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some pittsburgh flavor

chemistry help PLEASE

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* chemistry help thread *

 

i have a chemistry test tomorrow (wednesday 11:05 AM) and my teacher is a dolt and will not help me figure shit out. he does not teach me correctly. i know some of you out there are intelligent. moreso than myself, perhaps. i would appreciate if someone could help me in a few areas. any help before the given time would be greatly appreciated and you will get mounds of e-love from me. (step-by-step if you have the time/care?)

 

1)how would i go about finding the [H3O]+ count of a solution that contains 0.478 moles of sulfuric acid H2SO4 and 0.264 moles of aluminum hydroxide Al(OH)3 and has a total volume of 200.7 L?

 

2)If 12.5 mL of a 0.512 molar acetic acid solution (CH3COOH) and 25.0 mL of a .314 molar magnesium hydroxide solution react, how do i figure out the resulting pH of the solution?

 

3)how do i figure out the [H3O]+ of a 0.0500 molar sulfuric acid solution?

balanced equation: H2SO4 + H2O --> H3O + HSO4

i have no idea what to do beyond the point of setting up the reaction

 

4)if 24.5 mL of 0.765 molar hydrochloric acid (HCl) is titrated to its endpoint with 15.8 mL of aluminum hydroxide solution, what is the concentration of the base? (Al(OH)3)

 

i can't fail this test and i've been studying for the past 6 hours.

 

with much gratitude,

SPF

------------------------------------------

*edit-

sounds corny, but i will be back come quiz or test time. any of you other high school kids who can't understand a damn thing, ask away. shit, i might even be able to help you out.

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Fuck, i just realized that i mixed up the formula i wrote down here.. so just ignore allll the shit i posted for #3 earlier (even though i still couldn't figure that bitch out)

 

 

If i don't fall asleep, i'll see if i can take another crack at it.

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Guest -MOE LESTER-

my school tought me that 1 mole = 1.062 x 10'23

 

Al is short for aluminum

 

the lab has some cool electric scales too

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no, there was no volume for the sulfuric acid solution... maybe that's why it's so puzzling?

 

thank you, McPhoo

 

further help on the remaining questions is still able to be given, don't be shy

 

----------

SPF currently owes bitch-hood to:

McPhoo

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Guest xKillmodEx

--I Studied To Death In AP Chemistry--

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Ok, i'm still bored outta my skull, and since my own work seems trivial, i'll take another stab at yours.

 

#1 and 2 still baffle me, so i'll skip them for a bit.

 

#3 still eludes me due to the fact that it seems there isn't enough information given to solve the question (or at least i would think so). Does it say anything about STP or SATP?

 

#4 Try this: (*NOTE: I think this is the way to do it, but again, i'm not sure.)

 

Step 1: Set up the balanced equation. I did this quick, so check my balancing:

 

3 HCL + 1 Al(OH)3 ---> 1 AlCl3 + 3 H2O

 

Step 2: Calculate the moles of HCL. Since you are given the volume used and the concentration, plug the values into the forumla [ n=©x(v)] where 'n' = moles, 'c' = concentration and 'v' = volume

 

n = c x v

= (0.765 mol/L) x (0.0245 L) <-- gotta change mL to L

= 0.0187425 moles HCL

 

Step 3: Now that you know the moles of one of the substanes, you can set up a ratio to figure out the number of moles of the unknown substance (in this case, Al(OH)3 ). The ratio is as follows:

 

Moles of known substance x (moles of unknown substance [get this from the balanced equation] / Moles of known substance)

 

=0.0187425 mol HCL x (1 mol Al(OH)3 / 3 mol HCL) <--- both those numbers come from the coefficients in the balanced equation - use them to set up the ratio

 

= 0.0062475 mol Al(OH)3 (the HCL on the top and bottom cancel out)

 

Step 4: Now that you have the moles of the Al(OH)3 and you are already given the volume (amount titrated), you can again use the [ n = ©x(v)] formula to solve for the concentration.

 

n = c x v (manipulate the formula for 'c')

c = n / v

c = (0.0062475 mol / 0.0158 L) <-- again, change mL to L

 

c = 0.395... <-- then round off to whatever significan digit is asked.

 

Soooo, if i've done everything correctly, then concentration of the Al(OH)3 solution SHOULD be 0.395 mol/L or somewhere around there.

 

Hope i helped a bit.

 

Ps - i don't want/need yer bitch-hoodness.. i should know this stuff too, it's good practice. (nerds unite)

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mcphoo, thank you very much, that seems to be correct.

i'm still lost on thousands of other things but now i know at least one. heh.

 

tee hee bitch-hood is only a pretend term for people who would not like to help one SPF with his chemistry without some sort of insentive;)

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Guest Dr. Drew
Originally posted by some pittsburgh flavor

 

1)how would i go about finding the [H3O]+ count of a solution that contains 0.478 moles of sulfuric acid H2SO4 and 0.264 moles of aluminum hydroxide Al(OH)3 and has a total volume of 200.7 L?

 

2)If 12.5 mL of a 0.512 molar acetic acid solution (CH3COOH) and 25.0 mL of a .314 molar magnesium hydroxide solution react, how do i figure out the resulting pH of the solution?

 

3)how do i figure out the [H3O]+ of a 0.0500 molar sulfuric acid solution?

balanced equation: H2SO4 + H2O --> H3O + HSO4

i have no idea what to do beyond the point of setting up the reaction

 

i'm at work so i can't go into detail and try to figure them out, but, the only way i remember (at this time) to figure out these problems is by finding a table of IONIZATION CONSTANTS. usually called "K" with some type sub letter after it (Ka, Kb, etc) see if there is one in your text book. it should be in the same chapter that these types of problemas are given in, or in the apendix in the back. it will list the reaction, and the K value for each reaction

 

these problems involve setting up a table of ICE--Initail, Change and Equilibrium. that should also be in your text. this table can't be typed out on this forum, so i can't show you, but all i'm saying is go look in the text for it, its in any chemistry text book. it involves giving variables to new concentrations and using the K value and a little basic algebra to get the new concentrations after the reaction at equilibrium.

 

for problems 1 and 2 you need to set up the reaction like how you did in problem 3. the initial concentrations can be figured out using the volume and moles in the same way McPhoo showed you for problem 4, next you can determine how many moles are in each compound after the reaction at equilibrium by the ICE method, then use the equation K = ([P][P]/[R][R]) where [P]=concentration of products and [R] is concentration of reactants.

 

these should give you the concentrations you need for problem 1 and 3, and for problem 2, figure out the concentration of [H+] and then take log based 10 of that concentration and take away the negative sign and you get the pH.

 

sorry i couldn't go into better detail, but i'm at work, and i'm not even supposed to be in the internet. i dont wanna get caught.....

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Guest imported_El Mamerro

That just gave me a terrible, awful, and simply no-good high school flashback. I hate you.

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damn......

 

general chemistry..hadnt thought about this stuff in years..

 

looks like the test is over though..shoulda asked for help sooner!! so, a general strategy..

 

 

most chemistry problems need a balanced chemical equation, which will tell you a lot about whats going on..keep everything very clear and all information laid out in front of you..

 

come up with the balanced equation..which tells you the mole ratio...whole numbers which designate the relative amounts that go into, and come out of a chemical reaction...remember, matter is conserved..so nothing disappears or appears, just converted..that's why the equation is so informative.

 

the problem should give you exact amounts of either reactants or products..write these amounts just above the proper chemical symbol above the equation..

 

make a list

this should include all knowns, all unknowns, and ALL CONSTANTS...

 

(for some problems avogadro's number might be a needed constant, for others ionization constants, etc.)

 

the trick is to develop a problem solving strategy..i wish i could tutor you in some basic chemistry, cuz i was good at it, but damn..don't have the time here in a forum..

 

try...

www.chemtutor.com

http://www.chemtutor.com/roadmap.gif'>

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ok.. i think i got a D on the test so i'm excited for passing. Dr. Drew knows his shit... the chemistry textbooks we have don't have the acid constant tables in them and i was probably supposed to have a sheet with the table on it... oh well. chemtutor.com.... i never knew about that. i suppose we can keep this thread alive as the "chemistry help thread" because lord knows i'll be back. if not me, someone else.

 

thank you to mcphoo, dr. drw, and [expletive].

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Guest PHYNE

make lsd............................................and sell it to dirty hippies who smell like patrooly [sp]....i dunno.....man i seen the chemical make up of lsd on some sit yesterday...i love google searchs.........im barred out ......blah ..........

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2mg alprazolam;)

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Guest uncle-boy
Originally posted by El Mamerro

That just gave me a terrible, awful, and simply no-good high school flashback. I hate you.

 

argh....

 

 

 

and word to the DREW, you smart ass.

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