[FuturePharm.D.]

Hi everybody.  This is my first post here.  I'm studying for finals using previous exams.  I found three questions I don't understand why the given answer is correct.  I would appreciate some explanation as to why the answer is correct.

Thank you,
FuturePharm.D.


Question 1:

NH3, Kb = 1.8X10^-10

HCLO, Ka = 3.2X10^-8

HOAc, Ka = 1.8X10^-5

HCN, Ka = 4.0X10^-10

Given the above information arrange the following salts in the order of their increasing pH:

a) NH4NO3 < NaOAc < NaCN < HCLO

b) NH4NO3 < NaOAc < NaClO < NaCN

c) HClO < NH4NO3 < NaOAc < NaCN

d) NaCN < NaOAc < HClO < NH4NO3

e) NaOAc < NaCN < NH4NO3 < HClO


The correct answer is:  

b) H4NO3<NaOAc<NaCN<NaCN

Why?




Question #2:

2.  A solution containing 5.60g of a solute in 100g of benzene produced a freezing point of 4.35 degrees celsius.  Calculate the molar mass of the solute.  (Benzene: F.P. = 5.45 degrees celsius, and kf = 5.07 degrees celsius/molal)

a) 124g/mol

b) 352g/mol

c) 98.0g/mol

d) 51.6g/mol

e) 258g/mol


The answer is e) 258g/mol


Here's what I did:

molal = delta T/Kf = 1.1/5.07 = .216 molal

(5.6g/.1kg)(.216mols/1kg)(1000kg/g)

This obviously isn't correct.  Could someone please show me the way?


Last one:

In the following reaction, the rate at which O2 reacts is .50 mols/L/s.

4NH3 + 7O2 ----> 4NO2 + 6H2O

The overall rate of this reaction is:

a) 2.0 moles/L/s

b) 0.071 moles/L/s

c) 3.5 moles/L/s

d) 0.13 moles/L/s

e) 0.083 moles/L/s


I know the answer is b) 0.071  and I know it's 0.50/7.  But why?  Is it because there are more mols of oxygen than any other product and that determines the overall rate?  Why do you divide and not multiply?


Thanks again,
FuturePharm.D.

[Borek]

Hi everybody.  This is my first post here.

But you have posted the same questions at different forum, didn't you?

Given the above information arrange the following salts in the order of their increasing pH:

Weak acid salts hydrolyse. Calculate Kb's for conjugate bases and compare them to find out order. This gives only parttial answer, as NH3 is not in its basic form, but is present as conjugate acid NH4^+ - but that means its salt solution will be slightly acididc and the pH will be the lowest.

molal = delta T/Kf = 1.1/5.07 = .216 molal

OK

(5.6g/.1kg)(.216mols/1kg)(1000kg/g)

That's one of those fancy methods of making calculations that I have never learned. You have 56 g of solute in 1 kg of solvent and you know it is 0.216 mole. Use proportions to find out molar mass.

The overall rate of this reaction is:

Either my English or memory fails - I have no idea how the overall rate is defined.

[GCT]

Hi everybody.  This is my first post here.  I'm studying for finals using previous exams.  I found three questions I don't understand why the given answer is correct.  I would appreciate some explanation as to why the answer is correct.

Thank you,
FuturePharm.D.


Question 1:

NH3, Kb = 1.8X10^-10

HCLO, Ka = 3.2X10^-8

HOAc, Ka = 1.8X10^-5

HCN, Ka = 4.0X10^-10

Given the above information arrange the following salts in the order of their increasing pH:

a) NH4NO3 < NaOAc < NaCN < HCLO

b) NH4NO3 < NaOAc < NaClO < NaCN

c) HClO < NH4NO3 < NaOAc < NaCN

d) NaCN < NaOAc < HClO < NH4NO3

e) NaOAc < NaCN < NH4NO3 < HClO


The correct answer is:  

b) H4NO3<NaOAc<NaCN<NaCN

Why?




Question #2:

2.  A solution containing 5.60g of a solute in 100g of benzene produced a freezing point of 4.35 degrees celsius.  Calculate the molar mass of the solute.  (Benzene: F.P. = 5.45 degrees celsius, and kf = 5.07 degrees celsius/molal)

a) 124g/mol

b) 352g/mol

c) 98.0g/mol

d) 51.6g/mol

e) 258g/mol


The answer is e) 258g/mol


Here's what I did:

molal = delta T/Kf = 1.1/5.07 = .216 molal

(5.6g/.1kg)(.216mols/1kg)(1000kg/g)

This obviously isn't correct.  Could someone please show me the way?

The definition of molality is moles of solute per liter of solvent.  The solvent here is benzene, what you need is the moles of solute.  dT=Kf(x/liter of benzene), everything is given (or can be deduced), allowing you to solve for x, the moles of solute.  

now molar mass is grams/mole, I'm sure you can do the rest.

Last one:

In the following reaction, the rate at which O2 reacts is .50 mols/L/s.

4NH3 + 7O2 ----> 4NO2 + 6H2O

The overall rate of this reaction is:

a) 2.0 moles/L/s

b) 0.071 moles/L/s

c) 3.5 moles/L/s

d) 0.13 moles/L/s

e) 0.083 moles/L/s


I know the answer is b) 0.071  and I know it's 0.50/7.  But why?  Is it because there are more mols of oxygen than any other product and that determines the overall rate?  Why do you divide and not multiply?


Thanks again,
FuturePharm.D.

First write out the proper rate equation, and you'll probably obtain a better understanding from there.

FuturepharmD huh, where are you planning to attend for pharmacy school?

[FuturePharm.D.]

"The definition of molality is moles of solute per liter of solvent.  The solvent here is benzene, what you need is the moles of solute.  dT=Kf(x/liter of benzene), everything is given (or can be deduced), allowing you to solve for x, the moles of solute.  

now molar mass is grams/mole, I'm sure you can do the rest."


Isn't molality moles of solute per Kg solvent?  I thought the definition of molarity was moles per Liter?




"FuturepharmD huh, where are you planning to attend for pharmacy school?"

Texas A&M in Kingsville, TX is my first choice.  Actually, with admission being very competitive at most schools I'll go where-ever I can get accepted.  Hopefully it'll be somewhere in Tx.

Thanks for your help.
FuturePharm.D.

[Borek]

Isn't molality moles of solute per Kg solvent?  I thought the definition of molarity was moles per Liter?

And you were right.

[GCT]

"The definition of molality is moles of solute per liter of solvent.  The solvent here is benzene, what you need is the moles of solute.  dT=Kf(x/liter of benzene), everything is given (or can be deduced), allowing you to solve for x, the moles of solute.  

now molar mass is grams/mole, I'm sure you can do the rest."


Isn't molality moles of solute per Kg solvent?  I thought the definition of molarity was moles per Liter?




"FuturepharmD huh, where are you planning to attend for pharmacy school?"

Texas A&M in Kingsville, TX is my first choice.  Actually, with admission being very competitive at most schools I'll go where-ever I can get accepted.  Hopefully it'll be somewhere in Tx.

Thanks for your help.
FuturePharm.D.

yeah, you're right about the molality.

You're pretty close to California, one of the Universities there (I forgot which) has one of the top pharmacy schools in the nation.  

[FuturePharm.D.]

You're pretty close to California, one of the Universities there (I forgot which) has one of the top pharmacy schools in the nation.  

Since when is Texas close to California?  

[GCT]

Definitely closer than I am, Georgia , most pharmacy schools offer the same programs and don't differ too much in terms of the quality of their programs but I think that the one in California is exceptional (again, I don't recall the name, either Berkely, or U of California)

[jdurg]

If you look at a map and completely ignore the scale, Texas is only about a few fingers' width away from California.