The AP Chemistry Study Thread

[tryingmybest]

My last question is just too hard to post up with numbers so here is another one...

Question: 2000 AP FR

.345 g BeC2O4 dissolved in H2O to produce 100. mL solution. 20 mL portion of solution titrated with KMnO4. Eqtn for reaction:

16H+ + 2MnO4- + 5C2O4---> 2 Mn 2+ + 10CO2 + 8H2O

Volume of .015 M (MOLARITY) KMnO4 required for equiv. point is 17.8 mL

a) Calc. # moles of MnO4- and C2O4 2- of each reacted at equiv. point.
b) Calc. # moles of C2O4 2- present in the 100. mL prepared solution.

[stanfordream]

new question:
design an experiment to create a buffer of pH 5, 100mL that will be capable of buffering 6.0*10^-5 mol of acid. you are given "unlimited" HC2H3O2 powder and "unlimited" .100M NaOH. (you need to detemine the amount of powder and solution you actually need for the buffer).

[stanfordream]

ahhhhhhh!!! you posted before me, i'll do yours later when i get more time, unless some1 else does before me.

[sristi]

tryingmybest~ at equivalence point, moles C2O4 should equal the moles of MnO4
MaVa=MbVb. Also M x L = moles
.015 M) (.0178 L)= 2.67 x 10^-4 moles of MnO4= 2.67 x 10^-4 moles C2O4

so if there are 2.67 x 10^-4 moles C2O4 in .020 L, there are X in .1 L
X= 1.34 x 10 ^-3 moles in .100 L.

Is it me, or is there too much extra information. Well since this question came from an AP, I will guess that there were more question on this problem.

[Scareya]

Here's an easy question for you guys:

Indicate whether the following balanced equations involve oxidation-reduction. If they do, identify the elements that undergo changes in oxidation number and the oxidation numbers.

a. PBr3(l) + 3H2O(l) -> H3PO3(aq) + 3HBr (aq)
b. NaI(aq) + 3HOCl(aq) -> NaIO3(aq) + 3HCl(aq)
c. 3SO2(g) + 2HNO3(aq) + 2H2O(l) -> 3H2SO4(aq) + 2NO(g)
d. 2H2SO4(aq) + 2NaBr(s) -> Br2(l) + S)2(g) + Na2SO4(aq) + 2H2O(l)

Does anybody know of a site where I can find info on the color of different types of acid/base indicators? (ie penophthalein sp?)

sry scareya, i'll get back to you q in a sec.

[Scareya]

Is this what you wanted?
http://www.101science.com/PH.html
http://en.wikipedia.org/wiki/PH_indicator

[Evilbooyaa]

stanfordream- HC2H3O2 is actually a solution (like NaOH), but i'll go with a powder I suppose..... maybe it is. never seen HC2H3O2 powder.

wait a second.
how do you make a buffer out of HC2H3O2 and NaOH?
you need NaC2H3O2 or something of the sort...... you need the acid and the anion/cation to produce a buffer..... you can't make it out of HC2H3O2 and NaOH...... can you?

help please!

Scareya- ill go thorugh the first one step by step
A)left- P=+3, Br=-1, O=-2 H=+1
right- H=+1, O=-2, P=+3, Br=-1

therefore, don't involve oxidation/reduction

2)'I' goes from a -1 to +5--> oxidation
'Cl' goes from +1(?) to -1--> reduction

3)'S' goes from +4 to +6--> oxidation
'N' goes from +5 to +2 --> reduction

4)'Br' goes from -1 to 0 --> oxidation
'S' goes from +6 to +4(in the S)2, which I assume to be SO2)--> reduction

stanfordream's question is still on the board, so I won't post a new one.

btw, sristi, don't feel bad about posting questions! it helps those too lazy to look for questions themselves

[stanfordream]

hint: to get NaC2H3O2(aq), HC2H3O2 + NaOH --> NaC2H3O2 + H2O

[Evilbooyaa]

ah...... true.

meh someone else will do it. I gotta go do other stuff real quick.

Ah thank you scareya.

[Scareya]

You're welcome.

What's the difference between a galvanic cell and an electrovoltaic cell?

[Scareya]

Aren't they the same thing?

[sristi]

galvanic cell and a voltaic cell are the same, I think. not 100% sure. I never heard of electrovoltaic. I think you are mixing it up electrolytic cells. voltaic cells produce electricity. electrolytic produces electricity.
Edit: I meant electrolytic requires electricity. Sorry!