Lab, Gravimetric Determination of Phosphorus in Plant Food · AP Chemistry

A fertilizer label reads three numbers, N–P–K. The middle number is the phosphorus content, reported as percent P₂O₅ by mass. In this lab you dissolve a measured mass of plant food, precipitate the phosphorus out as a single pure solid (MgNH₄PO₄·6H₂O), isolate and dry it, weigh it, and use stoichiometry to work backward to the % P₂O₅ in the product, then compare to the label.

Unit 4 (Chemical Reactions & Stoichiometry) · precipitation + mole ratios. Hands-on: ~one block to dissolve, precipitate, vacuum-filter, and alcohol-wash a small (1.0–1.5 g) sample; then just a 5-minute re-mass next class, after the alcohol-washed solid air-dries across the Period-8 rotation gap. · GRADED · TEAM formal lab report: Bennett's "Laboratory Investigation #1," written up and scored on the shared AP Chem lab rubric.

Background CED Unit 4

Analytical chemistry asks how much of a component sits inside a mixture. Gravimetric analysis answers that question with a balance, in four moves:

The gravimetric pattern: (1) chemically convert the target into a pure, low-solubility substance; (2) isolate and separate that substance from everything else; (3) measure the mass of the purified solid; (4) use the balanced equation's mole ratios to convert that mass back to the amount of the original component.

What "phosphorus" means on the label. Plant food reports phosphorus as diphosphorus pentoxide, P₂O₅the middle of the three N–P–K numbers. A 20–20–20 all-purpose fertilizer is 20% N, 20% P₂O₅, 20% K₂O by mass; a 15–24–18 reads 24% P₂O₅. You will find that middle number yourself.

Step 1, re-dissolve as phosphoric acid. P₂O₅ is the dehydration product of phosphoric acid, so dissolving plant food in water re-creates soluble H₃PO₄:

P₂O₅(s) + 3 H₂O(l) → 2 H₃PO₄(aq)

Step 2, precipitate the phosphorus. Add a magnesium sulfate solution plus concentrated ammonia. In the basic environment the phosphoric acid sits as hydrogen phosphate HPO₄²⁻, and a single low-solubility solid drops out, magnesium ammonium phosphate hexahydrate:

5 H₂O(l) + HPO₄²⁻(aq) + NH₄⁺(aq) + Mg²⁺(aq) + OH⁻(aq) → MgNH₄PO₄·6H₂O(s)

Two notes on the equation. (1) In base, H₃PO₄ loses protons to form HPO₄²⁻ in a 1:1 ratio, so one P stays one P all the way through. (2) Concentrated ammonia in water sits as NH₄⁺ + OH⁻; the ammonia both supplies the NH₄⁺ for the solid and makes the solution basic. Every P atom you started with ends up locked in one formula unit of MgNH₄PO₄·6H₂O.

Step 3 & 4, isolate, dry, weigh, and convert back. Filter off the white solid, rinse with isopropyl alcohol to pull out water and speed drying, dry to constant mass, and weigh. Because the mole ratio is 1 P per precipitate and 2 P per P₂O₅, the weighed mass of MgNH₄PO₄·6H₂O converts cleanly back to mass of P₂O₅, and then to % P₂O₅ in the plant food.

Safety read before you start

Concentrated & 3 M ammonia (NH₃), irritant, corrosive vapour. Ammonia gives off a sharp odour and can cause severe skin burns, eye damage, and respiratory irritation. All ammonia additions happen in a well-ventilated area / fume hood: never lean over the beaker and inhale. Do not wave it toward your face to "smell" it.

Isopropyl alcohol (75%), highly flammable. Keep it well away from any flame or heat source; there are no open flames in this lab. It is also an eye/skin irritant and is harmful if swallowed, wash hands after the rinse step.

PPE, every student, the whole period: splash goggles, lab coat or apron, nitrile gloves when handling ammonia or the dissolved solution. Tie back hair. No food or drink.

Spills & disposal: alert Dr. Mulinta to any ammonia spill and ventilate. Filtrate and rinses go in the labelled aqueous waste container at the bench, not down the sink unless Dr. Mulinta directs otherwise.

Materials & Equipment low-gear setup

Standard school chem glassware plus a digital balance and a gravity-filtration rig. Nothing specialized, the technique is the instrument here.

Apparatus (per student): digital balance (± 0.01 g) · 250 mL beakers (3) · 100 mL beakers (2) · glass stirring rods (2) · rubber policeman · filter paper · 10 mL graduated cylinder · 50 mL graduated cylinder · Büchner funnel + filter flask + vacuum line (vacuum filtration; ring stand + ring clamp + glass funnel only as the gravity fallback) · watch glass for drying · splash goggles.

Chemicals (pre-measured, do not exceed): 20 mL of 75% isopropyl alcohol · 30 mL of 10% MgSO₄·7H₂O solution · distilled water · 60 mL of 3 M NH₃(aq) · plant food containing P₂O₅ (record its N–P–K label). Volumes are scaled to the smaller 1.0–1.5 g sample, still a large excess of Mg²⁺ and NH₃, with less filtrate to pull through.

Mrs. Kathy (lab tech) will pre-set: the 30 mL of 10% MgSO₄·7H₂O and 20 mL of 75% isopropyl alcohol in labelled 100 mL beakers; the 60 mL of 3 M NH₃ in a labelled 250 mL beaker staged at the ventilated station; the plant food (with its label visible) and distilled-water wash bottles; filter paper and a Büchner funnel + filter flask on a working vacuum line at each bench (gravity funnel + ring stand as fallback); the digital balance zeroed. The concentrated-ammonia dilution to 3 M is done ahead of time by Dr. Mulinta / Mrs. Kathy in the hood.

Procedure precipitate today · re-mass next session

Efficiency, why this version is fast. Bennett's original run bogged down at the filtering and drying. Four changes fix that without changing the chemistry: (1) a smaller 1.0–1.5 g sample makes ~half the precipitate, so there is less solid to filter and dry; (2) you pre-mass the filter paper while the precipitate settles (parallel task, not a sequential wait); (3) vacuum (Büchner) filtration pulls the liquid through in seconds instead of dripping through a gravity funnel; (4) the isopropyl-alcohol wash displaces water from the wet hexahydrate so it dries far faster. Finally, because Period 8 does not meet every day, the precipitate air-dries to constant mass across the free rotation gap: so Session 2 is just a 5-minute re-mass, not a drying wait.

Session 1: Dissolve, precipitate, vacuum-filter, alcohol-wash (one block)

Obtain your equipment and goggles. Collect your pre-set chemicals: the 3 M NH₃ in a 250 mL beaker (keep it at the ventilated station), and the MgSO₄·7H₂O and isopropyl alcohol in the two 100 mL beakers. Set up the Büchner funnel + filter flask on the vacuum line now so it is ready when the precipitate is (use the gravity funnel only if no vacuum line is available).
Zero a clean 250 mL beaker on the balance. Measure out 1.0–1.5 g of plant food into it and record the exact mass of plant food (± 0.01 g). Record the fertilizer's N–P–K label. (The smaller sample is deliberate, less precipitate to filter and dry.)
Add about 40 mL of distilled water and stir with a glass rod until the plant food dissolves as fully as it will.
If undissolved solids remain (fillers/coatings that are not phosphorus), filter them out: set the funnel over a clean 250 mL beaker, filter, and keep the clear filtrate: that liquid holds the dissolved phosphorus. Discard the insoluble residue.
To the clear solution, add the full 30 mL of 10% MgSO₄·7H₂O and stir.
At the ventilated station, slowly add the 60 mL of 3 M NH₃ with constant stirring. A white precipitate of MgNH₄PO₄·6H₂O forms. Stir well, then let the beaker stand ~5 min so precipitation goes to completion (the solid settles and the liquid above clears).
While it settles (parallel task): mass a piece of dry filter paper on the balance and record it as mass of filter paper. Label it with your initials. Seat it in the Büchner funnel and start the vacuum.
Vacuum-filter the precipitate through your pre-massed paper. Use the rubber policeman and small rinses of the 75% isopropyl alcohol (20 mL total) to transfer every bit of solid from the beaker onto the paper, and to wash the precipitate, the alcohol displaces water and speeds drying. Keep the vacuum on a few extra seconds to pull most of the liquid out of the cake. (Gravity funnel only as a fallback, it is much slower.)
Open the filter paper onto a labelled watch glass or your assigned drying spot and leave it to air-dry to constant mass. The alcohol-washed solid dries quickly, but do not weigh it this block: let it finish across the rotation gap to the next class. Optional speed-dry: a brief rest over a warm-water bath or in a low oven (≤ ~40 °C) reaches constant mass faster. Caution, keep it gentle: do NOT overheat MgNH₄PO₄·6H₂O, or it loses its waters of hydration and the mass (and your % P₂O₅) reads low.

Session 2: Quick re-mass (~5 min)

Retrieve your dried filter paper + precipitate. It should look dry and powdery with no damp patches; if it still looks wet, give it more time.
Mass the filter paper + dry precipitate together and record it.
Subtract the filter-paper mass to get the mass of MgNH₄PO₄·6H₂O precipitate. (For full constant-mass technique you would re-dry and re-weigh until two masses agree within ± 0.01 g, do this if time allows.)

Analysis Questions show all work · units · sig figs

Answer in your typed report. Show every formula and substitution, full credit needs the math, not just the answer. Molar masses: MgNH₄PO₄·6H₂O = 245.41 g/mol, P₂O₅ = 141.94 g/mol.

Track the phosphorus. Write the two reactions (dissolution of P₂O₅; precipitation of MgNH₄PO₄·6H₂O). State the mole ratio of P to precipitate, and of P to P₂O₅. Explain in one sentence why every phosphorus atom from the plant food ends up in the weighed solid.
Moles of precipitate. Use your measured precipitate mass and its molar mass (245.41 g/mol) to find moles of MgNH₄PO₄·6H₂O.
Moles of P, then moles of P₂O₅. There is 1 P per formula unit of precipitate, so moles of P = moles of precipitate. There are 2 P per P₂O₅, so moles of P₂O₅ = (moles of P) ÷ 2.
Mass of P₂O₅. Convert moles of P₂O₅ to grams (× 141.94 g/mol).
Percent P₂O₅. Calculate % P₂O₅ by mass in the plant food: (mass P₂O₅ ÷ mass plant food) × 100%.
Compare to the label. Does your % P₂O₅ match the middle N–P–K number on the package? Compute a percent error against the label value, and comment on whether your result is reasonable.
Why these reagents/steps? (a) Why add an excess of ammonia rather than a stoichiometric amount? (b) Why rinse the precipitate with isopropyl alcohol instead of more water before drying?
Error analysis. Identify one source of error that would make your % P₂O₅ read too high and one that would make it read too low. For each, trace the effect through the mass → moles → % chain. (Hint: think about precipitate that is still wet, solid lost in transfer, or co-precipitated impurities.)

Fertilizer N–P–K label (and brand)	__________
Mass of plant food (± 0.01 g)	__________ g
Mass of dry filter paper	__________ g
Mass of filter paper + dry precipitate	__________ g
Mass of MgNH₄PO₄·6H₂O precipitate (by subtraction)	__________ g

Lab · Phosphorus in Plant Food (Gravimetric)