measuring question

[moosemanty]

I am playing around with tissue culturing plants, and I am trying to mix a certain media mixture and it calls for 0.5 micromolar concentration of thidiazuron. I cannot find any info on how to measure that. can someone at least point me in the right direction on how to learn what I need to know to mix that concentration? I've never taken any chemistry classes so this leaves me a little confused.

[John Cuthber]

Well, this will tell you what micromolar means

https://en.wikipedia.org/wiki/Molar_concentration

 

Fundamentally, it's a bit like counting eggs in dozens, rather than one at a time.

However, because molecules and atoms are really small chemists use a rather bigger number than 12.

You might imagine that we would choose a "simple" number like a million billion billion.

 

That would be fine if we counted molecules- but we don't (usually).

We measure out stuff by weight.

So we picked a number that's not a dozen, or a million billion billion, we chose

602,214,857,000,000,000,000,000

 

which seems an odd choice, but it's (ideally) the number of atoms in a gram of hydrogen. It's called Avogadro's number- named after an Italian scientist

(strictly it's the number of atoms in 12 grams of carbon- because it's easier to measure that accurately but it's very nearly the same).

Why choose that?

Well, it's very difficult to weigh single atoms and molecules- but relatively easy to measure how much they weigh relative to one another.

​It makes sense to use the lightest atom- hydrogen- as a unit of mass. (it's sensible to have a unit that's roughly the same size as the things you are weighing-

you don't weigh supertankers in ounces.)

We call that unit the amu (atomic mass unit)

So we know that the carbon atom, for example, weighs 12 times more than the hydrogen (strictly 12.0107)

There are tables of atomic weights like this on the web.

http://www.lenntech.com/periodic/mass/atomic-mass.htm

 

The other thing you need to know is that atoms combine together (at least most of the time) in fixed ratios. So, for example, methane always has exactly 4 hydrogen atoms for each carbon atom.

So we know that, since the hydrogens weigh 1 AMU and the carbon weighs 12 AMU the methane must weigh 12 +4 =16 AMU.

 

Now let's imagine doing some chemistry- we burn the methane in air to form carbon dioxide and water.

CH₄ + 2 O₂ --> CO₂ + 2 H₂O

Each molecule of methane reacts with two molecules of oxygen to form a molecule of carbon dioxide and two molecules of water

(oxygen atoms hang round in pairs- so they are O₂ rather than just O. Quite a lot of gases do that)

 

Well, that's fine- we know the numbers of molecules involved- but how could we work out the weights of the materials?

The first step is to realise that you can multiply up that equation by any number you like

So, for example

Each zillion molecule of methane reacts with two zillion molecules of oxygen to form a zillion molecule of carbon dioxide and two zillion molecules of water.

 

It also works (of course) with Avogadro's number which is referred to as L (I have no idea why it's L)

So

L molecule of methane reacts with two times L molecules of oxygen to form L molecule of carbon dioxide and two times L molecules of water.

 

And by convention, we call that number of molecules a mole so

 

 

One mole of methane reacts with two moles of oxygen to give one mole of carbon dioxide and two moles of water.

Here's the clever bit.

We chose L so that it was the number of atoms in a gram of hydrogen.

So a mole of hydrogen atoms weighs 1 gram.

By adding up the numbers we know that a mole of methane weighs 16 grams

We can do the same with the other things in the equation

Each oxygen atom weighs (from that table) 16 grams (near enough)

So each O₂ molecule weighs 32 grams

And in the same way you can find that a mole of carbon dioxide weighs 12 + 28(16) = 44 grams and the water weighs 18 grams per mole.

 

So we can see that 16 grams of methane reacts with 64 (that's two lots of 32) grams of oxygen to give 36 (that's twice 18) grams of water and 44 grams of carbon dioxide.

 

So knowing the weights of the molecules in this way lets us calculate the weights of materials involved in a reaction.

Instead of having lots of tiny masses to keep track of, we divide by the mass of a hydrogen atom right at the start.

 

It's even more useful when you start doing reactions with chemicals in solution.

You make the solutions with odd looking masses of chemicals and in doing so you get solutions where the volumes that react with each other are in simple simple ratios.

You do that by making solutions with sensible numbers of moles per litre rather than sensible number of grams.

For the stuff you are looking at they chose a two millionth of a mole per litre.

I can look on the web and find out about thidiazuron.

The formula is C₉H₈N₄OS

and I can add the masses of the atoms together to get the mass of the molecule - or I can cheat and find it on line

There's a wiki page for the stuff

Ihttps://nl.wikipedia.org/wiki/Thidiazuron

It's in Dutch I think, but I can still find the molecular weight

220.25 odd grams per mole.

So if I wanted to make a solution that has a 2millionth of a mole per litre I could weigh out about 110.125 micrograms of it and dissolve it in a litre of water.

Well, I guess I could, but I'd need a good balance to weigh that small a quantity.

I could also try weighing out 110.125 miligrams and dissolving it in a thousand litres of water- that's fine if I want a tonne of solution.

 

Or I could take 110.25 mg of it and dissolve it in a litre of water (I suspect it wouldn't dissolve in water at this concentration so I would use a different solvent. One that's a good solvent, but not very toxic. I'd try alcohol)

So I's have a solution that was a thousand times more concentrated than I wanted.

But I could easily take a millilitre of this with a calibrated dropper, and dilute it to a litre with water.

that would give me the right concentration without having to weigh out an absurdly small amount or make up a huge volume of solution.

Edited August 28, 2016 by John Cuthber

[moosemanty]

wow, thank you so much for the detailed response. it is very much appreciated.

 

oh, and i'm an idiot. the concentration I needed for the thidiazuron was 2.5_uM, not 0.5 . so does that mean i'd need 550.63 mg/l for a 1000x solution?

 

Also I have a couple other ingredients. if you could check my math and answer my question below I would be extremely grateful.

 

chemical- gibberellic acid

molar mass- 346.37 g/mole

concentration desired- 7 _uM

my answer- 2.43 grams a liter for a 1000x solution

 

the final ingredient is indole-3-butyric acid. this one confuses me. the molecular mass is 203.24 g mol^-1

what is the superscript -1? does that indicate that the atom has a negative ionic charge? if so does it change the way you would calculate it? if not then if I wanted a 2.5 _uM concentrate would it come out to be 508.1mg per liter for a 1000x solution?

 

 

oh and one final question, the recipe calls for "500mg l^-1 of activated charcoal^". what does the negative superscript 1 mean with that?

 

Thank you again!

Edited August 30, 2016 by moosemanty