Question

Pure solid NaH2PO4 is dissolved in distilled water, making 100.00 ml of solution. 10.00ml of this solution is diluted to 100.00 ml to prepare the original phosphate standard solution. Three working standard solutions are made from this by pipetting 0.8ml, 1.5ml and 3.0 ml of the original standard solution into 100.00 ml volumetric flasks. Acid and molybdate reagent are added and the solutions are diluted to 100.00 ml.

You may assume that all these absorbance measurements have already been corrected for any blank absorbance. The absorbance of each is measured in the spectrophotometer.

Mass of NaH2PO4 (mg) 512.2

Absorbance

Standard 1 (0.8 ml) 0.1922

Standard 2 (1.5 ml) 0.3604

Standard 3 (3.0 ml) 0.7209

Calculate the following:

Concentration of original phosphate standard (mM)

Concentration of Standard 1(mM)

Concentration of Standard 2(mM)

Concentration of Standard 3(mM)

Slope of calibration line

Answer 1

Ans. # Step 1: Stock solution:

            Moles of NaH₂PO₄ = Mass / MW = 0.5122 g / 119.97701 g mol^-1 = 0.00427 mol

Now, [Stock soln.] = Moles /Vol. in liters = 0.00427 mol / 0.10 L = 0.0427 M

#Step 2: Original phosphate standard solution:

  Using C1V1 (stock soln.) = C2V2 (standard soln.)

            Or, C2 = (0.0427 M x 10.0 mL) / 100.0 mL = 0.00427 M

# Step 3: Working standards (WS)-

            # WS 1: Using C1V1 (standard soln.) = C2V2 (working soln.)

                        C2 = (0.00427 M x 0.8 mL) / 100 mL = 0.00003416 M = 0.03416 mM

            # WS 2: Using C1V1 (standard soln.) = C2V2 (working soln.)

                        C2 = (0.00427 M x 1.5 mL) / 100 mL = 0.00006405 M = 0.06405 mM

            # WS 3: Using C1V1 (standard soln.) = C2V2 (working soln.)

                        C2 = (0.00427 M x 3 mL) / 100 mL = 0.0001281 M = 0.1281 mM

# Slope of Calibration Curve: 5.6281