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Equilibrate the equation by oxidation numbers (oxidation states o.n.): $..P$ $+$ $..Cl_2$ $\longrightarrow$ $PCl_5 $
$2P$ $+$ $5Cl_2$ $\longrightarrow$ $2PCl_5 $
Write down the oxidation numbers of all atoms concerned?
o.n. ($P$) in $P$ = 0 o.n. ($P$) in $PCl_5$ = 5 o.n. ($Cl$) in $Cl_2$ = 0 o.n. ($Cl$) in $PCl_5$ = -1
How many electrons are exchanged by one phosphorus atom?
5 electrons are lost by each phosphorus atom.
How many electrons are exchanged by one chlorine atom?
1 electron is absorbed by each chlorine atom.
What is the ratio of phosphorus and chlorine?
5 atoms of chlorine to 1 atom of phosphorus.
What is therefore the ratio of molecules of phosphorus pentachloride and molecules of chlorine Cl2?
5 molecules of chlorine (totalising 10 atoms of chlorine) to 2 molecules of phosphorus pentachloride (totalising 2 atoms of phosphorus).
Equation: $2P$ $+$ $5Cl_2$ $\longrightarrow$ $2PCl_5 $.