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A soluble fertiliser contains phosphorus in the form of phosphate ions (PO₄³⁻) - VCE - SSCE Chemistry - Question 3 - 2006 - Paper 1
Question 3
A soluble fertiliser contains phosphorus in the form of phosphate ions (PO₄³⁻). To determine the PO₄³⁻ content by gravimetric analysis, 5.97 g of the fertiliser powd... show full transcript
Worked Solution & Example Answer:A soluble fertiliser contains phosphorus in the form of phosphate ions (PO₄³⁻) - VCE - SSCE Chemistry - Question 3 - 2006 - Paper 1
Step 1
Calculate the amount, in mole, of Mg₂P₂O₇.
Answer
To find the amount of Mg₂P₂O₇ in moles, we use the molar mass of Mg₂P₂O₇. The molar mass is calculated as follows:
- Molar mass of Mg₂P₂O₇ = 2(24.31 g/mol) + 2(30.97 g/mol) + 7(16.00 g/mol)
- Molar mass = 48.62 g + 61.94 g + 112.00 g = 222.56 g/mol
Now, using the mass of the precipitate:
- Moles of Mg₂P₂O₇ = mass / molar mass = 0.352 g / 222.56 g/mol = 0.00158 mol.
Step 2
Step 3
Step 4
Calculate the percentage of phosphate ions (PO₄³⁻) by mass in the fertiliser.
Answer
To find the mass of PO₄³⁻, we calculate:
- Mass of P = moles of P × molar mass of P = 0.0395 mol × 30.97 g/mol = 1.224 g.
Next, the mass of PO₄³⁻ can be derived:
- Mass of PO₄³⁻ = 1 P + 4 O = 30.97 g + 4(16.00 g) = 62.97 g.
Now, to find the mass percentage:
- Percentage of PO₄³⁻ = (mass of PO₄³⁻ in fertiliser / mass of fertiliser) × 100 = (1.224 g / 5.97 g) × 100 = 20.5%.
Step 5
Several actions which could occur during this analytical procedure are listed below (A–D). For each action, indicate the likely effect on the calculated percentage of phosphate ions in the fertiliser by placing a tick in the appropriate box.
Answer
A. The MgNH₄PO₄ precipitate was not washed with water.
- Calculated result would be too high.
B. The conical flask had been previously washed with water but not dried. - Calculated result would be too low.
C. A 25.00 mL pipette was unknowingly used instead of a 20.00 mL pipette. - Calculated result would be too low.
D. The mass of the fertiliser was recorded incorrectly. The recorded mass was 0.2 g. - Calculated result would be too high.
Step 6
In the case of action B above, explain your reasoning for the answer that you have given.
Answer
If the conical flask was washed with water but not dried, residual water could dilute the solution, leading to a lower concentration of phosphate ions when measured. This would result in a calculated lower content of phosphate ions in the fertiliser than actually present.