Holy Mole-y I have a whole MOLE of Tris friends in this bottle! I don’t have imaginary friends, but I do have invisible ones (at least invisible to the naked eye) – and when I *concentrate* while I going about my work in the lab I visualize the molecules like little friends of mine whose stories I’m trying to tell and I’m always wanting to know how many of these friends I have! Instead of “follows” and “likes” I’m more worried about things like MOLALITY and MOLARITY. Can the bumbling biochemist use candy to bring a MOLE FRACTION of clarity to this CONCENTRATION-REPORTING hilarity and shed some light on ELECTROLYTES?!
A mole (mol) is like the biochemist’s “dozen” – it is just a set number of things – anything – but if you ordered a mole of bagels you’d get 6.02 x 10^23 of them… (that’s 602 and and then 21 0s…) which, if you were to divide up among the human population would be about 86 trillion bagels per person. Talk about carbo-loading!
Yet, in this bottle of Tris buffer (pH-stabilizer) there’s a whole mole of Tris molecules (which are obviously way smaller than a bagel). I know there’s this many because I looked at the chemical bottle’s equivalent of a nutrition label & used the FORMULA WEIGHT (F.W.) to see that 1 mole of Tris weighs 121.14 grams & I weighed that much out. And then I put that in ~800mL of water, added HCl to get it to the right pH, then topped off the volume to 1L to get a 1M solution (M stands for molar and it means mol/L).
I stuck 1 mol of particles (“friends”) in and now there are 1 mol of particles hanging out in the water. I didn’t gain any friends, just moved them & got them to hang out with my water friends. But if I looked at the “nutrition label” for table salt (NaCl), saw it had a F.W. of 58.44 g/mol, measured that much out & stuck it in 1L of water, my “friend count” would double – NaCl is an ELECTROLYTE, so when it dissolves it also dissociates so you get more particles than you put in. To explain more, let’s step back a bit so I can assure you this is legit… And bring in that candy I promised you.
If we want to know how much of some chemical we have, we usually describe it in terms of how many FORMULA UNITS we have 👉 FORMULA UNIT’s kinda like “what you buy it as” 👉 we buy TABLE SUGAR as SUCROSE & TABLE SALT as SODIUM CHLORIDE (NaCl), and if you look at the “nutrition label” on a chemical bottle this is what things like MOLAR MASS and CHEMICAL FORMULA are based on.
MOLAR MASS tells how much 1 mole of the formula unit weighs. Other terms for molar mass include formula weight or molecular weight, so you might see it abbreviated F.W. or M.W. 👍
Take sucrose 👉 it has a chemical formula of C12H22O11 👉 This tells us that each sucrose contains 12 carbons, 22 hydrogens, & 11 oxygens. If you were to add up the atomic weights of each atom it contains (you can find these on the periodic table of elements) you’d get the weight of 1 molecule of sucrose (which would be a really tiny number😬) BUT if you multiplied that by 6 x 10^23 you’d get the weight of 1 mole of sucrose (an easier-to-work-with number 😅) & we call this the MOLAR MASS 👍
So this sucrose label tells me the formula unit is sucrose & 6 x 10^23 sucroses weigh 342.3 grams (g). So if I want 1 mol of sucrose, I need to weigh out 342.3g 👍
But what am I going to do with that mole of sucrose? 🤔 If I dissolved it in a glass of water, I’d get a super-sweet solution (⚠️ but don’t taste anything in the lab!) BUT if I dissolved it in a pool I’d probably not even be able to taste it (don’t drink that either…)⚠️
Clearly, when it comes to solutions, just knowing # of mol is NOT sufficient! 😩 Instead, what we really care about is the # of mol COMPARED to amount of solvent (the thing it’s dissolved in – often water) 👉 We want to know its MOLAR CONCENTRATION 👉 Some common ones are molarity, molality, & mole fraction, & we’ll get back to these later, but when it comes to particle concentrations, it’s important to know how many particles you’re gonna get (cuz it might be different than what you put in…)
I’ve been talking about particles as “friends” but what really is a particle? a PARTICLE is basically just a thingamabob that’s held together. Can water take apart a particle? It depends on what’s holding it together.
In a MOLECULE, all the bonds are STRONG, COVALENT bonds. In this type of bond, neighboring atoms are actually sharing electrons so they “need” each other Covalent bonds *can* be broken, but it’s not easy. To split up a covalent bond you need to go through an energetically costly “divorce process” that often requires helpers like proteases (protein-cutting proteins) or nucleases (DNA or RNA-cutting proteins)
Dissolving CAN’T take apart a MOLECULE into atoms but it CAN can take apart a particle into molecules
Not all particles are held together by strong bonds. in WEAK bonds, neighboring atoms don’t “need” each other, they just “want” each other. They’re held together by +/- charge attraction but they’re not “committed”
IONIC BONDS like those holding together the sodium & chloride are strong *attractions* (because the charges are full not just partial), but they’re still just attractions – they’re still “weak,” non-covalent bonds – they’re happy to leave if they find better partners, like water, so things held together by these bonds come apart when you dissolve them
You can visualize this with candy – if your formula units are a Rolo roll & a KitKat bar – if you unwrap them (dissolve) the Rolos will separate (dissociate), but the Kit-Kat pieces will stay stuck together. You’ll need to put in some real effort to break off the pieces of KitKat bars!
Similarly, if you dissolve a NON-ELECTROLYTE (like sucrose), you don’t separate its formula unit, but when you dissolve an ELECTROLYTE (like sodium chloride), you separate its formula unit into IONS (positively charged particles)
That’s cool & all but often what we really want to know how many particles there are in a certain space or compared to a certain amount of total particles. Especially because the # of PARTICLES in a solution affects that solution’s COLLIGATIVE PROPERTIES http://bit.ly/2P3pTN7
COLLIGATIVE PROPERTIES are properties of a solution that only depend on # of dissolved particles, NOT the identity of those particles – e.g. when you have more particles stuffed in there are fewer water molecules at the surface where they have the best chance of escaping into the air – so you get boiling point elevation – if you want to know more, check out my post on why you should bring salt if you want to cook spaghetti on top of Mt. Everest…) http://bit.ly/2Ruek3w
When it’s outside of water, a grain of sugar could be considered a “particle” – it’s made up of lots of individual sucrose MOLECULES bound to each other. But these bonds are weak, so when you put it in water it starts to dissolve – the sucrose MOLECULES come apart – now each of these is a particle – but the sucrose doesn’t come apart further – we have the same number of particles as we have formula units, so any calculations we did stand
Now consider a grain of salt – it too is made up of smaller parts that come apart when we put it in water (dissolve it) – but these parts are smaller than the formula unit we calculated based on -> for each mole of NaCl we put in we got 1 mole of Na⁺ AND 1 mole of Cl⁻ – in terms of particles, we have twice as many, so 1 mole of NaCl
And that’s important because it means it’ll have 2X the effect on COLLIGATIVE PROPERTIES (remember these depend ONLY on the # of particles) as 1 mole of sucrose
To account for this, instead of just using the formula unit concentration when calculating colligative properties, we adjust that concentration with that unit’s Van’t Hoff factor (i). For NON-ELECTROLYTES, “no” adjustment needed, so i = 1. BUT for ELECTROLYTES, i depends on the # of ions it breaks into. So NaCl would have an i of 2
So multiplying by the Van’t Hoff factor converts the number of formula units we put in into the number of dissolved particles we get
These are for ideal solutions – If we were to measure it experimentally, these numbers would actually be lower
👉 concentrated solutions, oppositely-charged ions may be close enough together that they pair back up – so ION PAIRING *Impairs* our ability to predict
We have several different ways we can report on concentration using moles.
Molarity is a measure of concentration which tells you about how many of a thing is in a volume of solvent (usually water). MOLARITY is moles per liter (mol/L) & we represent it w/capital M (e.g. a 1M solution has 1 mol/L so 1 L has 1 mol, 2L has 2 mol, 0.5L has 0.5 mol etc.)
The mole fraction (aka molar fraction) is the relative amount of 1 component of a mixture compared to the whole mixture. This might sound like molarity, BUT there’s a key difference
Molarity is the moles of something compared to the VOLUME of the solvent
The MOLARITY of one chemical won’t change if I add another chemical, but the MOLE FRACTION will!
And speaking of changing, the volume of a liquid can change w/changing temperature 👉 at a higher temp, molecules can move around more, so they take up more space 👉 To account for this, we can use molaLity instead of molaRity 👇
🔹 molaLity is moles per kg solvent & we represent it with a lowercase m
🔹🔹 Helpfully, at room temperature, the density of water is 1.00 kg/L so the molarity & molality for water-based (aqueous) solutions at room temp are basically the same 😅
Sorry today’s post was a bit of a mis-mash of past posts – It’s been a really exciting time for me in the lab because my experiments are finally starting to produce some cool results.