The AP Chemistry Study Thread

[shanianwang]

AP Chemers unite!


Which of the following dissolved would produce a slightly basic solution:

A) KNO3 B) FeCl3 C) KCN D) NH4Cl E) CH3NH3


For confused people: you answer the question, then pose your own question in the same post.

[sristi]

I am going to say C) KCN.

pretty sure as CN will form a weak acid with by snatching a hydrogen from water thus liberating some OH. The others except for E) are conjugate bases of strong acids so they wont do anything. I really cant explain anything about CH3NH3. can anyone help me with that?

My question:
A 22.0 gram sample of an unknown gas occupies 11.2 Liters at standard temperature and pressure. Which of the following could be the identity of the gas?
A) CO2 (carbon dioxide)
B) S03 (sulfite)
C) O2 (oxygen gas)
D) N2 (nitrogen gas)
E) He (Helium)

Good Luck !

[stanfordream]

This was posted on another thread by Evilbooya:
"you have solutions of AgNO3, Na2CO3, NaCl, and H2S in solutions of W,X,Y,and Z. using only test tubes and graduated cylinders, explain how you would determine which solution is what (no Ph paper allowed)"

Solution:
1. Add HCl: AgNO3 + HCl will form precipitate (AgCl); Na2CO3 will produce CO2 gas
2. i'm not sure about H2S, but i think maybe if you add NaOH, you'll get SO2 gas.
3. the one remaining must be NaCl

Edit: to the above question. (A) CO2 22g/.5mol=44g/mol

Why is water the most dense at 4 degrees C? (compared to its density at other temperatures)

[smder99]

something to do with IMF...not sure tho
H-bonds or something go figure

[theoneo]

Where's the question?!

[youknowme]

stanfordream, if you can only use test tubes and graduated cylinders, where did the HCl come from.
My question:
Mg(s) + NO3¯(aq) + H+(aq) --->Mg2+(aq) +NH4+(aq) + H2O(l)

When the skeleton equation above is balanced and all coefficients reduced to their lowest whole-number terms. what is the coeficient for H+

[Evilbooyaa]

stan thanks for bringing my question to this thread, but unfortunately your answer is incorrect (you can't use HCl or NaOH, but something else is there if that's a hint for ya)

the first one is KCN - CN- + H2O <--> HCN + OH-

2nd one- PV = nRT T = 273, P = 1, V = 11.2
n = 11.2/ (.0821*273)
n = .5 mol.
22g/.5mol = 44g MM = CO2.

stan for yours, I actually have no idea. wow, nice question.

you know me- damn redox..... gotta do this out on paper.
Mg + NO3- --> Mg+2 + NH4+
breaks down into
Mg --> Mg+2 and NO3- --> NH4+
Mg --> Mg+2 + 2e- = 4Mg --> 4Mg+2 + 8e-
NO3- + 10H+ + 8e- --> NH4+ + 3H2O

combine the two, the 8e- cancel
4Mg + NO3- + 10H+ --> NH4+ + 3H2O + 4Mg+2

coefficient for H+ is 10 assuming the solution is acidic.

[stanfordream]

sorry, i miss intepreted the question. lemme try again
X+Y yields precipitate and X+Z also yields precipitate; X+W has no precipitate. Thus, X is AgNO3, Y is either Na2CO3 or NaCl, and Z is the other. This leaves W as H2S.
Now add H2S to both Y and Z. the one thats Na2CO3 will produce CO2. the one remaining is NaCl

actually, about the question regarding water density, i have no idea why thats the case (its something about IMF) - i just know that it's 4 degrees C and have pondered about that for a while (did some book research and couldnt find a solid reasoning)

now for a question,
What is the electron configuration of Au?

[mrniceguy]

[Xe] 6s^2 4f^14 5d^9

Q) Write a balanced equation for the dissociation of Iron(II) hydroxide (Ksp=8.0x10^-16). Then, calculate the molar solubility of Fe(OH)2.

[theoneo]

Fe(OH)2 --> Fe2+ + 2OH-

(x)(2x)^2 = 8.0e-16 --> 4x^3 = 8.0e-16 --> x = 5.8e-6

-

What volume of dry carbon dioxide, measured at 25 degrees Celcius and 785 mm Hg, will result from the complete combustion of 2.50 g of pentane?

[youknowme]

mole of C5H12=2.5g/72(g/mole)
mole of CO2=5*(2.5/72)
Pv=nRT
V=nRT/P= 5*(2.5/72)*8.31*(25+273) / (785/760) = a number

don't feel like to do the math. lol

I have a couple of questions


1. Activation energy for an elementary process

1) Is part of the net enthalpy change for the reaction
2) None of these
3) Is smaller for slower reactions and larger for faster reactions
4) Is the energy required to produce the transition state from the reactants



2. A reaction mechanism has several steps. The rate of the overall reaction is equal to

1) the average rates of the steps
2) the rate of the slowest step
3) the rate of the fastest step
4) the difference between the rate of the fastest step and the rate of the slowest step

I put 3 for the first one and 2 for the second one. Can anyone verify?

1. Consider the multi-step reaction that has the overall reaction
2A + 2B ---- C + D

What is the rate law expression that would correspond to the following proposed mechanism?

A + B <-------> I (fast)
I + B -------> C + X (slow)
X + A --- D (fast)

1. Rate = k [b]
2. Rate = k [A]2[b]2
3. Rate = k [A]
4. Rate = k [A]2[b]
5. rate = k [i][b]
6. Rate = k [A]2
7. Rate = k [A][i][b]
8. Rate = k [A][b]2
9. Rate = k[A][b]

I put 5.


2. The reaction 3A + 2B + 4C -----> products

Is known to be second order in A, first order in B, and independent of the concentration of C. What is the value of the rate constant for this reaction if its rate is 0.695 M*s^-1 when [A] = 5.00 M, [b] = 7.00 M, and [C] = 3.00 M?

I got 0.00397 M^-2 s^-1



3. For the reaction 2NO2 ----> 2NO + O2

Rate = 1.4 * 10^-10 M*s [NO2]2 at 25.0 C.
If 2 mol of NO2 are initially present in a sealed 1.00 liter vessel at 25 C, what is the concentration of NO2 after 140 years?

I got 0.894 M.





The decomposition of sulfuryl chloride into sulfur dioxide and chlorine

SO2Cl2(g) ----> SO2(g) + Cl2(g)

Follows first-order kinetics. At 320 C the rate constant is 2.2 * 10^-5 sec^-1. If one started with a sample containing 0.16 moles of sulfuryl chloride per liter at 320 C what concentration would be left after 6.00 hours?

1) 0.10 M
2) 0.15 M
3) 0.052 M
4) 0.12 M
5) 0.26 M

I put 1.

Consider the reaction
2NOCl (g) -----> 2 NO (g) + Cl2(g)

With rate constant 0.0480 M^-1 s^-1 when conducted at 200 C. The initial concentration of NOCl was 0.521 M. What is the concentration of NOCl after 0.535 minutes and at 200 C?

1. 0.289 M
2. 0.0239 M
3. 0.112 M
4. 0.508 M
5. 0.200 M

I put 3 for this one.


Can anyone verify these answers? Thanx.

[celebrian25]

I have a question:

how do I figure out the units of K for molecular collisions?

You take the rate and divide it by the concentration of the reactants.