Sciencemadness Discussion Board

Mixed Questions

Claisen - 14-3-2011 at 04:23

1. When H2S gas is passed through acid solutions containing CuCl2, HgCl2, Pb(NO3)2 and Ni(SO4), all the metal ions are precipitated as their sulphides except one. Whis one is it?

Is is that one of the metal sulphide formed dissolves in solution?
How do I find that out?

2. Which halogen atom is comparatively more reactive - vinyl chloride or chloro benzene?

The only difference arises in the stability of vinyl carbocation and phenyl carbocation as the two halides are same in all other respects (I think so). I think V.C. is least stable though I don't have a valid reason.

3. What is the 'percent strength' of 5 volume H2O2 solution?

This question seems unclear to me. Is that percent strength by mass or by volume?

Normality of the given H2O2 solution = 0.89N
Molarity= 0.45M

1 mole H2O2 contains 34g H2O2
0.45 mole H2O2 contains 15.3g H2O2

So % strength (assuming by weight) = 0.153%

But the answer is 1.52 :|

4. Is the unit of molar conductance and molar conductivity same in any case?

[Edited on 14-3-2011 by Claisen]

eniv62 - 14-3-2011 at 07:39

Hi Claisen,

By now let me answer the number 3:

If 15.3 g H2O2 are contained in 1 L solution (Molarity = moles/Liter of solution)
then in 100 mL (1000mL/10) it should have 1.53 g (simply divided everything by 10)

So percentage means per cent, that's why is better take it based on 100 mL (1.53 g of H2O2 + 98.47 g of H2O).

Then % by weight is 1.53 %

I 'll need to check the other questions.

Best Regards :)

eniv

[Edited on 14-3-2011 by eniv62]

Magpie - 14-3-2011 at 08:07

Quote: Originally posted by Claisen  
1. When H2S gas is passed through acid solutions containing CuCl2, HgCl2, Pb(NO3)2 and Ni(SO4), all the metal ions are precipitated as their sulphides except one. Whis one is it?

Is is that one of the metal sulphide formed dissolves in solution?
How do I find that out?


It is Ni++. This is part of the identification of cations in the classical wet qualitative analysis of cations.

The specification is that [H+] be 0.3M. That way the [S--] is just high enough to take down all of those cations except Ni++. This is the Group II precipitation.

In the Group III precipitation, where the [S--] is higher, Ni++ is precipitated.

bbartlog - 14-3-2011 at 08:53

Quote:
1 mole H2O2 contains 34g H2O2
0.45 mole H2O2 contains 15.3g H2O2

So % strength (assuming by weight) = 0.153%


15.3g / 1000g (by weight) = 1.53%, not 0.153%. You put the decimal in the wrong place. The answer of 1.52% might be from using a more precise molecular weight of H2O2 than 34.

entropy51 - 14-3-2011 at 09:02

Quote: Originally posted by Claisen  
What is the 'percent strength' of 5 volume H2O2 solution?

This question seems unclear to me. Is that percent strength by mass or by volume?
5 volume H2O2 means that 1 volume of it releases 5 volumes of O2 gas when decomposed.

5 volume H2O2 = 1.5 % by weight. You can do the math.

[Edited on 14-3-2011 by entropy51]

eniv62 - 14-3-2011 at 13:20

Quote: Originally posted by eniv62  
Hi Claisen,

By now let me answer the number 3:

If 15.3 g H2O2 are contained in 1 L solution (Molarity = moles/Liter of solution)
then in 100 mL (1000mL/10) it should have 1.53 g (simply divided everything by 10)

So percentage means per cent, that's why is better take it based on 100 mL (1.53 g of H2O2 + 98.47 g of H2O).

Then % by weight is 1.53 %

I 'll need to check the other questions.

Best Regards :)

eniv

[Edited on 14-3-2011 by eniv62]


Actually this is only a good approximation because the weight of H2O2 is low.

If you want to get the exact value you need to know the density of the solution; Molarity = Moles/Liter of solution,
but we don't know how much 1L of this solution weighs, unless we have the density, we can get the weight.
From that, it is easy to calculate the percent in weight.

Regards

eniv

ScienceSquirrel - 15-3-2011 at 04:06

I think it is a legitimate approximation, 3% hydrogen peroxide is called 10 volume and 6% peroxide is called 20 volume.

Claisen - 15-3-2011 at 06:15

Thanks to all.
1.52 or 1.53 is not a big deal. There are usually slightly varying answers in numericals.

Any one for 2) and 4) ?

eniv62 - 15-3-2011 at 07:43

Hi Claisen,

Let me try number 2. As far as I remember from my Organic Chemistry course: I think chloro benzene molecule is quite more stable that vinyl chloride. Benzene molecule presents resonance, which increases its stability, even though it is attached to a Cl atom, meanwhile vinyl has a double bond which augment its reactivity.
I think the Cl atom attached to both molecules should have the same reactivity.
Please correct me if I am wrong.

Thanks

eniv