[iamasciencenoob]

I found a 50.0 ml sample of tap water(25.0 ml tap water and 25.0 ml super distilled water) from my home to have 16 ppm of Phosphorus. I am trying to determine how many ppm of PO₄^3- is in the sample. My lab instructor gave us some steps to follow that are not clear to me.

" To find ppm PO₄^3-
  ppm P>mg/L P> MM P, >stoichiometric ratio P:KH₂PO₄^3- > MM KH₂PO₄^3- > mg/L KH₂PO₄^3- > ppm KH₂PO₄^3- = ppm PO₄^3-

I understand 1ppm = 1 mg/L, but I am confused on how to continue.

I changed 16 ppm to 16 mg/L
I then converted that to .016 g/L
Next I divided by molar mass of P to get 5.2 x 10^-4 mol/L, but now I am stuck.

[Dan]

" To find ppm PO₄^3-
  ppm P>mg/L P> MM P, >stoichiometric ratio P:KH₂PO₄^3- > MM KH₂PO₄^3- > mg/L KH₂PO₄^3- > ppm KH₂PO₄^3- = ppm PO₄^3-

I disagree with the part in red.

I understand 1ppm = 1 mg/L, but I am confused on how to continue.

I changed 16 ppm to 16 mg/L
I then converted that to .016 g/L
Next I divided by molar mass of P to get 5.2 x 10^-4 mol/L, but now I am stuck.

Ok so far. Consider:


How many mol P are in 1 mol PO₄^3-?
So given the molar concentration (mol/L) of P, what is the molar concentration of PO₄^3-?
Convert the molar concentration (mol/L) to mass concentration (g/L) of PO₄^3- [hint, you already did the reverse conversion for P].
Convert to ppm.

[iamasciencenoob]

Its a 1:1 molar ratio so I would just take into account the molar masses of P and PO₄^3- when converting?

[Borek]

Its a 1:1 molar ratio so I would just take into account the molar masses of P and PO₄^3- when converting?

That's the idea.