Chemistry Question (1 Viewer)

[hit patel]

Hi guys, just found a question that got me thinking eventhough it was simple or may be in didnt get enough sleep.

Question: When solid lead nitrate was added to 200 ml of water, it resulted in a saturated solution. Then the student took exactly 20 ml of the saturated solution. An excess of sodium sulphide solution was added to precipitate all the lead as lead sulphide. The resulting mixture was filtered and the mass of solid lead sulfide was found to be 8.61g.

Calculate the molarity of lead nitrate in mol/L in original saturated solution.

My Working:

Equation: Pb(NO3)2+Na2S--->2NaNO3+PbS
Moles of PbS= 8.61/(207.2+32.07)=0.03598445mol
There are therefore from reaction equation: 0.03598445 mol of Pb(NO3)2 in 20 ml.

From Now the the problem is that I donot know which is the correct way to use:
Way1: THere are 10x20ml in 200ml so moles of Pb(NO3)2 is 0.3598445.

OR

Way2: The solution is saturated therefore the number of moles is the same in 20ml as in 200ml.


Which Way is correct? I dont have the answers unfortunately. Please reply ASAP.

Thanks

 

[Kurosaki]

Way2: The solution is saturated therefore the number of moles is the same in 20ml as in 200ml.
_
Incorrect, they have the same concentration not same number of moles.

 

[Kurosaki]

and your first method got the right answer, multiply it by 5 though

 

[HSC2014]

To argue your Way2 (which is incorrect): Suppose that there is 0.06mol of Pb(NO3)2 in 20mL of water - forming a saturated solution. If I added 180 mL of water, wouldn't there be more room to "saturate"? There is a clear distinction between saturation at 20mL and 200mL. Yes the molarity (concentration) will be the same if you take a 20mL sample from the saturated 200mL, however the actual molar content of Pb(NO3)2 will differ in each case.

That was probably confusing, but anyway, first method is correct. Make sure to size your measurements up to get moles/L

 

[hit patel]

Way2: The solution is saturated therefore the number of moles is the same in 20ml as in 200ml.
_
Incorrect, they have the same concentration not same number of moles.

Yes true, I was thinking that if we kept molarity constant then multiplying by a larger volume would get more moles. Thanks probably didnt get enough sleep to think clearly

and your first method got the right answer, multiply it by 5 though

Why 5? DOnt i multiply by 10 to get 200ml?

To argue your Way2 (which is incorrect): Suppose that there is 0.06mol of Pb(NO3)2 in 20mL of water - forming a saturated solution. If I added 180 mL of water, wouldn't there be more room to "saturate"? There is a clear distinction between saturation at 20mL and 200mL. Yes the molarity (concentration) will be the same if you take a 20mL sample from the saturated 200mL, however the actual molar content of Pb(NO3)2 will differ in each case.

That was probably confusing, but anyway, first method is correct. Make sure to size your measurements up to get moles/L

Yes thats what i thought but then i saw saturated so I thought its better to ask then end up getting it wrong in any exams.
Thanks guys. PLease clarify why is it x5 and not x10?

 

[hit patel]

Way2: The solution is saturated therefore the number of moles is the same in 20ml as in 200ml.
_
Incorrect, they have the same concentration not same number of moles.

Yes true, I was thinking that if we kept molarity constant then multiplying by a larger volume would get more moles. Thanks probably didnt get enough sleep to think clearly

and your first method got the right answer, multiply it by 5 though

Why 5? DOnt i multiply by 10 to get 200ml?

To argue your Way2 (which is incorrect): Suppose that there is 0.06mol of Pb(NO3)2 in 20mL of water - forming a saturated solution. If I added 180 mL of water, wouldn't there be more room to "saturate"? There is a clear distinction between saturation at 20mL and 200mL. Yes the molarity (concentration) will be the same if you take a 20mL sample from the saturated 200mL, however the actual molar content of Pb(NO3)2 will differ in each case.

That was probably confusing, but anyway, first method is correct. Make sure to size your measurements up to get moles/L

Yes thats what i thought but then i saw saturated so I thought its better to ask then end up getting it wrong in any exams.
Thanks guys. PLease clarify why is it x5 and not x10?

 

[HSC2014]

Why 5? DOnt i multiply by 10 to get 200ml? PLease clarify why is it x5 and not x10?

"Way1: THere are 10x20ml in 200ml so moles of Pb(NO3)2 is 0.3598445."

In your case, you have 0.36 mol/200mL but you want mol/L, so times further by 5 i.e. (0.36 x 5)/(200mL x 5) = 1.80 mol/L

Though, there was an unecessary jump from 20mL to 200mL to 1000mL (x10 x5).

 

[Kurosaki]

let the number of moles you first added be x.
then the concentration originally would be 5x (x/0.2)=5x M
Then you take 20ml. n=cv=0.02(5x)=0.1x

find the moles of lead sulphide. from the equation u see its equal to the moles of lead nitrate in solution. then from this you find x, which is 0.35 roughly. multiply by 5, throw in M, and you get approx. 1.75M. It said calculate molarity not number of moles.

 

[Kurosaki]

alternatively, you calculated the number of moles of lead nitrate in your 20ml, and since the concentration is same as in the 200ml one, then simply use c=n/v, and that will get you the same answer.
you have the moles of lead nitrate, and the volume is 20ml=0.02L, so yeah...