O LevelChemistryCambridgeExam Questions10. Chemistry of the Environment10.2 Fertilisers

Fertilisers (Cambridge O Level Chemistry)

Exam Questions

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10.2 Fertilisers

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10.2 Fertilisers

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1a1 mark

Fertilisers are concentrated forms of plant nutrients

Which of the following ions do plants need for growth?


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Cu2+  
   
PO43–  
   
SO42-  
   
OH–   

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1b1 mark

Potassium nitrate is used as a fertiliser. The potassium ion has the formula K+ and the nitrate ion has the formula NO3–. 

Give the formula for potassium nitrate.

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1c1 mark

What property must fertilisers have in order for plants to take up the ions through their roots.

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1a3 marks

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

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

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1b1 mark

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.

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1c
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Using your answer to part (b) calculate the percentage by mass of nitrogen in calcium nitrate.

(Ar: Ca = 40, N = 14, O = 16) 

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1d2 marks

Explain why salts from fertilisers end up in bodies in groundwater.

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1a3 marks

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 ..................................................

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1b4 marks

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.

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1c1 mark

State the essential plant nutrient not supplied by ammonium phosphate.

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1d3 marks

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, PO43–, from the rock phosphate is changed into the dihydrogenphosphate ion, H2PO4–.
 
PO43– + 2H2SO4→ H2PO4–+ 2HSO4– 
 
What type of reagent is the phosphate ion? Give a reason for your choice.
 
[2]

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2a4 marks

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]

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2b
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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]

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3a4 marks

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]

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3b
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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.

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3c2 marks

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]

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3d
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Phosphates can be found in reservoirs from which drinking water is obtained. A maximum of 0.025 mg / 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.

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