molar concentrations

[Joe Farina]

I've always been confused by molar concentrations. Sometimes I've encountered them in papers on holographic chemistry, but didn't know how to convert them to my layman's understanding. I found this definition which I like, in a book on marine aquariums:

[Din]

It's just the molecular weight dissolved in one litre. You look up the atomic weights in, for example, any periodic table, add them up and dissolve that amount in grams into one litre of water.So ammonium dichromate - [(NH4)2]Cr2O7 is two nitrogens with atomic wt 14, 8 hydrogens with atomic wt 1, two chromiums with at wt 52 and seven oxygens with atomic wt 16. So, MW of ammonium dichromate is 28 + 8 + 104 + 112 = 252. Thus a 1M solution of the stuff is 252 gms dissolved in 1 litre of water.

Thus, for dcg holography, if you dissolve 3 gms of dichromate in 100 mL of water, you would have a 30% by weight of solution, or. a concentration of 30 gms/litre. This would be approximately a 30/252 ~ 0.1M solution.

[Joe Farina]

It's just the molecular weight dissolved in one litre. You look up the atomic weights in, for example, any periodic table, add them up and dissolve that amount in grams into one litre of water.So ammonium dichromate - [(NH4)2]Cr2O7 is two nitrogens with atomic wt 14, 8 hydrogens with atomic wt 1, two chromiums with at wt 52 and seven oxygens with atomic wt 16. So, MW of ammonium dichromate is 28 + 8 + 104 + 112 = 252. Thus a 1M solution of the stuff is 252 gms dissolved in 1 litre of water.

Thus, for dcg holography, if you dissolve 3 gms of dichromate in 100 mL of water, you would have a 30% by weight of solution, or. a concentration of 30 gms/litre. This would be approximately a 30/252 ~ 0.1M solution.

Thanks, I like that even better

[Colin Kaminski]

Thank you Dinesh, that is a really easy to teach it. I am going to steal it. Do you have a concise way to teach mEq?

[Din]

There's really no set answer. The simplest way is to look up the atomic weight and divide by the valency. But, elements may have several valencies, so you have to choose the appropriate valency for the reaction of interest. For example, the atomic wt of potassium is 39, the valency of potassium is 1 (you know this because it bonds with one OH ion in KOH). Thus the equivalent wt of potassium is 39. But, in potassium dichromate, the equivalent weight in an acid reaction is given by the number of electrons it loses or gains. So, in an acid reaction Cr -> Cr{6+}, so the equivalent wt of potassium dichromate is it's mol wt/6 = 294/6 = 49. So, to make a 1N solution of pot dichromate, you'd dissolve 49 gms in a litre of water.

I'm not sure that equivalent weights are a good measure for the actinic reaction of dichromates in holography, since the chain of events is fairly complex. You'd have to go through the chain of events (Cr6+ -> Cr3+ etc) adding and subtracting the number of electrons ("equivalemts") for every event. Equivalent weights are generally used to determine how many electrons are gained or lost, or conversely, how many H+ and OH- are gained or lost in a redox reaction. In an actinic reaction, one of the reactants - light - loses or gains no electrons (unless you happen to be doing your holography near a black hole, so pair production occurs. In this case, you're pretty much on your own!)

By the way, I don't know if anyone noticed that I made a mistake in my post on molar solutions:

Thus, for dcg holography, if you dissolve 3 gms of dichromate in 100 mL of water, you would have a 30% by weight of solution,

3 gms in a 100 mls of water is a 3% solution. It's still true that this is a 0.1M solution, since a 3% solution is still 30 gms/litre

[Colin Kaminski]

mEqs are very useful for water chemistry. I have to use them all the time. I often have to calculate resulting pH from adding organic acids to water. The pKs are such that there are buffer pHs to overcome.