G.Chem, O.Chem questions!!!!

[apbshah]

Can anyone clarify the following? Answers are in bold.

1. What is the hybridization and geometry of carbonyl carbon in an methanal?
a. sp, linear
b.sp2, pyramidal
c. sp2 trigonal planar
d. sp3, tetrahedral.

2.Consider a first order reaction where the concentration of the reactant decreases to 60% of the initial concentration in 5 minutes. The value of the rate constant in per minutes is:
a.0.0044
b.0.0080
c.0.10
d.0.18
e.None of the above.

3. A mixture of nitrogen and oxygen gas is collected by displacement of water at 25C and 600torr Hg pressure. If the partial pressure of nitrogen is 500 torr Hg, what is the partial pressure of oxygen? (Vapor pressure of water at 25 degrees is C=30)
a. 30 torr.
b. 120 torr.
c. 490 torr.
d. 70 torr.
e. 110 torr.

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[makwendo]

Can anyone clarify the following? Answers are in bold.

1. What is the hybridization and geometry of carbonyl carbon in an methanal?
a. sp, linear
b.sp2, pyramidal
c. sp2 trigonal planar
d. sp3, tetrahedral.

O
II
C
/ \
H H


Carbon hybridization sp2
Oxygen hybridization sp2

sp2 bond angles = 120 degrees = trigonal planar

2 lone pairs of electrons from oxygen pushing down on the H's to create a trigonal planar molecule

 

[714guy]

Can anyone clarify the following? Answers are in bold.


2.Consider a first order reaction where the concentration of the reactant decreases to 60% of the initial concentration in 5 minutes. The value of the rate constant in per minutes is:
a.0.0044
b.0.0080
c.0.10
d.0.18
e.None of the above.


first order reaction equation for kinetics = ln[final/initial] = -kt

ln [60%/100%] = - k (5 mins)

ln [60/100] = -.510825 = - k (5)

.102165 = k = reaction rate


3. A mixture of nitrogen and oxygen gas is collected by displacement of water at 25C and 600torr Hg pressure. If the partial pressure of nitrogen is 500 torr Hg, what is the partial pressure of oxygen? (Vapor pressure of water at 25 degrees is C=30)
a. 30 torr.
b. 120 torr.
c. 490 torr.
d. 70 torr.
e. 110 torr.


total pressure = PP (nitogen) + PP (oxygen) + PP (water)

NOTE: there is pressure of water also b/c it was collected over water.

600 torr = 500 torr (nitrogen) + PP (oxygen) + 30 torr (water)

600 - 500 - 30 = 70 torr = PP (oxygen)

.

 

[SMC2UCLA2_]

.

Kaplan blue book doesnt teach the 3 equations for the reaction rate constant of first, second and third order equations. I figured that is because these do not show up on the DAT...anyone else know otherwise?

 

[apbshah]

Thanks a lot. I thought the last question would involve some weird mathematics and formulas. I did try it by just subtraction, but I think I forget to subtract the pp of water. Thanks again.

Do they expect us to calculate ln(60/100) without calculator?

 

[714guy]

Kaplan blue book doesnt teach the 3 equations for the reaction rate constant of first, second and third order equations. I figured that is because these do not show up on the DAT...anyone else know otherwise?

the ln equation was derived from the first order reaction equation

[A final] = [A initial] e ^ -kt <-- that is in the kaplan book.

just divide by [A initial] and Ln both side.

apbshah - I don't know how you'd figure it out without a calculator, where did you get the questions from? I'll ask around if theres a better way to solve it and report back.

 

[SMC2UCLA2_]

the ln equation was derived from the first order reaction equation

[A final] = [A initial] e ^ -kt <-- that is in the kaplan book.

just divide by [A initial] and Ln both side.

apbshah - I don't know how you'd figure it out without a calculator, where did you get the questions from? I'll ask around if theres a better way to solve it and report back.

ahh true, but the second order and third order reaction equations are not in kaplan book. I could memorize them for the DAT but i assumed that we wouldnt need to know it. I havent come across a question before this that required using the reaction order equations.