Discrepant volume c...

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# Discrepant volume changes in Unit 2, Sim 3, Set 1

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I only added 126.11g of water, but the output reads volume of solution is 122.55ml and volume of solvent is 126.11ml. They should be the same right? I added 37.40g of NaCl, volume of solution is now 158.90ml; this seems like an unnaturally large increase in solution vol. There is also a disparity in the symbolic sim: 2 NaCl molecules=0.64 moles; 7 H20 molecules=6.8 moles.

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Posted by: @ryim

I only added 126.11g of water, but the output reads volume of solution is 122.55ml and volume of solvent is 126.11ml.

Thank you for catching this! I forgot to include the edge case where we have only solvent particles in the container. I was diving the by the density value we have been using for solutions rather than the density of the solvent itself.

Posted by: @ryim

I added 37.40g of NaCl, volume of solution is now 158.90ml; this seems like an unnaturally large increase in solution vol.

Thank you for bringing this up! The volume value of the solution you see reflects two quantities: the dissolved solution volume + the undissolved volume of the solid. In this scenario, there is still undissolved NaCl. Using the mass remaining in solid form and the known density of NaCl, this value is added to the solution volume.

Posted by: @ryim

There is also a disparity in the symbolic sim: 2 NaCl molecules=0.64 moles; 7 H20 molecules=6.8 moles.

This is a choice made specifically in response to teacher comments. One of the issues teachers brought up during the Fall 2019 PD was that single particles moving into solution at a 1 particle = 1 mol correspondence were causing too large of swings in aqueous mass. To solve this problem, I implemented an asymmetric scaling technique whereby water molecules maintain a 1:1 particle:mol ratio, but the solute particles can be scaled to any quantity needed to obtain a reasonable pedagogical model.

I've detailed the mathematical technique here for anyone curious to know more about the underlying model:

Postdoctoral Research Associate
Simulation Developer

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