Need some help with gen chem chem questions

[flybry2000]

So I've been going through an old addition of the Barron's DAT study guide, and I'm just not sure if some of the answer choices are wrong or it's me (which could be a strong possibility) Anyone care to take a stab at these and see what you come up with, with a short explanation? Thanks!

1) What is the mass in grams, of 245 milliliters of SO2 at STP?

A) 7.01
B) 0.694
C) 0.651
D) 0.732
E) 6.51

2) For the reaction A+B==> C, the rate of formation of C is given by the formula Rate=k[A]^2. Doubling the concentration of A will:

A) Quadruple the initial rate
B) Double the initial rate
C) Have no effect on the initial rate
D) Reduce the rate to a 1/4 of its value
E) Reduce the rate to a 1/2 of its value

3) Catalysts have which of the following effects on a chemical reaction

A) Lower the activation energy
B) They increase the free energy
c) They cause the reaction to proceed spontaneously
D) They increase the rate at which the product is formed
E) They shift the equilibrium so the reverse rxn proceeds easier

4) Which has the lowest percent of carbon by mass?

A) CH4
B) C2H4
C) Na2C2O2
D) C6H12O6
E) CH3CO2H

5) How many atoms of carbon are in 4.0x10^-8 grams of C3H8?

A) 1.64x10^15
B) 2.34x10^11
C) 1.89x10^14
D) 4.66x10^2
E) 4.00x10^4

6) If all the chloride in a 5 gram sample of an unknown metal chloride is precipitated as AgCl with 70.9 mililiters of 0.201M AgNO3, what is the percentage of cholride in the sample?

A) 50.55%
B) 20.22%
C) 1.43%
D) 10.10%
E) none of the above





Answers:
1)B 2)B 3)B 4)C 5)E 6)E
My Answers:
1)? 2)A 3)A 4)? 5)A 6)?

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[305602]

1. Use the equation PV = nRT, and n=grams/MM. With this, we have that: grams/64.05 = (1 atm)(.245 L) / (0.08206)(298 K)

2. This uses Rate Law. You are given the Rate Law as:
Rate=k[A]^2. All you have to worry about in this problem is [A], because [A] is technically [A]^1, if you double [A], Rate must then also double.

 

[UCB05]

1. At STP, 1 mol of any gas occupies 22.4L of volume. Therefore using ratios (0.245/22.4) = x/1. x = 0.0109 mols SO2. MM = 64, therefore there are .7g SO2. Closest answer is B

2. The rate formula indicates that the reaction is FIRST order with respect to A and second order with respect to B. Doubling [A] will double the overall rate of reaction. Doubling will quadruple the rate of reaction. Answer would be B

3. I would have said A as well. I don't know what makes a catalyst increase free energy.

4. Strictly eyeballing, carbon is the dominant constituent of A and B, and exists in a 1:1 ratio with O in D and E. In option C, two carbons look to be outweighed by two O's and two Na's much more than any other option, therefore I would pick C without any math. If you actually calculate out the % by mass the answer should indicate C as well.

5. I got A) 1.64x10^15
4.0x10^-8g = 9.1x10^(-10) mol = 5.47x10^(14) molecules of C3H8 = 1.64x10^15 atoms of C

6. 70.9 mililiters of 0.201M AgNO3 reacts with Cl. Therefore
(0.0709)(.201) = 0.014mols Ag+ reacted = 0.014mols Cl- reacted
MM of Cl is 35.5, therefore (.014)(35.5) = 0.506g Cl- reacted.
Since you started with 5g of the sample, %Cl = (100)(0.506)/5 = 10.11%
Closest answer is D. Either I set up the problem wrong, the answer is wrong, or one unit off the 4th significant digit isn't good enough for them.

Someone let me know if I did something wrong in any of these. Otherwise Barron's DAT is batting 3/6

 

[UndergradGuy7]

I got the same as UCB05's answers.

 

[JBarr29]

Yeah 3 is definitely incorrect.
A catalyst won't change the free energy - catalyst has no effect on delta G, but does lower activation energy. Answer book must be wrong.