Reef Chemistry Question of the Day # 175 Ion Concentration

[Randy Holmes-Farley]

Reef Chemistry Question of the Day # 175

Which of the following represents the largest number of ions?

A. 100 ppm phosphate
B. 100 ppm nitrate
C. 100 ppm potassium
D. 100 ppm magnesium

Super bonus question: does the answer above have more ions than 10 dKH of alkalinity?

Good luck!































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[Cory]

I think its no3, i added the weights of individual elements.

 

[Taservices]

I say A, bonus no.

 

[JimWelsh]

The "number of ions" of a particular ion in a solution, i.e., the molarity, is determined by dividing the weight in grams of that ion per liter of solution by the molecular or atomic weight of the ion. Since the concentration by weight is constant in each case, then the ion with the smallest molecular or atomic weight will have the greatest molarity, and hence the greatest number of ions. Of all these ions, magnesium is by far the lightest, with an atomic weight of 24.3 (yes, I have that memorized), so the answer is D.

Regarding the Super bonus question: The number of magnesium ions in 1 liter of a solution with 100 PPM (assuming PPM means "w/v") of magnesium is 0.1 g / 24.3 = 0.00412 moles. Now, 10 dKh is the same as 10 / 2.8 = 3.57 meq/L (yes, I have that conversion memorized), which represents 0.00357 mole equivalents worth of negative charge per liter. That negative charge may be in the form of singly-charged ions, such as HCO3- or OH-, or it may be in the form of doubly-charged ions such as CO3-- (in which case the number of ions is one half of the mole equivalents represented), or even ions like PO4---, or any combination thereof, but in any event, the total number of ions represented by that 10 dKh of alkalinity will be equal to or less than 0.00357 moles, which is less than the 0.00412 moles of magnesium present per liter, so the Super bonus question answer is, "Yes."

 

[gtbarsi]

I do not have the answer but I am going to ask the stupid question. If there is 100 PPM of each how can there me more ions of one than the other? The only way I see that it is possible is if some are not ionic.

*Edit, Clearly I should have waited for the post above...

 

[Cory]

Looks like i did it backwards Jim lol

 

[Randy Holmes-Farley]

And the answer is...D. 100 ppm magnesium

When measured with a unit like ppm (which can be equated to milligrams per kilogram of seawater), the chemical with the most ions per unit weight, or per milligram, wins. Magnesium is by far the lightest of the ions listed, so it wins (as Jim showed)

Jim also showed that the number of ions contributing to the 10 dKH of alkalinity cannot be as much as the number of magnesium ions in 100 ppm. So the answer is yes.

Happy Reefing!

 

[JimWelsh]

Hey, I finally got one right!

 

[beaslbob]

And the answer is...D. 100 ppm magnesium

When measured with a unit like ppm (which can be equated to milligrams per kilogram of seawater), the chemical with the most ions per unit weight, or per milligram, wins. Magnesium is by far the lightest of the ions listed, so it wins (as Jim showed)

Jim also showed that the number of ions contributing to the 10 dKH of alkalinity cannot be as much as the number of magnesium ions in 100 ppm. So the answer is yes.

Happy Reefing!

dern
I looked up malecular weight looking for the lightest and came up with magnesium.
But by the time I did all that randy already gave the answer.
phosphate 94.9, magnesium 24.3, potassium 39 and nitrate 64.0049

had no idea on the super bonus tho