convert concentration to mole fraction

[ridingbio]

Hi there
I'm a biologist working on my graduate thesis project that involves comparing the elemental signature in otoliths (fish ear bones) to concentrations of the same elements found in water. In particular, I am looking at Strontium. I have a water concentration of Sr at .008g/L, and I need to convert that to micro-moles/mole. Ive been searching through old texts and on the internet for methods on how to do this, yet I haven't found anything. Its been several years since Ive taken chemistry, though I still recall how to calculate molar concentration, but I can't figure out how convert that to a mole fraction. Any help on this would be appreciated. And/or if you have useful links or titles to credible resources that would be helpful as well.

[JohnS]

Originally Posted by ridingbio

Hi there
I'm a biologist working on my graduate thesis project that involves comparing the elemental signature in otoliths (fish ear bones) to concentrations of the same elements found in water. In particular, I am looking at Strontium. I have a water concentration of Sr at .008g/L, and I need to convert that to micro-moles/mole. Ive been searching through old texts and on the internet for methods on how to do this, yet I haven't found anything. Its been several years since Ive taken chemistry, though I still recall how to calculate molar concentration, but I can't figure out how convert that to a mole fraction. Any help on this would be appreciated. And/or if you have useful links or titles to credible resources that would be helpful as well.

You have to convert the species of interest and the overall mixture each to moles. For dilute mixtures, it is common to just consider the solvent (water) rather than the mixture; technically it is a minor error, but it is insignificant for dilute mixtures.

You have 0.008 g Sr in a liter of water. The liter of water is usually approximated as 1 kg (not exact, but close enough as long as the water is cool).

.008 g of elemental strontium has molar mass of 87.62 g/mol (unless you are dealing with particular istopes), so 91.3 µmol.

The liter or kilogram of water has a molar mass of 18.015 g/mol, hence 55.5 mol. (If you are doing anything high precision, you may wish to look up the density of water at the temperature of interest)

So you have 91.3 µmol/55.5 mol = 1.645 µmol/mol

[Mrs X]

This is quite confusing, but i'm not convinced John's method (while correct) is the way you need to look at it. If you are comparing strontium in bone to strontium in the ambient water, then you need to take into account the strontium levels in both, and present them as a ratio.

Do you have comparison levels? (moles Sr per kg of bone, compared to moles Sr per litre of sea water?)

[ridingbio]

Thank you both for your replies. The otolith molar fraction was calculated by dividing the molar concentration of Sr by the molar concentration of Calcium. I came across some literature since my last post where the element:Ca ratios were compared in both the otoliths and the water, so I'm thinking I will have to take the same approach that John mentioned, but use calcium instead of water in my calculation.