Fertilisers (Cambridge (CIE) O Level Chemistry): Theory Questions

Plants require three main elements for healthy growth which can be provided by fertilisers.

Give the formulae of three ions used to make fertilisers which provide the three required elements.

Calcium nitrate is used as a fertiliser to help prevent blossom end rot in tomatoes which causes browning of the bottom of the fruit. It provides both calcium and nitrogen.

Write the formula for calcium nitrate.

Using your answer to part (b) calculate the percentage by mass of nitrogen in calcium nitrate.

(A_r: Ca = 40, N = 14, O = 16) 

Explain why salts from fertilisers end up in groundwater.

A garden centre sells three fertilisers.

Fertiliser A is ammonium nitrate. Fertiliser B is potassium sulfate. Fertiliser C is ammonium phosphate.

State which fertiliser is an ammonium salt that also supplies phosphorus.

A field is short of nitrogen and potassium only.

State which two fertilisers the farmer should use to supply only the elements the field is short of.

Give a reason for each choice.

Explain why none of fertilisers A, B and C on its own is described as an NPK fertiliser.

State the benefit to a crop of using a fertiliser that supplies all three elements.

A fertiliser bag is labelled:

Contains ammonium nitrate and potassium chloride

State which of the three elements supplied by an NPK fertiliser is supplied by ammonium nitrate.

State which of the three elements supplied by an NPK fertiliser is supplied by potassium chloride.

A gardener needs a fertiliser for a plant that requires all three of the elements supplied by an NPK fertiliser.

Explain whether the fertiliser in this bag is suitable for the gardener.

Name a salt that the manufacturer could add to the bag so that the fertiliser could be described as an NPK fertiliser.

A farmer tests the soil in three fields. The table shows which elements each soil lacks.

The farmer can buy these four fertilisers.

ammonium sulfate potassium phosphate ammonium phosphate potassium nitrate

State which single fertiliser supplies the element that field 1 lacks and no other of the three elements.

State which single fertiliser is suitable for field 2.

Explain why none of the four fertilisers on its own is suitable for field 3.

The farmer decides not to use ammonium sulfate or potassium phosphate on field 3.

Explain how the two remaining fertilisers, used together, supply field 3 with all three elements.

A fertiliser is made by mixing ammonium sulfate and potassium phosphate.

Explain how this mixture supplies all three of the elements in an NPK fertiliser.

Potassium nitrate could be used in the mixture in place of potassium phosphate.

Explain why the mixture would then no longer supply all three elements.

A gardener spreads the original mixture on a vegetable crop.

State the effect that the mixture has on the crop.

Fertilisers are used to promote plant growth.

Two fertilisers are ammonium phosphate, (NH₄)₃PO₄, and calcium dihydrogenphosphate,Ca(H₂PO₄)₂.

Describe a test to distinguish between these two fertilisers.

test ..................................................

result ..................................................

Many fertilisers are manufactured from ammonia.

Describe how ammonia is made in the Haber process. Give the essential conditions and an equation for the process.

State the essential plant nutrient not supplied by ammonium phosphate.

The soluble compound, calcium dihydrogenphosphate is made by heating the insoluble mineral rock phosphate, Ca₃(PO₄)₂, with sulfuric acid.  

i) Why would rock phosphate not be effective as a fertiliser?

[1]

 ii) The phosphate ion, PO₄^3–, from the rock phosphate is changed into the dihydrogenphosphate ion, H₂PO₄^–. 

PO₄^3– + 2H₂SO₄→ H₂PO₄^–+ 2HSO₄^–  

What type of reagent is the phosphate ion? Give a reason for your choice. 

[2]

The Atacama desert in Chile has deposits of the salt sodium nitrate.

Very large amounts of this salt were exported to Europe for use as a fertiliser. After the introduction of the Haber process in 1913, this trade rapidly diminished. 

i) Explain why the introduction of the Haber process reduced the demand for sodium nitrate. 

[2]

ii) Suggest why surface deposits of sodium nitrate only occur in areas with very low rainfall such as desert areas. 

[1]

iii) The desert has smaller surface deposits of potassium nitrate. Suggest why potassium nitrate is a better fertiliser than the sodium salt. 

[1]

The equation for the decomposition of copper(II) nitrate is given below.

2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂

Copper(II) nitrate forms a series of hydrates with the formula Cu(NO₃)₂.xH₂O.

All these hydrates decompose to form copper(II) oxide.

1 mole of Cu(NO₃)₂.xH₂O forms 1 mole of CuO.  

i) What is meant by 1 mole of a substance? 

[1]

ii) 7.26 g of a hydrate, Cu(NO₃)₂.xH₂O, formed 2.4 g copper(II) oxide.

number of moles of CuO formed = ......................
number of moles of Cu(NO₃)₂.xH₂O in 7.26 g = ......................
mass of 1 mole of Cu(NO₃)₂.xH₂O = ...................... g
mass of 1 mole of Cu(NO₃)₂ is 188 g
the value of x in this hydrate = ...................... 

[4]

Groundwater can contain fertilisers washed from fields. An example of a fertiliser that can be found is ammonium nitrate. 

i) Write a balanced equation, including state symbols, to show how ammonium nitrate can be made by a student via a neutralisation reaction.  

[2]

ii) Describe how the student could perform a chemical test for the presence of nitrate ions. 

[2]

Drinking water extracted from rivers in agricultural areas often contain nitrates. The first legal standard for nitrate in drinking water was set in 1980. The current UK drinking water standard is 50 mg / dm³ to safeguard against cases of methaemoglobinaemia or blue baby syndrome. 

Calculate the maximum mass in grams of nitrate that would be safe in a 240 cm³ glass of water. Give your answer to two significant figures.

An alternative fertiliser is ammonium phosphate.   

i) Give the formula for ammonium phosphate. 

[1]

 ii) Which ion that plants need is missing from this fertiliser? 

[1]

Phosphates can be found in reservoirs from which drinking water is obtained. A maximum of 0.025 g / dm³ of phosphate ions is allowed by water companies.

A sample of reservoir water has a concentration of 0.00014 mol / dm³.

Calculate the percentage of the maximum mass allowed of phosphate ions in the sample of reservoir water.