Question

1. A wastewater sample contains 7.4 mg P/L PO, and 4.5 mg P/L P,0*. a. Calculate the total phosphorus as mg PL b. Find the concentrations of Poland P0,* in wastewater as mg PO/L and mg P.0. L, respectively.

Answer 1

1.

a. Given that the wastewater sample contains 7.4 mg P/L PO₄^3- and 4.5 mg P/L P₂O₇^4-.

The phosphorous in the wastewater is contributed by these two types of ions

The contribution of phosphorous by PO₄^3- per litre of wastewater is 7.4 mg and the contribution of phosphorous to 1 litre wastewater by P₂O₇^4- is 4.5 mg.

Thus 1 litre wastewater contains 7.4mg of Phosphorous from PO₄^3- and 4.5 mg of Phosphorous from P₂O₇^4-

Therefore a total of 7.4 + 4.5 = 11.9 mg of phosphorous is present in 1 litre of wastewater.

Thus the required total phosphorous as mg/L is 11.9 mg/L

b.

7.4 mg phosphorous present in the 1 lire of water as PO₄^3-

This means the weight of Phosphorous in PO₄^3- is 7.4 mg in 1 litre of wastewater.

7.4 mg = 0.0074 grams (1 mg = 0.001 grams)

The molar mass of PO₄^3- is 94.9714 g/mol

The molar mass of Phosphorous is 30.97 g/mol

This means 94.9714 grams of PO₄^3- contains 30.97 grams of Phosphorous

Or we can say

30.97 grams of phosphorous is present in 94.9714 grams of PO₄^3- then

0.0074 grams of phosphorous is present in

0.0074 x 94.9714 / 30.97 = 0.02269 grams = 22.69 mg

Thus 7.4 mg of phosphorous is contained 22.69 mg of PO₄^3-

Therefore the required concentration of PO₄^3- is 22.69 PO₄^3- mg/L

Likewise the concentration of P₂O₇^4- can be calculated from the molar masses

Given concentration of phosphorous as P₂O₇^4- is 4.5 mg = 0.0045 grams

The molar mass of P₂O₇^4- is 173.943

Molar mass of Phosphorous is 30.97 grams

Here two molecules of phosphorous is present in P₂O₇^4- and hence 2 x 30.97 grams = 61.94 grams of phosphorous is present in 173.943 grams of P₂O₇^4-

Thus 61.94 grams of phosphorous is present in 173.943 grams of P₂O₇^4- then

0.0045 grams of phosphorous is present in

0.0045 x 173.943 / 61.94 = 0.01264 grams = 12.64 mg of P₂O₇^4-

Thus the required concentration of P₂O₇^4- is 12.64 mg P₂O₇^4- / Litre