Solumbility of Barium hydroxide

[dcowboys107]

The Solubility of barium hydroxide in water at 20 degrees C is 1.85g/100g Water. A solution is made up of 256 mg in 35.0 g of water. Is the solution saturated? If not, how much more needs to be added to make a saturated solution?

 

This is what I have done so far: ?g of Ba(OH2)2=35.0g water x (1.85 g/100g)=.6475 g is needed to form a saturated solution. .6475-.256= .392g is needed to make is saturated.

 

Is this right? Some of my classmates got conflicting answers. Thanks for the help!

[insane_alien]

looks fine yeah. i'll go with that.

[dttom]

The Solubility of barium hydroxide in water at 20 degrees C is 1.85g/100g Water. A solution is made up of 256 mg in 35.0 g of water. Is the solution saturated? If not, how much more needs to be added to make a saturated solution?

 

This is what I have done so far: ?g of Ba(OH2)2=35.0g water x (1.85 g/100g)=.6475 g is needed to form a saturated solution. .6475-.256= .392g is needed to make is saturated.

 

Is this right? Some of my classmates got conflicting answers. Thanks for the help!

 

concentration of Ba(OH)2= (1.85/171.3)/0.1 = 0.107997664 mol/dm3

solubility product of Ba(OH)2= (0.107997664)*(2*0.107997664)^2

= 5.03852117E-3 mol3/dm9

concentration of Ba(OH)2 in that 35g solution= (256/1000)/171.3/0.035

= 0.04269869 mol/dm3

ionic product = 0.04269869*(2*0.04269869)^2 = 3.113892857E-4 mol3/dm9

as ionic product<solubility product, hence the solution is not saturate.

(0.04269869+x)*4*(0.04269869+x)^2 = 5.03852117E-3

(0.04269869+x) = 0.107997664

x= 0.065298974

amount of Ba(OH)2 needed further= 0.065298974/1000*35= 2.28546409E-3 mol= 0.391499998 g = 0.3915g

[John Cuthber]

Or you could do it the easy way like the original poster did.

[insane_alien]

i wonder if dttom has ever heard of false precision.

[dttom]

I know the method of the original poster is correct, while he said that his classmate got a conflicting answer, and I just post another method and wonder if that conflicting answer as this...

[jdurg]

Guys, please don't EVER put someone down for performing a calculation in a different way than you would have done it, as long as their result is correct as well. Everybody should know multiple ways of solving problems otherwise they get stuck in a "plug and chug" type of mindset and if a problem is slightly different than the ways they're used to solving problems they sit there like a retarded rabbit.

[John Cuthber]

Well, if I was paying them by the hour I'd prefer Dcowboys' method to Dttom's and If I was stuck in an exam with only however many hours to finish the paper I know which method I'd use.

[insane_alien]

i wasn't putting him down for performing the calculation, just that "0.391499998 g"

 

is more precise than it could possibly be within the scope of the question. while if this was in mathematics it wuld be fine but its mathematically dishonest when dealing with figures given to an accuracy of 2 decimal places. if this was an engineering clas he would have been flayed alive by the lecturer. while this example doesn't produce any spectacular inaccuracies it can happen and is a bad practise to get into.

[jdurg]

Well, if I was paying them by the hour I'd prefer Dcowboys' method to Dttom's and If I was stuck in an exam with only however many hours to finish the paper I know which method I'd use.

 

Since when are we paying people here on these forums? Have you ever thought about taking the time to explain WHY your method might be better? I don't know about you, but I'm not about to listen to what some random schmuck on an internet messageboard says unless there is some logic behind it. Take a mere five minutes to explain WHY your method is probably the better one and not come across as an arrogant cock and perhaps people won't find each and every one of your posts to be incredibly demeaning and offensive?

[shadowacct]

i wasn't putting him down for performing the calculation, just that "0.391499998 g"

 

is more precise than it could possibly be within the scope of the question. while if this was in mathematics it wuld be fine but its mathematically dishonest when dealing with figures given to an accuracy of 2 decimal places. if this was an engineering clas he would have been flayed alive by the lecturer. while this example doesn't produce any spectacular inaccuracies it can happen and is a bad practise to get into.

What dttom does is correct, and what you propose is incorrect. YOU will introduce inaccuracies during the calculations, not dttom. If you ever do any calculations with finite precision input data, then use higher precision for intermediate results and only perform rounding steps at the end. This is exactly what dttom does!

 

The only point of debate might be whether the rounding in the last step should be 0.392 g or 0.3915 g, but that's all and it only is a minor point.

[John Cuthber]

"Have you ever thought about taking the time to explain WHY your method might be better?"

Yes, that's why I pointed out that in the real world, or in an exam, one method would be better because it's faster.

 

Did you notice the word "if" in my post- it's used to refer to things like hypothetical cases. For example, rather than the absolute case of answering the question in this forum (for which nobody does get paid) I was inviting people to consider the case where someone was paying by pointing out what my preference would be in that case.

 

I'm also amused that you refer to it as my method. If you had read the thread firstly you might have spotted that it was Dcowboy's method. (The hint there is that I called it "Dcowboys' method " if you look carefully you might notice the proper noun with an apostrophe and a s- this form is called a possessive).

Secondly you might have noticed that I didn't provide a method for calculating the answer, I just commented on the 2 that had been given.

Thirdly, you might have noticed that I was tacitly stating why I preferred one method to the other. The clear implication was that I preferred it because it was quicker.

Perhaps, if you had taken 5 minutes to look at what had been written, you might "not come across as an arrogant cock".

 

Incidentally, the method DCowboy used gives as accurate an answer as it's ever going to if you work to 4 decimal places.

On the other hand dttom's method (because it involves squaring things) requires more places of decimals to get the right answer.

Since your calculator will do this for you it doesn't matter much but, if you were repeatedly doing something like this inside a loop in a computer program, the one with less maths would get the right answer quicker.

 

OK, that's a second reason why the quick easy method is better.

And, just in case jdurg missed it again, its a second reason why the quick easy method is better. (That's not so much an arrogant cock as a patronising one BTW)