Add new unit: Molarity

[bibbca]

Add Molarity (mol/L) as a unit, also add M to regex.

[drlippman]

Thanks for this. I have a feeling this is going to conflict with #318 which I've been slow to merge and review. Perhaps @nick-chura can weigh-in on whether the M molarity unit is going to cause problems in the revised units scheme?

[nick-chura]

Adding M as a unit doesn't interfere with #318 -- it just would need to be added to javascript/assess2.js, javascript/AMhelpers2.js and the .min files.

Just two thoughts on adding it:

1. Wolfram|Alpha and Google both define 1 M as 1 mol/L, so there would be some conflict in our defining it as 1 mol/m^3. Wikipedia agrees that 1 mol/m^3 is the SI definition, though says it is less popular than 1 mol/L in chemistry literature.
2. A personal fear is that adding M (by either definition) may cause a lot of reasonable answers to be marked wrong, such as a student wanting to answer "5 meters" but typing "5 M" and not getting an answerbox warning message (like they do now).

So, I think it's a reasonable add, though I have some reservations because the system doesn't (yet) have a way to tell a student how it is interpreting a unit such as "m" or "M".

[nick-chura]

Also, in the format of #318, I think M would look like

'M' => [1,array(0,-3,0,0,0,0,0,1,0,0),1,0,0], // Molarity

because it would allow only abbreviated metric prefixes, we don't want to allow it to be pluralized with an ending 's', and it is case sensitive.

[drlippman]

Yeah, I had the same concern about a student accidentally entering "5 M" when they mean "5 m", but there will be cases with units where we'll run into that.

Wikipedia does suggest that 1 mol/L is commonly designated 1 M, while it seems to suggest mol/m^3 is designed c. And indeed it later says that 1 mol/m^3 = 10^-3 M. Given that, it sounds like the definition of M should instead be

'M' => [0.001,array(0,-3,0,0,0,0,0,1,0,0),1,0,0], // Molarity

Caleb, as the science teacher, does that seem right to you?

[drlippman]

Wait, I think I have that backwards. 1km = 1000m gets coded as 'km' => [1000,array(0,1,0,0,0,0,0,0,0,0)],

Then since 1 mol/m^3 = 10^-3 M, we'd have 1 M = 10^3 mol/m^3, which would be 'M' => [1000,array(0,-3,0,0,0,0,0,1,0,0),1,0,0], // Molarity

1 M = 10^3 mol/m^3

[bibbca]

Honestly, I wrote that line copying the line for Liters. Since in my mind, 1 M = 1 mol / L.
'L' => [0.001,array(array(0,3,0,0,0,0,0,0,0,0)],

So, long story short, that does seem right to me.

[drlippman]

Great. I need to finish reviewing Nick's contribution, which will conflict with this one. Once I get that one done, we can figure out how to merge this change in. Hopefully next week.

[bibbca]

Thanks so much!