# Amount of substance of a molecule in a solute the same as amount of substance of constituent elements?

I’ve done absolutely no chemistry since high school. Now, 16 years later, I’ve been asked to look at some chemical analyses and I’m having trouble!

Context: I’ve got a series of spectroscopy results giving the concentrations (in mg/l) of chlorine and $\ce{SO4}$ in a sample. I want to find the molar and mass ratios of $\ce{S}/\ce{Cl}$.

What I’ve done: I convert the mg to g, then (mass) concentration into a molar concentration by dividing by the molar mass (35.5 g/mol for $\ce{Cl}$ and 96 g/mol for $\ce{SO4}$).

Question: am I right in thinking that the molar concentration of $\ce{S}$ will be the same as the molar concentration of $\ce{SO4}$? My reasoning is that 1 mol $\ce{SO4}$ contains 1 mol $\ce{S}$ and 2 mol $\ce{O2}$.

Furthermore, to get the mass concentration of $\ce{S}$ alone, can I just multiply my concentration of $\ce{SO4}$ by the molar mass ratio, $\frac{M(\ce{S})}{M(\ce{SO4})}$?

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• Your approach seems solid. – Lighthart Mar 27 '15 at 20:48

• Just a small correction at the end: the molar ratio of $\ce{S}$ and $\ce{SO_4^{2-}}$ is exactly 1, but the molar mass ratio is approximately $\frac{1}{3}$, as you mention above – Nicolau Saker Neto Apr 2 '15 at 14:04