**Procedure:**

1) Pipette 10 mL of cola sample into a 100 mL volumetric flask. Make up the solution to the mark using deionized water. From this solution, pipette 5 mL aliquots into three separate volumetric flasks. Set these samples aside for now.

2) Prepare a 20 μg/mL standard phosphate solution in a 250 mL volumetric flask. You are provided with a 1000 ppm standard.

3) To a series of 100 mL volumetric flasks, add 0.0, 8.0, 16.0, 24.0, 32.0, and 40.0 mL of the dilute phosphate solution using a 50 mL burette. These are your standards, so be careful with measurements. Do not make solutions up to the mark yet.

4) You now have nine flasks: 6 standards and 3 replicate samples. To each flask, successively add 10 mL of nitric acid solution, 10 mL of vanadate solution, and 10 mL of molybdate solution. Mix each solution well and dilute the volume with deionized water.

5) Let the solutions react for 20 minutes before measuring absorbance at 400 nm.

**Table 1:** Volumes of standard phosphate solution added, concentration of PO4^3- solution in μg/mL, measured and corrected absorbances of the samples.

a) Please calculate the total mass of phosphate (in μg) added to each flask.

**Explanation:**

Phosphate standard provided, C1 = 1000 ppm = 1000 mg/L = 1000 μg/mL (1 mg = 1000 μg and 1 L = 1000 mL).

The volume of 1000 μg/mL added = V1 = ?

The concentration of the stock solution, C2 = 20 μg/mL.

The volume of stock solution, V2 = 250 mL.

The dilution formula is C1 × V1 = C2 × V2.

V1 = (C2 × V2) / C1 = (20 μg/mL × 250 mL) / 1000 μg/mL = 5 mL.

The volume of 1000 ppm standard added to prepare 250 mL of 20 μg/mL was 5 mL.

**Standard Flasks:**

- **Standard Flask 1:**
C1 = 20 μg/mL, V1 = 0 mL.
Mass = 20 μg/mL × 0 mL = 0 μg.
C2 = (20 μg/mL × 0 mL) / 100 mL = 0 μg/mL.

- **Standard Flask 2:**
C1 = 20 μg/mL, V1 = 8 mL.
Mass = 20 μg/mL × 8 mL = 160 μg.
C2 = (20 μg/mL × 8 mL) / 100 mL = 1.6 μg/mL.

- **Standard Flask 3:**
C1 = 20 μg/mL, V1 = 16 mL.
Mass = 20 μg/mL × 16 mL = 320 μg.
C2 = (20 μg/mL × 16 mL) / 100 mL = 3.2 μg/mL.

- **Standard Flask 4:**
C1 = 20 μg/mL, V1 = 24 mL.
Mass = 20 μg/mL × 24 mL = 480 μg.
C2 = (20 μg/mL × 24 mL) / 100 mL = 4.8 μg/mL.

- **Standard Flask 5:**
C1 = 20 μg/mL, V1 = 32 mL.
Mass = 20 μg/mL × 32 mL = 640 μg.
C2 = (20 μg/mL × 32 mL) / 100 mL = 6.4 μg/mL.

- **Standard Flask 6:**
C1 = 20 μg/mL, V1 = 40 mL.
Mass = 20 μg/mL × 40 mL = 800 μg.
C2 = (20 μg/mL × 40 mL) / 100 mL = 8.0 μg/mL.

**The table becomes Absorbance vs. Phosphate.**

**The linear calibration curve is y = 0.0497x + 0.0321**, where y is Absorbance, and x is concentration.

**Samples:**

- **Sample R1:**
Corrected absorbance, y = 0.140.
Concentration, x = (y - 0.0321) / 0.0497 = (0.140 - 0.0321) / 0.0497 = 2.17 μg/mL.
Volume of sample R1 = 100 mL.
Mass of phosphate in sample R1 = 2.17 μg/mL × 100 mL = 217 μg.

- **Sample R2:**
Corrected absorbance, y = 0.138.
Concentration, x = (y - 0.0321) / 0.0497 = (0.138 - 0.0321) / 0.0497 = 2.13 μg/mL.
Volume of sample R1 = 100 mL.
Mass of phosphate in sample R1 = 2.13 μg/mL × 100 mL = 213 μg.

- **Sample R3:**
Corrected absorbance, y = 0.140.
Concentration, x = (y - 0.0321) / 0.0497 = (0.140 - 0.0321) / 0.0497 = 2.17 μg/mL.
Volume of sample R1 = 100 mL.
Mass of phosphate in sample R1 = 2.17 μg/mL × 100 mL = 217 μg.