Need help with these gen chem questions

[Chrono]

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Hey guys,

I'm trying to figure out how they got the answer for these questions, and need some help.

43.) For the reaction between A and B to form C it
is found that when one combines 0.6 moles of
A with 0.6 moles of B, all of the B reacts, 0.2
moles of A remain UNREACTED and 0.4
moles of C are produced. What is the
balanced equation for this reaction?
A. A + 2B -------->C
B. A + 3B ------>2C
C. 3A + 3B-------> 2C
D. 3A + 2B------->3C
E. 2A + 3B-------->2C

(the answer is E)


54.)

What is the hydroxide ion concentration,
[OH– ] , of a solution having a pH of 5.0?
A. 5 x 10–5 M
B. 5 x 10 M
C. 1 x 10–5 M
D. 1 x 10–9 M
E. 5 x 10–9 M

(the answer is D)


56.)
For the reaction:
AgCl(s) + 2NH3(aq) Ag(NH3)2+(aq) + Cl–(aq),
the equilibrium constant K = 4 x 10–3, which
of the following statements is true?
[Ksp for AgCl is 1.0 x 10–10]

A. The addition of NH3 decreases the
solubility of AgCl.
B. AgCl is more soluble in aqueous NH3
than in water.
C. AgCl is more soluble in aqueous
solution containing Cl– than in water.
D. AgCl is less soluble in aqueous NH3
than in water.

(the answer is B)


58.)

(the answer is E)


64.

Which of the following represents the
CHANGE in oxidation state of nitrogen during
the chemical reaction?
2NO + 3S + 4H2O--------->2HNO3 + 3H2S ?
A. 1
B. 2
C. 3
D. 4
E. 5

(the answer is C)



Thanks!

 

[azar87]

Hey guys,

I'm trying to figure out how they got the answer for these questions, and need some help.

43.) For the reaction between A and B to form C it
is found that when one combines 0.6 moles of
A with 0.6 moles of B, all of the B reacts, 0.2
moles of A remain UNREACTED and 0.4
moles of C are produced. What is the
balanced equation for this reaction?
A. A + 2B -------->C
B. A + 3B ------>2C
C. 3A + 3B-------> 2C
D. 3A + 2B------->3C
E. 2A + 3B-------->2C

(the answer is E)


54.)

What is the hydroxide ion concentration,
[OH– ] , of a solution having a pH of 5.0?
A. 5 x 10–5 M
B. 5 x 10 M
C. 1 x 10–5 M
D. 1 x 10–9 M
E. 5 x 10–9 M

(the answer is D)


56.)
For the reaction:
AgCl(s) + 2NH3(aq) Ag(NH3)2+(aq) + Cl–(aq),
the equilibrium constant K = 4 x 10–3, which
of the following statements is true?
[Ksp for AgCl is 1.0 x 10–10]

A. The addition of NH3 decreases the
solubility of AgCl.
B. AgCl is more soluble in aqueous NH3
than in water.
C. AgCl is more soluble in aqueous
solution containing Cl– than in water.
D. AgCl is less soluble in aqueous NH3
than in water.

(the answer is B)


58.)

(the answer is E)


64.

Which of the following represents the
CHANGE in oxidation state of nitrogen during
the chemical reaction?
2NO + 3S + 4H2O--------->2HNO3 + 3H2S ?
A. 1
B. 2
C. 3
D. 4
E. 5

(the answer is C)



Thanks!

Sorry i can't help with all the problems right now but i can help you with prob 54 since the pH=5 that measures the amount of H+ in solution... so now that we have the ph is 5 the -log of 5 will give you 1 x 10^-5 will give you the amount of H+ and that leaves 1x10^-9 the remainder for the concentration of OH-......since the pOH +ph =14
Hope that helps

 

[hausee]

43.) For the reaction between A and B to form C it
is found that when one combines 0.6 moles of
A with 0.6 moles of B, all of the B reacts, 0.2
moles of A remain UNREACTED and 0.4
moles of C are produced. What is the
balanced equation for this reaction?
A. A + 2B -------->C
B. A + 3B ------>2C
C. 3A + 3B-------> 2C
D. 3A + 2B------->3C
E. 2A + 3B-------->2C

(the answer is E)
========
0.2 A unreacted so only 0.4 A react with 0.6 B and form 0.4 C. A:B->C equ. 0.4:0.6->0.4 and you will get 2:3->2, thus 2A+3B->2C

 

[hausee]

What is the hydroxide ion concentration,
[OH– ] , of a solution having a pH of 5.0?
A. 5 x 10–5 M
B. 5 x 10 M
C. 1 x 10–5 M
D. 1 x 10–9 M
E. 5 x 10–9 M

(the answer is D)
=======
in any solution Kwater=KH*KOH=10^-14, so pH5 solution has [H]=10^-5 and thus [OH]=10^-9.

 

[hausee]

For the reaction:
AgCl(s) + 2NH3(aq) --> Ag(NH3)2+(aq) + Cl–(aq),
the equilibrium constant K = 4 x 10–3, which
of the following statements is true?
[Ksp for AgCl is 1.0 x 10–10]

A. The addition of NH3 decreases the
solubility of AgCl.
B. AgCl is more soluble in aqueous NH3
than in water.
C. AgCl is more soluble in aqueous
solution containing Cl– than in water.
D. AgCl is less soluble in aqueous NH3
than in water.
(the answer is B)
===========
For A, B and D. basically means addition of reactant NH3, so will increase the solubility of AgCl. A and D are wrong and B is right.

For C, you are adding product Cl- so will be less solube

 

[hausee]

For 58 , reverse second equ. and you get H=-13.5. now add the both eqution and add H too. 21.6+ (-13.5)=8.1. hard to explain on this one without picture.

 

[hausee]

N in NO is +2 and in HNO3 is +5, so changed 3.

 

[Chrono]

Thank you Hausee!!!

My test is in a couple of days so I'll be posting some more questions before the big day, I'd be grateful for your help!

So on question 64 you don't have to account for the coefficient of the compounds? It said 2NO so I was multiplying -2 (for oxygen by 2) and same for N which was wrong.

btw were these questions similar to what you encountered on the DAT?

 

[wantVCUdental]

For the reaction:
AgCl(s) + 2NH3(aq) --> Ag(NH3)2+(aq) + Cl–(aq),
the equilibrium constant K = 4 x 10–3, which
of the following statements is true?
[Ksp for AgCl is 1.0 x 10–10]

A. The addition of NH3 decreases the
solubility of AgCl.
B. AgCl is more soluble in aqueous NH3
than in water.
C. AgCl is more soluble in aqueous
solution containing Cl– than in water.
D. AgCl is less soluble in aqueous NH3
than in water.
(the answer is B)
===========
For A, B and D. basically means addition of reactant NH3, so will increase the solubility of AgCl. A and D are wrong and B is right.

For C, you are adding product Cl- so will be less solube

I arrived at B as well, but is AgCl really more soluble in NH3? and if so, why?

 

[hausee]

I arrived at B as well, but is AgCl really more soluble in NH3? and if so, why?

forming ligands?

 

[K Niner]

The question asked if AgCl was more soluble in water or NH3, it didn't compare solubility of AgCl and NH3.

 

[AndyMay777]

43.) For the reaction between A and B to form C it
is found that when one combines 0.6 moles of
A with 0.6 moles of B, all of the B reacts, 0.2
moles of A remain UNREACTED and 0.4
moles of C are produced. What is the
balanced equation for this reaction?
A. A + 2B -------->C
B. A + 3B ------>2C
C. 3A + 3B-------> 2C
D. 3A + 2B------->3C
E. 2A + 3B-------->2C

(the answer is E)

Solution:
1.start by writing out the equation A+B > C
2.Now write in the coefficients you know, the problem tells you that .4m C are produced, and .6m B are used. So now you have A + .6B > .4C
3.Since you started with .6m A and .2 are not used, you used .4m A
4. youre equasion now looks liek .4A + .6B > .4C
5. so your ratio is 4 to 6 to 4, divide by two and you have a 2 to 3 to 2 ratio.
6. final equation looks like E



54.)

What is the hydroxide ion concentration,
[OH– ] , of a solution having a pH of 5.0?
A. 5 x 10–5 M
B. 5 x 10 M
C. 1 x 10–5 M
D. 1 x 10–9 M
E. 5 x 10–9 M

(the answer is D)

Solution:
1.Ph is the inverse log of [H+]
2.POH is the inverse log of [OH-]
3.PH plus POH = 14
4.so to find poh, 14-5 = 9
5. inverse log [OH] = POH =9
6.[OH] = 1x 10^-9


56.)
For the reaction:
AgCl(s) + 2NH3(aq) Ag(NH3)2+(aq) + Cl–(aq),
the equilibrium constant K = 4 x 10–3, which
of the following statements is true?
[Ksp for AgCl is 1.0 x 10–10]

A. The addition of NH3 decreases the
solubility of AgCl.
B. AgCl is more soluble in aqueous NH3
than in water.
C. AgCl is more soluble in aqueous
solution containing Cl– than in water.
D. AgCl is less soluble in aqueous NH3
than in water.

(the answer is B)




58.)

Solution: you are trying to get No2 gas on the products side and the basic elements to make it on the reactants side
1.write the second equation in reverse NO + O > NO2 and the deltaH is -13.5 bcuz its reversed
2.now deal with the second equasion, N+O> No and delta h is 21.6
3. No are on opposite sides so they can be crossed out
4. you have left N+ O+ o > NO2
5. add the delta h toether 21.6 + -13.5 and you get 8.1

(the answer is E)


64.

Which of the following represents the
CHANGE in oxidation state of nitrogen during
the chemical reaction?
2NO + 3S + 4H2O--------->2HNO3 + 3H2S ?
A. 1
B. 2
C. 3
D. 4
E. 5

(the answer is C)

Solution:
1.On the reactants side, O = -2 so N = +2, dont worry that there are two moles, just think in terms of one molecule
2. On the products side 3*O = -6 plus H which is +1 = -5.
3. N must be +5 to balance out the charges
4. N has gone from 2 to 5. which is a change of 3


Thanks![/QUOTE]

 

[UCB05]

I arrived at B as well, but is AgCl really more soluble in NH3? and if so, why?

The addition of NH3 favors the forward reaction, causing more AgCl to dissolve than if NH3 were not present (in just water)

 

[Chrono]

Could someone walk me through this one? The answer is B. I don't why they add them the way they do.

 

[AndyMay777]

Could someone walk me through this one? The answer is B. I don't why they add them the way they do.

step by step:
1. get rid of the pesky 1/2O2 and rewrite it as O. so it looks liek CO + O > C02
2.Now get the CO on the products side, so reverse the reaction CO2> CO + O
3. becuase you reverse the rxn, change -283.3 kj to +283.3 kj
4. Becuase you need 2 CO on the products side, multiply everything by 2 so the eq looks liek 2co2 > 2co + 2O, becuase that was multiplyed by 2, do the same to the 283.3kj to get 566.6
5.now you have 2O on the products side to get rid of and 2 co2 on the reactsnts side to get rid of
6. take the second equasion and also multiply it by 2 to get 2c + 2o2 > 2co2
7. multiply the -393.7 by 2 also to get -787.4
8.take the 2 o2 in the equasion and rewrite the equasion "2c + 2o2 > 2co2" to 2c + 4o > 2c02
9.now 2co2 is on opposite sides of each reaction and can be crossed out.
10. 2 o can be crossed out from the products side, and that leaves 2O or O2 on the reactants side
11. what you are left with is 2C + O2 > 2CO
12. now that you have the formula add up the two dulta h , -787.4 +566.6
and you get -220.8 or answer B

 

[Chrono]

the answer is C.

I am very thankful for your help everyone.

 

[payme2]

Fe2O3 + MgCl2 ------> FeCl3 + MgO unbalanced reation
Fe2O3 + 3MgCl2 -----> 2FeCl3 + 3MgO balanced reaction (but not important)

When you approach this problem... write down all the ions in the soup solution.

Fe(3+), O(2-) , Mg(2+) and Cl(1-)

Now take a look at the multiple choice....it has iron reacting with chloride.

so Fe(3+) x Cl(1-) = FeCl3
and same thing with magnesium oxide.



Oh yeah, if you don't know Le Chatelier's principle down cold, then you're pretty much screwed on the PS section on the MCAT. lol It always shows up. A + B ----> C If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. So AgCl, being a reactant, is more soluble when you add more NH3 (another reactant). You'll probably see this form. Also know that Le chatelier's principle works with heat as a product.
A+B ----> C + heat (exorthermic reaction)....If you take heat out of the solution, then more product will form.

And for the enthalpy of formation question. The above explanations are good, but I think they forgot to tell you that O2 = 0 enthalpy. Any diatomic molecule of gas has 0 enthalpy of formation.


OFF topic...are these the medium/hard level questions on the DAT? Or on the easier side. I took the MCAT and we had questions like this too.