any chemistry buffs awake?

[MrDudeMan]

i have a test in a few hours and im not sure about a lot of this stuff...

ive got a practice test to work from and she said it will be very similar. ive done 3/4 of it and im pretty sure those are right, but i have no idea how to do the last few parts.

ill list these things in order and if you can help with any part of any of them i would appreciate it.

**answered** 14) a 25.0mL sample of HCL is titrated with a 0.139 M NaOH sultion. the equivalence point is reached with 15.4mL of base. the concentration of HCL is ____ M. this seems easy enough, but i cant get any of the answers listed. it takes more than half of the original amount of the base to make it reach the equivalence point, so i figure its on the higher end of the molar concentration of this strong acid. am i even close? the answers have a pretty broad range.

15) in which of the following aqueous solutions would you expect PbCL2 have the lowest solubility?

answers: .020 M BaCL2, .020 M KCl, .015 M PbNO3, .015 M NaCl, or pure water

ok, i think PbCL2 is a base, but im not sure. that means it would have the lowest solubility in a base like it? i was thinking the answer is .015 M NaCl, but i dont know how similar that is to PbCL2.

20) which below best describes the behavior of an amphoteric hydroxide in water?

answers: with conc. aq. NaOH, its suspension dissolves.
with conc. aq. HCl, its suspension dissolves.
with conc. aq. NaOH, its clear solution forms a precipitate
with conc. aq. HCl, its clear solution forms a precipitate
with conc. aq. of both NaOH and HCl, its suspension dissolves.

i think its the last one (both dissolve) because amphoteric means it can act as an acid or base. am i right thinking that way?

 

[Analog]

Are you building a bomb?

 

[MrDudeMan]

Originally posted by: yellowfiero
Are you building a bomb?

yes, it will be planted in nashville this saturday. i thought id pitch in and hit another city because the terrorists missed one in their plan.

 

[hypn0tik]

HCl + NaOH -> NaCl + H2O

C (NaOH) = 0.139 mol/L
V (NaOH) = 0.0154 L
n (NaOH) = 2.14 x 10^-3 mol

Since HCl and NaOH occur in a 1:1 ratio, you need 2.14 x 10^-3 mol of HCL.

V (HCl) = 0.025L

C (HCl) = n / V = 0.0856 M

No idea about the other two.

 

[MrDudeMan]

Originally posted by: hypn0tik
HCl + NaOH -> NaCl + H2O

C (NaOH) = 0.139 mol/L
V (NaOH) = 0.0154 L
n (NaOH) = 2.14 x 10^-3 mol

Since HCl and NaOH occur in a 1:1 ratio, you need 2.14 x 10^-3 mol of HCL.

V (HCl) = 0.025L

C (HCl) = n / V = 0.0856 M

No idea about the other two.

*light bulb*

ah that makes perfect sense. thank you...i see now how you just used the givens and a few implied things to come up with that answer. that is the exact answer listed. thanks.

 

[kitkat22]

The first one is based on the equation V1 * C1 = V2 * C2 where v=volume an c=concentration.

You need to memorize your solubility rules. (I certainly can't remember them!)

I believe the last one you are good.

 

[MrDudeMan]

Originally posted by: cscpianoman
The first one is based on the equation V1 * C1 = V2 * C2 where v=volume an c=concentration.

cool, didnt know that equation. my chemistry class sucks hardcore. i dont care though because im taking it to just get it out of the way.

do you have any idea about the other 2?

 

[hypn0tik]

Originally posted by: Bigsm00th

Originally posted by: hypn0tik
HCl + NaOH -> NaCl + H2O

C (NaOH) = 0.139 mol/L
V (NaOH) = 0.0154 L
n (NaOH) = 2.14 x 10^-3 mol

Since HCl and NaOH occur in a 1:1 ratio, you need 2.14 x 10^-3 mol of HCL.

V (HCl) = 0.025L

C (HCl) = n / V = 0.0856 M

No idea about the other two.

*light bulb*

ah that makes perfect sense. thank you...i see now how you just used the givens and a few implied things to come up with that answer. that is the exact answer listed. thanks.

No problem. One thing to keep in mind though. ALWAYS make sure you work with the BALANCED chemical equation, but I'm sure you know that already.

 

[MrDudeMan]

Originally posted by: hypn0tik

Originally posted by: Bigsm00th

Originally posted by: hypn0tik
HCl + NaOH -> NaCl + H2O

C (NaOH) = 0.139 mol/L
V (NaOH) = 0.0154 L
n (NaOH) = 2.14 x 10^-3 mol

Since HCl and NaOH occur in a 1:1 ratio, you need 2.14 x 10^-3 mol of HCL.

V (HCl) = 0.025L

C (HCl) = n / V = 0.0856 M

No idea about the other two.

*light bulb*

ah that makes perfect sense. thank you...i see now how you just used the givens and a few implied things to come up with that answer. that is the exact answer listed. thanks.

No problem. One thing to keep in mind though. ALWAYS make sure you work with the BALANCED chemical equation, but I'm sure you know that already.

yeah i just didnt think to look at it that way. i know, i am stupid.

 

[Siva]

What chem class is this for?

Well for the titration one wouldn't it just require a 1:1 mol ratio since its a strong acid and strong base? Isn't that just basic stoichiometry. For number 20 it should dissolve in both, that is the definition of amphoteric. I am not sure about number 15 though. Its definately not NaCl, because .02M KCl is a higher concentration salt solution, so if anything .02M KCl would be more likely to salt out the PbCl2. Same thing with BaCl2. Based on the process of elimination I'd have to guess PbNO3 just because its so different from the other possibilities, but I couldn't tell you why.

 

[MrDudeMan]

Originally posted by: Siva
What chem class is this for?

Well for the titration one wouldn't it just require a 1:1 mol ratio since its a strong acid and strong base? Isn't that just basic stoichiometry. For number 20 it should dissolve in both, that is the definition of amphoteric. I am not sure about number 15 though. Its definately not NaCl, because .02M KCl is a higher concentration salt solution, so if anything .02M KCl would be more likely to salt out the PbCl2. Same thing with BaCl2. Based on the process of elimination I'd have to guess PbNO3 just because its so different from the other possibilities, but I couldn't tell you why.

gen chem 2. we figured out number 14, and yes you are right. are you sure about number 20? i mean it seems right, but i have no clue.

and number 14, yeah i see now how wrong NaCl is. why would it not be pure water?

 

[Siva]

Pure water has the lowest concentration of solute in it. Since I'm assuming PbCl2 dissolves into ions, having ions in solution would make it less likely to dissolve, ie lowest solubility.

As for the amphoteric question... I'm just basing it on the knowledge of what amphoteric means, which is something that acts as an acid or a base. In base it will act as an acid and create a soluble salt, in acid it should act as base and the same thing will happen. Right?

Gen chem seems so far away and its only been a year. I don't even remember how to properly balance redox reactions

 

[kitkat22]

I just looked up the rules and here you go

Solubility Rules for ionic compounds in water
Rule 1 - Compounds of NH4 (1+) and group 1A metal ions are soluble
Rule 2 - Compounds of NO3 (1-), ClO4 (1-) ClO3 (1-) and C2H3O2 (1-) are soluble
Rule 3 - Compounds of Cl (1-), Br (1-) and I (1-) are soluble except those of Ag (1+), Cu (1+), Tl (1+) and Pb (2+)
Rule 4 - Compounds of SO4 (2-) are soluble except those of Ca (2+), Sr (2+), Ba (2+) and Pb (2+)
Rule 5 - Most other ionic compounds are insoluble

You know your PbCl2 is insoluble in water according to the rules the question lies in whether or not there may be advantageous mixings with the other compounds. Despite being insoluble in water PbCl2 is still somewhat soluble. Work out any patterns with the other compounds and compare them to the rules and see what happens. Looking over the other compounds they are all soluble in water. Hope this helps

 

[Siva]

Originally posted by: cscpianoman
I just looked up the rules and here you go

Solubility Rules for ionic compounds in water
Rule 1 - Compounds of NH4 (1+) and group 1A metal ions are soluble
Rule 2 - Compounds of NO3 (1-), ClO4 (1-) ClO3 (1-) and C2H3O2 (1-) are soluble except those of Ag (1+), Cu (1+), Tl (1+) and Pb (2+)
Rule 3 - Compounds of Cl (1-), Br (1-) and I (1-) are soluble
Rule 4 - Compounds of SO4 (2-) are soluble except those of Ca (2+), Sr (2+), Ba (2+) and Pb (2+)
Rule 5 - Most other ionic compounds are insoluble

You know your PbCl2 is insoluble in water according to the rules the question lies in whether or not there may be advantageous mixings with the other compounds. Despite being insoluble in water PbCl2 is still somewhat soluble. Work out any patterns with the other compounds and compare them to the rules and see what happens.

Where did you get that PbCL2 is insoluble? "Rule 3 - Compounds of Cl (1-), Br (1-) and I (1-) are soluble"

PbNO3 is the only insoluble compound, however that shouldn't affect the solubility of PbCL2, in fact it should be just as soluble as in pure water. Imagine dissolving an ionic compound in pure water. Then imagine the same volume of water with 5 stones from the street thrown in. Since the stones aren't soluble, the ionic compound's solubility is not affected.

I think the least soluble would be the .02M BaCL2, because wouldn't that have the highest concentration of ions already in solution, since there are two Cl- ions for every one Ba2+ ion.

 

[kitkat22]

Opps I realized I made a mistake in the chart, sorry!!!! Rules 3 and 5.

 

[pouncerchew]

so is the answer .02 BaCL2?

 

[kitkat22]

I still believe the answer to be water.

 

[Eli]

wow.

/head explodes

....

which is sad because this stuff interests me.

 

[Siva]

Originally posted by: cscpianoman
Opps I realized I made a mistake in the chart, sorry!!!! Rules 3 and 5.

Oh I forgot those stupid solubility rules. The answer answer BaCl2, this time for sure. If PbCl2 was soluble in water, it would take a whole lot of it to be saturated in any solution. Since it is insoluble, which only means the solubility is very low not completely insoluble, it takes a very low concentration of ions already in the solution for the equilibrium to be shifted from its ionic form to its solid form.

The equilibrium constant [Pb2+(aq)][Cl-(aq)]2 = Ksp = 2.4 ´ 10-4

That's very low. If you added either Pb2+ or Cl- that would cause there to be more solid PbCl2. Since Cl- is squared it is exponentially more important to the solubility. Since BaCl2 contributes 2Cl- ions for every molecule dissolved it will have the largest effect on the equilibrium.

Its slowly coming back

 

[kitkat22]

Cool, I was beginning to wonder about my water philosophy while I was mediatating in the shower. I was looking at the option between PbNO3 and BaCl2 and I completely forgot about the Ksp equation. I kind of wish Le Chatlier would take a hike, but seems there is no escape!

 

[Siva]

Originally posted by: cscpianoman
Cool, I was beginning to wonder about my water philosophy while I was mediatating in the shower. I was looking at the option between PbNO3 and BaCl2 and I completely forgot about the Ksp equation. I kind of wish Le Chatlier would take a hike, but seems there is no escape!

no there isn't, stupid equilibrium

 

[MrDudeMan]

you guys are the best thanks a lot for the help. the answer was BaCl2.