Difficulty: Intermediate | Prerequisites: Comfortable with mole calculations and percentage composition. Review the chemical formula determination lab notes.
This lab brings together gravimetric analysis, percentage composition, and the chemistry of fertilisers. The numbers on a fertiliser bag (like 20-5-10) look simple but are deliberately misleading: they do not represent the actual elemental percentages of nitrogen, phosphorus, and potassium. Understanding why requires you to know how those percentages are conventionally expressed as oxides (P₂O₅ and K₂O) rather than as pure elements. This lab teaches you to convert between the label's convention and actual elemental composition, and to verify it experimentally by precipitating phosphorus out of solution.
Fertiliser labels use the N-P-K system (e.g. 20-5-10), but the P and K numbers refer to oxide equivalents (P₂O₅ and K₂O), not pure elements. To find the actual percentage of phosphorus, you need to calculate the mass fraction of P within P₂O₅. In the lab, you isolate phosphorus using gravimetric analysis by making the solution basic and precipitating the phosphorus compound, then weighing it.
Gravimetric analysis
A quantitative technique that separates a target substance from everything else by precipitating it out of solution, filtering, drying, and weighing. Think of it as isolating one ingredient from a mixture by turning it into a solid you can collect and weigh.
N-P-K ratio
The three numbers on a fertiliser label, representing nitrogen (N), phosphorus (as P₂O₅), and potassium (as K₂O) by mass percentage. For example, 20-5-10 means 20% N, 5% P₂O₅, 5% is not 5% pure phosphorus.
Diphosphorus pentoxide (P₂O₅)
The oxide form used to express phosphorus content on fertiliser labels. To get the actual percentage of elemental phosphorus, you must calculate the mass fraction of P within P₂O₅.
Potassium oxide (K₂O)
The oxide form used to express potassium content on fertiliser labels, following the same convention as phosphorus.
Precipitate
An insoluble solid that forms when two solutions are mixed or when conditions (like pH) change. In this lab, making the solution basic causes the phosphorus compound to precipitate.
Basicity test
Red litmus paper turns blue when dipped in a basic solution. This confirms the solution is basic enough for the phosphorus to precipitate.
Litmus paper
A pH indicator strip. Red litmus turns blue in base; blue litmus turns red in acid. In this lab, you check that red litmus turns blue before proceeding.
A fertiliser labelled 20-5-10 contains 20% nitrogen, 5% phosphorus (expressed as P₂O₅), and 10% potassium (expressed as K₂O) by mass.
The P and K numbers are misleading because they report the oxide equivalent, not the pure element.
This convention dates back to 19th-century agricultural chemistry and persists today.
To find the true percentage of phosphorus:
Calculate the mass of phosphorus in one formula unit of P₂O₅.
Divide by the molar mass of P₂O₅.
Multiply by the N-P-K percentage for P.
The result is the absolute percentage of elemental phosphorus in the fertiliser.
Dissolve the fertiliser sample in solution.
Make the solution basic (add base until red litmus paper turns blue).
The phosphorus compound precipitates out as an insoluble solid.
Filter, dry, and weigh the precipitate.
From the mass of the precipitate, calculate back to the mass of phosphorus.
Use the red litmus paper test: if it turns blue, the solution is sufficiently basic.
If the solution is not basic enough, the phosphorus will not fully precipitate, and your result will be too low.
Percentage of elemental phosphorus from a fertiliser label:
\% P_{\text{actual}} = \frac{\text{mass of P in P}_2\text{O}_5}{\text{molar mass of P}_2\text{O}_5} \times \text{N-P-K label } \%Worked example for P₂O₅:
Molar mass of P₂O₅ = 2(30.97) + 5(16.00) = 141.94 g/mol
Mass of P in P₂O₅ = 2(30.97) = 61.94 g/mol
Mass fraction of P = 61.94 / 141.94 = 0.4364
For a fertiliser labelled 20-5-10: actual % P = 0.4364 x 5% = 2.18%
Every bag of fertiliser at a garden centre carries the N-P-K numbers. Farmers and agronomists use the actual elemental percentages (not the label numbers) to calculate how much fertiliser to apply per hectare. Over-application of phosphorus is a major cause of algal blooms in lakes and rivers, so getting the number right matters environmentally as well as economically.
Gravimetric analysis is also used in water treatment plants to measure contaminant levels and in mining to assay ore samples.
Students often assume the "5" in a 20-5-10 label means the fertiliser is 5% phosphorus by mass. It means 5% P₂O₅, which works out to a much lower actual percentage of elemental phosphorus (about 2.18%).
Forgetting to make the solution sufficiently basic is a common lab error. If the litmus paper does not turn blue, the phosphorus has not fully precipitated and your mass will be too low.
Students sometimes confuse red litmus and blue litmus. In this lab, you use red litmus paper and watch for it to turn blue (indicating a basic solution).
⚠️ Be able to convert from the N-P-K label percentage to actual elemental percentage for both phosphorus and potassium.
⚠️ Know that P is reported as P₂O₅ and K is reported as K₂O on fertiliser labels.
⚠️ Understand the purpose of making the solution basic: to precipitate the phosphorus compound.
⚠️ Remember the litmus test: red litmus paper turns blue in a basic solution.
True or false: A fertiliser labelled 20-5-10 contains 5% elemental phosphorus.
False. It contains 5% P₂O₅, which corresponds to about 2.18% elemental phosphorus.
Fill in the blank: Phosphorus on a fertiliser label is expressed as ______.
P₂O₅ (diphosphorus pentoxide).
True or false: Blue litmus paper turning red confirms a basic solution.
False. Red litmus paper turning blue confirms a basic solution.
Fill in the blank: Gravimetric analysis determines the amount of a substance by ______ it.
Precipitating, filtering, drying, and weighing.
True or false: K₂O on a fertiliser label represents pure potassium content.
False. It represents potassium expressed as potassium oxide.
Q: A fertiliser is labelled 10-10-10. What is the actual percentage of elemental phosphorus?
A: Mass fraction of P in P₂O₅ = 61.94 / 141.94 = 0.4364. Actual % P = 0.4364 x 10% = 4.36%.
Q: Why are fertiliser labels considered misleading?
A: Because the P and K values report oxide equivalents (P₂O₅ and K₂O), not the actual mass of elemental phosphorus and potassium. The real elemental content is lower than the label suggests.
Q: What must you do to the solution before the phosphorus compound will precipitate?
A: Make it basic. You confirm basicity by testing with red litmus paper, which should turn blue.
Q: A student performs the gravimetric analysis but forgets to check the litmus paper. The solution was still acidic. How will this affect the result?
A: The phosphorus will not fully precipitate, so the mass of the collected solid will be too low, and the calculated percentage of phosphorus will be an underestimate.
Q: Calculate the mass fraction of potassium in K₂O.
A: Molar mass of K₂O = 2(39.10) + 16.00 = 94.20 g/mol. Mass of K = 2(39.10) = 78.20 g/mol. Mass fraction = 78.20 / 94.20 = 0.8302, or about 83%.
The mole-to-mass conversions here are the same ones you used in the Avogadro's number and chemical formula labs. If you struggled with those, revisit the moles formula before tackling this topic.
Gravimetric analysis is conceptually similar to chromatography: both separate a mixture to identify or quantify one component. Chromatography works by differential migration; gravimetric analysis works by selective precipitation.
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