The AP Chemistry Study Thread

[sristi]

Tryingmybest~I think we need more information to solve the problem. If you got the problem from like an AP test, review book, etc. can you please give us exact question. I am completely stumped.

Answers to my question:
1) You are right
2) You are right
3) No
4)No
5) You are right

Anybody else?

[stanfordream]

1) i think N2 occupies 78%
3) H2
4) if the word "plar" is supposed to be "polar", then none

predicted the following reactions (2000 AP FRQ)
a) a small piece of calcium metal is added to hot distilled water.
e) a solution of tin(II) nitrate is added to a solution of sodium sulfide.
g) powdered strontium oxide is added to distilled water.

[youknowme]

for the desity part. is it at ATP? if not, then i don't think it can be solved.

[Evilbooyaa]

sristi
3) is H2
4) is CO2- the bonds between and C and O are the only bonds I could assume to be polar.

stanfordream, these are all net ionics (which I'm assuming where you are getting these from), I also balanced them, which is not necessary for net ionics
-2Ca + 4H2O --> 2Ca(OH)2 + 2H2
-Sn+2 + S-2 --> SnS
-Sr2O + H2O --> 2Sr+ + 2OH-

[shanianwang]

Since there's supposed be a lot of equilibrium on the test, here's one:


Calculate the pH at the equivalence point in the titration of 50 mL of .10 M methylamine (CH3NH2) with a .20M HCl.

Kb of CH3NH2 (methylamine): 4.4*10^-4 or Ka of CH3NH3+ : 2.3*10^-11



HINT: This is a weak base, strong acid titration, so you need to eventually find the pH for the hydrolysis of CH3NH3+ with ICE charts and approximation.

[sristi]

Stanford dream~ for #4: none of them are "polar" like water. But here you need to chose the best answer. only carbon dioxide has some degree of varying electonegativity.
#2 is H2

Shanianwang~ can you give the volume of the HCl. I think we need it to find the moles of H

[Evilbooyaa]

sristi, you don't need the volume.
your looking at equivalence point, where moles of CH3NH2 = moles of HCl.

M and moles, solve for volume.

50mL @ .1M CH3NH2 moles = .1*(.05) = .005 moles CH3NH2
.2M HCl and .005 moles = .005/.2 = .025L = 25mL.
Ch3NH2 + HCl --> CH3NH3+ + Cl-
.005 .005 0 0
-.005 -.005 +.005 +.005
0 0 .005 .005
^all moles^

new volume is 50ml + 25ml = 75mL. 005moles/.075L = .0666666666667M CH3NH3+

CH3NH3+ <----> CH3NH2 + H+
Ka = 2.3E-11 = [CH3NH2][H+]/[CH3NH3+]

[CH3NH2]=[H+]=x

2.3E-11 = x/.066666666666666666667
x = 2.3E-11*.066667 = some number
-log(x) = PH
don't have grapher on me, but there is the work.

[sristi]

Thanks. Didn't notice the part about the equivilance point. I got 5.9 for the PH

An ion containing only oxygen and chlorine is 31% oxygen by mass. What is its empirical formula?
NO CALCULATORS

A) Cl0
B) Cl02
C) ClO3
D) ClO4
E) Cl2O

[Evilbooyaa]

well Cl2O doesn't exist, so that's out.
31% oxygen means out of 100grams, 31 are oxygen. means 69g are clorine.

31g about equal to 32, which is 2 moles. 69 close enough to 71, which is 2 moles of Cl.
2moles of Cl
2moles of O

answer is A) ClO

let me know if i'm right.

[stanfordream]

new question please

[sristi]

EvilBooyaa~ you are right. give us a new questions

[Evilbooyaa]

meh not a fan of giving questions cause I make them unnecessarily hard.
but fine.

I-131 decays by emitting an alpha particle and an electron.
Write out the full equation representing the statement above.

[ziggytheman]

I-131-53 --->electron-0-(-1) + alpha-4-2 + Te-127-52.

correct me if im wrong, we're still on radiation (last section).

Balance the following half-rxn:

MnO4- ----> Mn+2

[theautumneffect]

MnO4- + 8H+ + 5e- ---> Mn 2+ + 4H2O

In a system that is at dynamic equilibrium:
a) increasing the temperature of the system will drive the reaction in which direction?
b) increasing the pressure of the system will drive the reaction in which direction?

[Scareya]

a) drive the reaction forward
b) drive the reaction forward - less moles

What is the pH of Acetic Acid (50 mL, .10 M) when it is titrated with NaOH (5 mL, .15 M)?