Water Chemistry
posted by Mary .
I have a water sample that contains 0.50 mg/l (or ppm) of phosphate.
We know that phosphate is a component of Total Phosphorous.
How do I calculate what the Total Phosphorous is from my sample that contains 0.50 mg/l of phosphate???
I have to do something with the atomic weights of P and O(4)...or divide P/O(4) or 30/64 but then what???
Many thanks.

a) Look up the atomic mass of P.
b) Calculate the formula mass of the phosphate ion, PO4^2.
Multiply 0.50 mg/L by the fraction of (a)/(b).
[The above fraction is roughly 31/95, not 30/64] 
So, Total Phosphorous is 0.16 mg/l?

That's correct.

I might point out here that 0.16 mg/L is the P in the phosphate which may or may not be the total P in the sample. IF all of the P in the sample was converted to phosphate, then phosphate determined, that represents the total P in the sample. However, if phosphate was determined without first making sure that ALL of the P was in the form of phosphate, then 0.16 may not represent total P. It's a picky point but worth pointing out, I think.