Reef Chemistry Question of the Day #58 Molecular Weight

Randy Holmes-Farley · Nov 11, 2014

Reef Chemistry Question of the Day #58

We use the molecular weight of chemicals for many purposes, as has been demonstrated in many of these questions of the day. For example, in determining how much a calcium boost raises salinity.

Molecular weight is, to a first approximation, simply a count of the number of neutrons and protons in an atom, ion, or molecule, since electrons have very little weight.

In the question a few days ago about calcium chloride, I noted the molecular weight of chloride used for such calculations is 35.45 and the molecular weight of the calcium is 40.

How can the chloride molecular weight not be a whole number if it is a count of things (protons and neutrons) which must be whole numbers?

Good luck!

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H2O · Nov 11, 2014

Sometimes its hard to get a count when the only constant thing in this world is change just kidding the later u wake up the harder the questions get I'm getting a water change ready my hands r wet I let u guys have fun with this one

Randy Holmes-Farley · Nov 11, 2014

H2O said:
Sometimes its hard to get a count when the only constant thing in this world is change

:lol:

Good answer.

A similar answer would be its hard to get an accurate measure when they are so small.

nautical_nathaniel · Nov 11, 2014

35.40 is an average of the molecular weights of several isotopes of the anion Chloride and 40 is actually a rounding of 40.078 +/- 0.004 of the average molecular weight of all known Calcium isotopes, its easier to just use 40 since most sig figs of general use measurement devices only go to the tenths place.

Cory · Nov 12, 2014

I'd really like to know the answer but don't have an idea! Less neutrons or protons or electrons probably. Or maybe because it's a molecule? Interesting to know for sure.

Randy Holmes-Farley · Nov 15, 2014

And the answer is...isotopes! (good answer, pensacola)

While the number of neutrons and protons is absolutely a fixed integer in any given atom or ion, there can be slightly different numbers of neutrons in many atoms.

Take, for example, carbon.

Every carbon atom has 6 protons. That is what makes it carbon, rather than something else, such as oxygen (with 8 protons).

But the number of neutrons can vary. With carbon, you can pack in 6, 7, or 8 neutrons (or other numbers if you make them in a lab).

If there are 6 neutrons, you have carbon 12, which is stable and is the most common form.

If there are 7 neutrons, you have carbon 13, which is stable but is much less common.

If there are 8 neutrons, you have carbon 14, which is unstable and breaks apart, releasing radiation (radioactive decay).

Scientists can also force in other numbers of neutrons, but all of those other forms are unstable.

So when scientists refer to the molecular weight of something, the value depends on the context. When using molecular weights for calculations involving aggregates of large numbers of atoms (such as a glass of water), then they are usually interested in the "average" molecular weight. Such an average is the weighted average of all of the naturally occurring isotopes. Each is weighted by the percentage of that isotope that occurs naturally.

Since carbon is mostly carbon 12 with just a tiny amount of carbon 13 and 14 naturally present in such samples, the "average" molecular weight of carbon is about 12.0107

For chlorine in the question at hand, there are a variety of isotopes, with chlorine 35 (75.8%) and chlorine 37 (24.2%) the stable, naturally occurring, isotopes.

Hence the molecular weight of chlorine is the weighted average of the two: 35 x 0.758 + 37 x 0.242 = 35.45

Cory · Nov 15, 2014

Thanks Randy. I never knew there were different elemental forms called isotopes. Can the neutrons be lower rather than higher in isotopes?

By the way have you seen the T.V series Breaking Bad? A high school chemist gets cancer and to pay his medical bills guess what he does. One of my favourite series.