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Watanuki
Newbie
Joined: 07 Oct 2012
Posts: 2
Location: United States

Posted: Sun Oct 07, 2012 12:53 pm Post subject: Chemistry Homework Questions 


I've worked out every question but I got 5 questions wrong and I don't know which ones, can someone help me out on which ones I got wrong?
Question 1
Give the major force between ethanol and water.
Answer
A. dispersion
B. iondipole
C. dipoledipole
D. ionion
E. hydrogen bonding
Question 2
What volume of a 0.716 M KBr solution is needed to provide 30.5 g of KBr?
Answer
184 mL
21.8 mL
42.7 mL
357 mL
Question 3
Determine the solubility of N2 in water exposed to air at 25°C if the atmospheric pressure is 1.2 atm. Assume that the mole fraction of nitrogen is 0.78 in air and the Henry's law constant for nitrogen in water at this temperature is 6.1 × 104 M/atm.
Answer
6.5 × 104 M
5.7 × 104 M
1.8 × 104 M
1.5 × 104 M
3.6 × 104 M
Question 4
Which of the following concentration units are temperature dependent?
Answer
mole fraction
none of the above
molarity
molality
mass percent
Question 5
Commercial grade HCl solutions are typically 39.0% (by mass) HCl in water. Determine the molality of the HCl, if the solution has a density of 1.20 g/mL.
Answer
6.39 m
9.44 m
10.7 m
17.5 m
39.0 m
Question 6
Calculate the mole fraction of total ions in an aqueous solution prepared by dissolving 0.400 moles of MgCl2 in 850.0 g of water.
Answer
0.00900
0.0248
0.0167
0.00841
0.0270
Question 7
A 4.55 L sample of solution contains 0.115 g of sodium ions. Determine the concentration of sodium ions in ppm if the density of the solution is 1.00 g/mL.
Answer
52.3 ppm
13.2 ppm
36.5 ppm
12.7 ppm
25.3 ppm
Question 8
A compound is found to have a molar mass of 598 g/mol. If 35.8 mg of the compound is dissolved in enough water to make 175 mL of solution at 25°C, what is the osmotic pressure of the resulting solution?
Answer
6.36 torr
8.36 torr
5.99 torr
5.01 torr
3.42 torr
Question 9
Identify the solute with the highest van't Hoff factor.
Answer
FeCl3
MgSO4
Nonelectrolyte
NaCl
MgCl2
Question 10
The boiling point of an aqueous 1.83 m salt solution is 102.5°C. Determine the value of the van't Hoff factor for this solute if the Kb for water is 0.512°C/m.
Answer
2.3
3.6
2.7
3.0
1.8
Question 11
1. Given the following balanced equation, determine the rate of reaction with respect to [NOCl]. If the rate of Cl2 loss is
4.84 × 102 M/s, what is the rate of formation of NOCl?
2 NO(g) + Cl2(g) → 2 NOCl(g)
Answer
2.42 × 102 M/s
4.84 × 102 M/s
1.45 × 101 M/s
1.61 × 102 M/s
9.68 × 102 M/s
Given the following balanced equation, determine the rate of reaction with respect to [SO2]. (D has been used to represent the Delta symbol in the answers because the font is not available)
2 SO2 (g) + O2(g) → 2 SO3(g)
Answer
Rate =  D[SO2]
Dt
Rate = +2D[SO2]
Dt
Rate =  1/2D[SO2]
Dt
Rate = + 1/2D[SO2]
Dt
Question 13
Give the characteristic of a first order reaction having only one reactant.
Answer
The rate of the reaction is proportional to the natural logarithm of the concentration of the reactant.
The rate of the reaction is not proportional to the concentration of the reactant.
The rate of the reaction is directly proportional to the concentration of the reactant.
The rate of the reaction is proportional to the square of the concentration of the reactant.
The rate of the reaction is proportional to the square root of the concentration of the reactant.
Question 14
What is the overall order of the following reaction, given the rate law?
2NO(g) + H2(g) → N2(g) + 2H2O(g) Rate = k[NO]2[H2]
Answer
2nd order
3rd order
0th order
1st order
4th order
Question 15
What are the units of k in the following rate law?
Rate = k[X][Y]2
Answer
1/(Ms^2)
1/(M^3 s)
1/(M^2 s)
M^2*s
M^2/s
Question 16
Given the following rate law, how does the rate of reaction change if the concentration of Y is doubled?
Rate = k [X]2[Y]3
Answer
The rate of reaction will increase by a factor of 9.
The rate of reaction will increase by a factor of 4.
the rate of reaction will increase by a factor of 8.
The rate of reaction will increase by a factor of 2.
The rate of reaction will remain unchanged.
Question 17
Determine the rate law and the value of k for the following reaction using the data provided.
CO(g) + Cl2(g) → COCl2(g)
[CO]i (M) [Cl2]i (M) Initial Rate (M1s1)
0.25 0.40 0.696
0.25 0.80 1.97
0.50 0.80 3.94
Answer
Rate = 18 M3/2s1 [CO]2[Cl2]1/2
Rate = 36 M1.8s1 [CO][Cl2]2.8
Rate = 4.4 M1/2s1 [CO][Cl2]1/2
Rate = 17 M2s1 [CO][Cl2]2
Rate = 11 M3/2s1 [CO][Cl2]3/2
Question 18
What data should be plotted to show that experimental concentration data fits a firstorder reaction?
Answer
1/[reactant] vs. time
[reactant] vs. time
ln(k) vs. 1/T
ln[reactant] vs. time
ln(k) vs. Ea
Question 19
How many halflives are required for the concentration of reactant to decrease to 12.5% of its original value?
Answer
2.75
1.75
1
2
3
Question 20
The halflife for the secondorder decomposition of HI is 15.4 s when the initial concentration of HI is 0.67 M. What is the rate constant for this reaction?
Answer
4.5 × 102 M1s1
3.8 × 102 M1s1
9.7 × 102 M1s1
2.2 × 102 M1s1
1.0 × 102 M1s1
Question 21
The firstorder decay of radon has a halflife of 3.823 days. How many grams of radon remain after 7.22 days if the sample initially weighs 250.0 grams?
Answer
76.3 g
183 g
67.5 g
4.21 g
54.8 g
Question 22
For a reaction, what generally happens if the temperature is increased?
Answer
there is no change with k or the rate
an increase in k occurs, which results in a faster rate
a decrease in k occurs, which results in a faster rate
a decrease in k occurs, which results in a slower rate
an increase in k occurs, which results in a slower rate
Question 23
The firstorder rearrangement of CH3NC is measured to have a rate constant of 3.61 x 1015 s1 at 298 K and a rate constant of 8.66 × 107 s1 at 425 K. Determine the activation energy for this reaction.
Answer
127 kJ/mol
160. kJ/mol
338 kJ/mol
240. kJ/mol
417 kJ/mol
Question 24
Given the following proposed mechanism, predict the rate law for the overall reaction.
A2 + 2B → 2AB (overall reaction)
Mechanism
A2 = 2A fast
A + B > AB slow
Answer
Rate = k[A2]
Rate = k[A2][B]1/2
Rate = k [A2]1/2[B]
Rate = k[A2]
(This one I choose) Rate = k[A][B]
Question 25
Carbon14, which is present in all living tissue, radioactively decays via a firstorder process. A onegram sample of wood taken from a living tree gives a rate for carbon14 decay of 13.6 counts per minute. If the halflife for carbon14 is 5720 years, how old is a wood sample that gives a rate for carbon14 decay of 11.9 counts per minute?
Answer
9.4 × 103 yr
7.6 × 102 yr
5.3 × 102 yr
1.1 × 103 yr 

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GrahamKemp
Distinguished Member
Joined: 19 Mar 2012
Posts: 396

Posted: Tue Oct 09, 2012 4:01 pm Post subject: 


Most of these look fine at first glance, the theory questions seems sound, and the calculations I checked are correct.
Which questions in particular do you lack confidence? 

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Watanuki
Newbie
Joined: 07 Oct 2012
Posts: 2
Location: United States

Posted: Tue Oct 09, 2012 7:23 pm Post subject: 


GrahamKemp wrote:  Most of these look fine at first glance, the theory questions seems sound, and the calculations I checked are correct.
Which questions in particular do you lack confidence? 
Fellow classmates have told me Question 8, 22, and 24 were wrong, that it is suppose to be 6.36 torr, an increase in k occurs, which results in a faster rate, and Rate = k [A2]1/2[B]. Supposedly, my answer 8.36 was in atm, and it was suppose to be in torr. I found the second one in the book. #24, I don't particularly understand, knowledge that I have of it is that the slow step is the rate determining step, that answer, I don't particularly understand. My professor said there might be an error in one of the questions, so i might of gotten one more right. That leaves one more question wrong unaccounted for. I can't guess as to which one in particular that I am lacking confidence in though. So I guess if you can explain #24, that would be great. 

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GrahamKemp
Distinguished Member
Joined: 19 Mar 2012
Posts: 396

Posted: Tue Oct 09, 2012 7:49 pm Post subject: 


The slow step is the rate determining step, but that means you ignore the subsequent steps. The rate is the combination of all elementary steps up to and including the bottleneck (and cancelling intermediates).
A2 + 2 B = 2 AB
========
{A2 > 2 A}, k1 fast
{A + B > AB}, k2 slow
Rate = 1/2 d[AB]/dt = k2 [A][B]
[A] can't be in the answer, as it's an intermediate, but can be derived from [A]^2 = k1 [A2].
Rate = 1/2 d[AB]/dt = k1k2 [A2]^(1/2)[B] 

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