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Chemistry of the Environment: Question 2

Syllabus 10.2

Multiple choice Core 1 mark

A hydroponics technician preparing a nutrient feed for a strawberry polytunnel dissolves three compounds together to make a single combined feed:

  • ammonium nitrate
  • calcium phosphate
  • ammonium chloride

Which element, needed to make this an NPK fertiliser, is missing from the combined feed?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall what an NPK fertiliser must supply

An NPK fertiliser must supply three elements for improved plant growth: nitrogen (N), phosphorus (P) and potassium (K).

Step 2: Work out which elements each compound in the feed supplies

  • Ammonium nitrate, NH4NO3\text{NH}_4\text{NO}_3: both parts of this compound (ammonium and nitrate) contain nitrogen, so this supplies nitrogen only.
  • Calcium phosphate, Ca3(PO4)2\text{Ca}_3(\text{PO}_4)_2: this supplies phosphorus (as well as calcium, which is not one of the NPK elements).
  • Ammonium chloride, NH4Cl\text{NH}_4\text{Cl}: this supplies nitrogen only.

Step 3: Identify which of nitrogen, phosphorus and potassium is missing

Combining all three compounds gives:

  • Nitrogen. Supplied (twice over, by both ammonium compounds)
  • Phosphorus. Supplied (by calcium phosphate)
  • Potassium. not supplied by any of the three compounds

So the combined feed is missing potassium.

Why the other options are wrong

  • A (Nitrogen): this element is supplied, by both ammonium nitrate and ammonium chloride.
  • B (Phosphorus): this element is supplied, by calcium phosphate.
  • D (Oxygen): oxygen is not one of the three elements (N, P, K) that define an NPK fertiliser.

Final answers

  • The missing element is potassium\boxed{\text{potassium}}, option C.