Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: FancySnow on August 16, 2018, 02:36:22 AM
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Hello Chemistry and Math wizards, I need your help.
Attached to this post is a problem in my textbook. Please help me understand something.
There seems to be a disconnect in my brain when I read "The solution contains 0.899g of solution per milliliter". Can anyone explain to me how it is understood or how do I mathematically come to the conclusion that there is 0.899g of solution? I take it that it's derived from the density given. But why? I don't get it.
From my understanding of percent mass/volume. The given information states that there there is 28.0 wt% of ammonia. OK, so, according the textbook in a previous page, by definition, 28.0 wt% means that there is 0.280g of ammonia in 100g of solution. So my mind understood that definition. But, when I read the solution in this problem, I don't understand where and how I'm supposed to accept "the solution contains 0.899g of solution per milliliter".
Please enlighten me. I am dense at times - I know. But, please be nice. I just want to learn and move on from this problem.
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It is how it is.
It means 1 ml has a weight of 0,899 g nothing more. The definition of density is given by Rho = m/V. So you can say Rho = 0,899g/1ml = 899 g/l
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Were you thinking it meant the solution contains 0.899g solute per mL? (Or solvent?) That would certainly be wrong. But a solution density of 0.899g/ml means there are 0.899 g of solution per millilitre. Perhaps the expression "the solution contains solution" confused you. It is certainly an unusual way of putting it.
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You have gone off in the wrong direction with respect to what weight percent means in this context, but it illustrates one thing that I have long believed, which is that there is much confusion about percent scales in chemistry and that teachers need to be at least aware of some of the pitfalls. I would interpret the value of 28% to mean that for every 100 g of ammonium hydroxide solution, there is a 28-gram portion of pure ammonia and a 72-gram portion of water. That notion is consistent with the problem that you posted.