'm primary epee but the college I'm at now seems to be primary anything but epee (three epeeist) so I'm looking into competively fencing all three weapons.

[u4ic]

6. What weapon am I?

Why, I'm a nuclear bomb! :nuke:



grrrrroan...



I should clarify I like watching fencing. A decade or so ago, while in theatre school, I had to learn stage fighting, and hence, swordfighting. I liked sabre best. :hi:

[akushuki]

11. Stage Fighting is a little different.

for example: right of way doesnt apply, so stage fencers when fencing sabre bring the sword behind their head to get a good drawback for slicing at an angle. Thats bad in real fencing. But it is the flashyest and lets the audience understand what is going on.

[ZombieNixon]

4. I think you need to multiply that

by the molar mass of oxalic acid (H2C2O4 -> 2*1 + 2*12 + 4*16 = 2+24+64 = 112 grams per mole)

I don't know that that's the final answer, but you're definitely missing a grams per mole in there somewhere.

[akushuki]

5. hmmm..

Thank you, but the molar mass of oxalic acid I thought is (COOH COOH 2H2O)= 126 grams per mole.

.05 X 126 = 6.3 and that isnt the right answer either. :(
So I'll go about trying to integrate that in somewhere else into the equation.

[ZombieNixon]

7. Oops, I hit * where I should have hit +

x(

[sir_captain]

9. That's not the right molecular mass

Oxalic acid is H2C2O4 which is ~90.03 g/mol

[akushuki]

12. Straight from my lab book.

oxalic acid (COOH* COOH* 2H20) has two acidic hydrogens (*) and two waters of hydration. Only the acidic hydrogens are included in claculating equivalents per mole. the waters of hydration are included in computing the molecular weight of the acid.
And thats how I got the extra 36 grams/mole because I got the grams in they water too.

[sir_captain]

17. Ah, well, you see, that's a key piece of information

Though I'm not sure why anyone would count some sort of hydrous version of the acid. The chemical formula of Oxalic acid is definitely COOH-HOOC. But no matter. I will edit my below post with answers using both masses.

See post below.

[sir_captain]

8. I don't think you don't have it quite right

What is this for? Normality is really a standard measure of proton exchange that is mostly useful in titrations.

Keeping in mind that I haven't done gen chem in about a decade...

Normality = Grams/(Equivalent Weight X Volume)

Equivalent weight is the amount of solute needed to be the equivalent of one mole of hydrogen ions

In your case, .200 = x/((90.03/2) x .250)

so x = .200 * (45.015 * .250)

x = 2.25 grams

that said, I could be remembering this totally wrong

[akushuki]

15. U R TEH 1337 GENIUS!

U PWNS

Thanks alot, once I plugged in my molecular weight of 126 I got 3.15
But i never would have gotten that without your understanding of normality
THANKS!!!

[sir_captain]

18. Heh...beat me to it

Glad to help. I still don't understand why two water molecules are being added on, but as long as it's the right answer for you, that's all that counts. ;-)

You see why you had to divide the molecular mass by 2, right? Since oxalic acid is a diprotic acid, you only need 1/2 mole equivalents to equal 1 mole of H+ ions.

[akushuki]

20. Actually I dont understand the concept well at all.

I'm only taking this Chem 111 because I guess I need it to be an engineer. I find chem a little boring to actually want to take it. But my lab book says that hydration waters are included in calculating molecular weight. The other engineers on my floor were having problems with this too because none of us knew what Normality was, we tried google and wikipedia but both gave this answer that wasnt your nice little formula.

But yes, Thank you Thank you Thank you again.

[sir_captain]

22. I guess if it's a solid at room temperature

it must exist in a solid hydrated form in which case you would indeed have to include the mass of the water. That's got to be it.

Normality is a sort of weird concept. Like I said, it's mainly useful in predicting titration volumes, and it's not as if one needs to do that every day.

I'm surprised your class didn't really explain the concept, though...weird.

Anyway, glad I could help.

[akushuki]

24. My lecture is 500 people and chem discussion is 2 days after my lab

so noone really got a chance to ask about Normality. This was one of my prelab questions (I got all the rest), that need to be completed correctly before the lab is due. Its all done online so it tells you if you are right or not after inputing the value. I also dont remember normality at all from highschool. So its possible we didnt cover it then.

[sir_captain]

26. Got it

Yeah, I bet normality is usually skipped. Particularly since for a monoprotic acid like HCl it will be the same value as the molarity.

[deucemagnet]

23. If I remember correctly...

...it is because in solution, the cations associated with the acid are not free protons, but hydronium ions (H30+).

[sir_captain]

25. Well, that's true of all acids

but the H20 part of the hydronium ion belongs to the aqueous solution, and not the acid. Upon further reflection, it's because oxalic acid is a solid at room temp and exists in a hydrous solid form.

[deucemagnet]

27. That sounds right.

Jars of solid acids often have +H20 or +2H20 after their formulae. It's one of those little chemistry things that most of us biology-types pay little attention to, and confuses the hell out of others. :crazy:

[deucemagnet]

21. A very common mistake...

...I did my thesis in a microbiology lab where even postdocs would screw up solutions of polyprotic acids because they assumed molarity=normality (which it does, for most of the acids we used). But now you will never make that mistake! :hi:

[Floogeldy]

10. .710

:)

[akushuki]

13. sorry, but that isnt right either N/T

[Reverend_Smitty]

14. Not good with chemistry at all

so I'd just like to say that I feel your pain...I'm glad that I got through that semester with a somewhat respectable and totally undeserved (thank God for curves) B-

I'll light a candle for you and hope everything works out well :P

[Lethe]

16. how bout this?

?g = (90g / 1 mol)*(1 mol / ? eq)*(0.200eq / 1L)*(0.250L)

the only thing i dont know is how many eq are in 1 mol of H2C2O4. it depends on how many moles of H+ ions that oxalic acid contributes.

[sir_captain]

19. Oxalic acid is diprotic

so the equivalent weight is the molecular mass divided by two.

[Lethe]

28. chemistry rocks!

it's all about, like, chemicals and stuff.