# Chemistry Practical Questions on Nitrogen and Nitrates

A factory uses 63.0 kg of 68% pure nitric (V) acid per day to produce an ammonium fertilizer for an agricultural county. If the density of the acid is 1.42 gcm-3, calculate:

(i) the concentration of the acid used in moles per litre.

Molar mass HNO3               = 63

Method 1

Moles of HNO3 in 1cm3 = Mass in 1cm3 1.42         =>    1.42       = 0.0225 moles

Molar mass HNO3                             63

Molarity = Moles x 1000=>0.0225 moles x10000 = 22.5molesdm-3/M

1 cm3

100%       =      22.5molesdm-3/M

68%          =              68 x 22.5                         = 15.3M/ molesdm-3

100

Method 2

Moles of HNO3 in 1000cm3 = Mass in 1000cm3 =>1.42 x1000

Molar mass HNO3                   63

=22.5397 molesdm-3/M

100%       =      22.5397 molesdm-3/M

68%          =              68 x 22.5397                   = 15.327 molesdm-3

100

(ii) the volume of ammonia gas at r.t.p used. (H=1.0, N=14.0, O=16.0, one mole of gas = 24 dm-3 at r.t.p)

Chemical equation

HNO3 (as)  + NH3 (g)  ->      NH4NO3 (as)

Mole ratio HNO3 (as):  NH3 (g) = 1: 1

1 mole HNO3 (as) -> 24dm3 NH3 (g)

15.327 mole HNO3 (as) ->15.327 mole x 24 dm3 = 367.848dm3

1dm3

(iii) the number of crops which can be applied the fertilizer if each crop requires 4.0g.

HNO3 (aq)  + NH3 (g)     ->   NH4NO3 (aq)

Molar mass NH4NO3 =80 g

Mole ratio HNO3: NH4NO3   = 1: 1

Mass of HNO3 in 63.0 kg = 68% x 63 =42.84kg

1 mole HNO3 (aq) =63g    -> 80g NH4NO3

(42.84×1000) gHNO3 (aq) -> (42.84×1000) g x 80

63

= 54400g

Mass of fertilizer =        54400g       = 13600 crops

Mass per crop                   4.0

E. NITRATE (V) NO3 and NITRATE (III) NO2 Salts

Nitrate (V) /NO3 and Nitrate (III) /NO2 are salts derived from Nitric (V)/HNO3 and Nitric (III)/HNO2 acids. Both HNO3 and HNO2 are monobasic acids with only one ignitable hydrogen in a molecule.

Only KNO2, NaNO2 and NH4NO2 exist. All metallic nitrate (V) salts exist.

All Nitrate (V) /NO3 and Nitrate (III) /NO2 are soluble/dissolve in water.

(a)Effect of heat on Nitrate (V) /NO3 and Nitrate (III) /NO2 salts (Test for presence of Nitrate (V) /NO3 ions in solid state)

1. All Nitrate (III) /NO2 salts are not affected by gentle or strong heating except ammonium nitrate (III) NH4NO2.

Ammonium nitrate (III) NH4NO2 is a colourless solid that decompose to form Nitrogen gas and water.

Chemical equation

NH4NO2 (s)         ->   H2O (l) + N2 (g)

This reaction is used to prepare small amounts of Nitrogen in a school laboratory.

2. All Nitrate (V) /NO3 salts decompose on strong heating:

Experiment

Put ½ spatula full of sodium nitrate (V) into a test tube. Place moist blue/red litmus papers on the mouth of the test tube.  Heat strongly when test tube is slanted.

Test the gases produced using glowing splint

Caution (i) Wear safety gas mask and hand gloves

(ii)Lead (II) nitrate (V) decomposes to Lead (II) oxide that reacts and fuses with the test tube permanently.

Repeat with potassium nitrate(V), copper(II) nitrate(V), Lead(II)nitrate(V), silver nitrate(V), Zinc nitrate(V), Magnesium nitrate(V) and Ammonium nitrate(V).

Observations

Cracking sound

Brown fumes/gas produced except in potassium nitrate (V) and Sodium nitrate (V)

Glowing splint relights/rekindles but feebly in Ammonium nitrate(V).

Black solid residue with copper(II) nitrate(V)

White residue/solid with sodium nitrate(V), potassium nitrate(V),silver nitrate(V), Magnesium nitrate(V)

Yellow residue/solid when hot but white on cooling with Zinc nitrate(V)

Brown residue/solid when hot but yellow on cooling with Lead(II)nitrate(V)

Explanation

1. Potassium nitrate(V) and Sodium nitrate(V) decomposes on strong heating  to form potassium nitrate(III) and Sodium nitrate(III) producing Oxygen gas. Oxygen gas relights/rekindles a glowing splint.

Chemical equation.

2KNO3(s)   ->      2KNO2(s)  +     O2 (g)

2NaNO3(s) ->      2NaNO2(s) +     O2 (g)

2.Heavy metal nitrate(V)salts decomposes to form the oxide, brown nitrogen (IV) oxide and  Oxygen gas.

Copper(II)oxide is black.Zinc oxide is yellow when hot and white when cool/cold. Lead(II)oxide is yellow when cold/cool and brown when hot/heated.

Hydrated copper(II)nitrate is blue. On heating it melts and dissolves in its water of crystallization to form a green solution. When all the water of crystallization has evaporated,the nitrate(V)salt decomposes to black Copper(II)oxide and a mixture of brown nitrogen(IV)oxide gas and colourless Oxygen gas.

Chemical equation

2Cu(NO3)2(s)        ->     2CuO (s)       +    4NO2(g)    +    O2(s)

2Ca(NO3)2(s)        ->     2CaO (s)       +    4NO2(g)    +    O2(s)

2Zn(NO3)2(s)        ->     2ZnO (s)       +    4NO2(g)    +    O2(s)

2Mg(NO3)2(s)        ->     2MgO (s)     +    4NO2(g)    +    O2(s)

2Pb(NO3)2(s)          ->     2PbO (s)      +    4NO2(g)    +    O2(s)

2Fe(NO3)2(s)          ->     2FeO (s)      +    4NO2(g)    +    O2(s)

Silver nitrate(V)and Mercury(II)nitrate decomposes to the corresponding metal and a mixture of brown nitrogen(IV)oxide gas and colourless Oxygen gas.

Chemical equation

2AgNO3 (s)          ->     2Ag (s)      +    2NO2(g)    +    O2(s)

Hg(NO3)2(s)        ->     Hg(l)          +    2NO2(g)    +    O2(s)

The production/evolution of brown fumes of Nitrogen(IV)oxide gas on heating a salt is a confirmatory test for presence of NO3 ions of heavy metals

(b)Brown ring test (Test for presence of Nitrate(V) /NO3  ions in aqueous/ solution state)

Experiment

Place 5cm3 of Potassium nitrate(V)solution onto a clean test tube. Add 8 drops of freshly prepared Iron(II)sulphate(VI)solution.  Swirl/ shake.

Using a test tube holder  to firmly slant and hold the test tube, carefully add 5cm3 of Concentrated sulphuric (VI) acid down along the side of test tube.Do not shake the test tube contents.

Caution: Concentrated sulphuric (VI) acid is highly corrosive.

Observation.

Both Potassium nitrate(V)and freshly prepared Iron(II)sulphate (VI)do not form layers

On adding Concentrated Sulphuric(VI)acid,two layers are formed.

A brown ring is formed between the layers.

Explanation

All nitrate(V)salts are soluble. They form a miscible mixture when added freshly prepared Iron(II)sulphate(VI)solution. Concentrated sulphuric(VI)acid is denser than the miscible mixture thus settle at the bottom.

At the junction of the layers, the acid reacts with nitrate(V)salts to form Nitric(V)acid/HNO3. Nitric(V)acid/HNO3 is   reduced to Nitrogen (II)oxide by the Iron(II)sulphate(VI) salt to form the complex compound Nitroso-iron(II)sulphate(VI)/FeSO4.NO . Nitroso-iron(II)sulphate(VI) is brown in colour.It forms a thin layer at the junction between concentrated sulphuric (VI)acid and the miscible mixture of freshly prepared Iron(II) sulphate(VI) and the nitrate(V)salts as a brown ring.

Chemical equation

FeSO4(aq)   +     NO(g)    ->    FeSO4.NO(aq)

(Nitroso-iron(II)sulphate(VI)complex)

The brown ring disappear if shaken because concentrated sulphuric (VI)acid mixes with the aqueous solution generating a lot of heat which decomposes Nitroso-iron(II)sulphate(VI)/FeSO4.NO to iron(II)sulphate(VI) and Nitrogen(II)oxide.

Chemical equation

FeSO4.NO(aq)                 ->FeSO4(aq)           +     NO(g)    ->

Iron(II)sulphate(VI) solution is easily/readily oxidized to iron(III)sulphate(VI) on exposure to air/oxygen. The brown ring test thus require freshly prepared Iron(II) sulphate(VI) solution

(c)Devardas alloy  test (Test for presence of Nitrate(V) /NO3  ions in aqueous/ solution state)

Experiment

Place 5cm3 of Potassium nitrate(V)solution onto a clean test tube. Add 5 drops of sodium hydroxide solution.  Swirl/ shake. Add a piece of aluminium foil to the mixture.Heat.Test any gases produced using both blue and red litmus papers.

Observation.                                                                                     Inference

Effervescence/bubbles/fizzing

colourless gas that has a pungent smell of urine              NO3

Blue limus paper remain blue

Red litmus paper turn red.

Explanation

The Devardas alloy test for NO3 ions in solution was developed by the Italian scientist Artulo Devarda(1859-1944)

When a NO3salt is added sodium hydroxide and aluminium foil, effervescence of ammonia gas is a confirmatory test for NO3ions.