2013: Advent Chemistry: Getting Started

by on December 1, 2013
A mole emerging from a molehill

Not one of these.

So here we are, day one, and first things should come first. Let’s talk about measurement.

Measuring out your reactants is fairly vital to doing chemistry. You can’t get very far if you don’t know what you’ve got and how much of it. So you stick a beaker on the scales, reset the numbers to zero, measure out your 2.37 grams each of sodium hydroxide and hydrochloric acid, and off you go.

Sadly, it’s not quite as simple as that. Well, no, it is. But you have to do some other, more complicated stuff as well.

2.37 grams of sodium hydroxide is not the same as 2.37 grams of hydrochloric acid, and not just because you generally encounter hydrochloric acid as a solution. Even if you had solid hydrochloric acid and measured it out weight for weight, you wouldn’t have the same amount. If you mix your 2.37 grams of each thing together, there’d be a simple one to one reaction – HCl + NaOH → NaCl + OH2 – and you’d still have some hydrogen chloride left over.

The key to this apparent paradox is that one molecule of HCl and one molecule of NaOH don’t weigh the same. HCl is lighter, so you get more of it per gram. It’s like getting more plums per kilogram than peaches. And this is true of almost any set of things you might react together. How much you have just isn’t a good guide to how many.

This is where that weird chemical thing known as a mole comes into it. One mole is 6.022×1023 molecules, atoms or repeat units of a substance. This is a very large number and not on the face of it very intuitive, until you know that one carbon atom is 12 atomic units of mass (six protons plus six neutrons, which are nearly equal in mass), and 6.022×1023 atoms of carbon is 12 grams…

That relationship is the solution to the entire measurement problem. One mole of a substance is equal to its molecular weight, in grams.

So you don’t say “I need 2.37 grams of each.” You say “I have 2.37 grams of sodium hydroxide, so how much hydrochloric acid do I need?” and then you do all your sums in moles. 2.37 grams divided by 40 grams per mole equals 0.059 moles. 0.059 moles of hydrochloric acid is… well, it depends on the strength of your solution, but moles also solve your problem there. You don’t define the strength of a solution by weight per volume. You define it by moles per volume. 0.1 molar hydrochloric acid, to take a fairly common laboratory strength, has 0.1 moles per litre of HCl (the rest is water). 0.059 moles divided by 0.1 moles per litre equals 0.59 litres, which is 590 millilitres and a workable quantity, though if I were doing a more complicated reaction I might want stronger acid, just to keep my total volume down.

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