Write formulas for the following compounds:
(a) Mercury (II) chloride
(b) Nickel (II) sulphate
(c) Tin (IV) oxide
(d) Thallium (I) sulphate
(e) Iron (III) sulphate
(f) Chromium (III) oxide

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Formula of Given Compounds are:
(a) HgCl₂ 
(b) NiSO₄ 
(c) SnO₂
(d) TI₂SO₄
(e) Fe₂(SO₄)₃
(f) Cr₂O₃.

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In a redox reaction, one of the reacting species is to undergo an increase in Oxidation Number and other must undergo a decrease in Oxidation Number based upon this, the given reaction is redox reaction.



Here F₂ is oxidised to HOF and reduced to HF, therefore it is a redox reaction.

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(a) Although the mechanism of photosynthesis is very complex but broadly speaking it may be visualised to occur in two steps. In the first step, H₂O decomposes to give H₂ and O₂ in the presence of chlorophyll and H₂ thus produced reduces CO₂ to C₆H₁₂O₆ in the second step. During the second step, some H₂O molecules are also produced as


Therefore, it is more appropriate, to write the equation for photosynthesis as (3) because it emphasises that 12H₂O are used per molecule of carbohydrate formed and 6H₂O are produced during the process.

(b) The purpose of writing O₂ two times suggests that O₂ is being obtained from each of the two reactants.
                  
The path of reactions (a) and (b) can determine by using  in reaction (a) or by using 

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In sulphur dioxide (SO₂) and hydrogen peroxide (H₂O₂), the oxidation states of sulphur and oxygen are +4 and -1 respectively. As these can increase as well as decrease when the compounds take part in chemical reactions, hence they can act as oxidising as well as reducing agents. Example,

In hydrogen peroxide H₂O₂, the oxidation number of O is -1 and the range of the Oxidation number that O can have are from O to -2 can sometimes also attain the oxidation numbers +1 and +2. Hence, H₂O₂ can act  as an oxidising as well as reducing agent.

In ozone (O₃), the oxidation state of oxygen is zero while in nitric acid (HNO₃), the oxidation state of nitrogen is +5. As both can undergo decrease in oxidation state and not an increase in its value, hence they can act only as oxidants and no as reductants.

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Let the oxidation number of P = x
Writing the oxidation number of each atom at the top of its symbol.
                 +1 + 1 x - 2
                 Na  H₂PO₄    
Sum of the oxidation number of various atoms = 0
                                +1+2(+1)+x+4(-2) = 0
                                        x - 5 = 0
or                                      x = 5
  Oxidation number of P in NaH₂PO₄ = 5