One litre of mixture of `O_2` and `O_3` at STP was allowed to react with an excess of acidified solution of KI. The iodine liberated required 40 " mL of " `(M)/(10)` sodium thiosulphate solution for titration. What is the mass per cent of ozone in the mixture? Ultraviolet radiation of wavelength 300 nm can decompose ozone. Assuming that one photon can decompose one ozone molecule, how many photons would have been required for complete decomposition of ozone in the original mixture?

Explanation

`O_2+Kito` no reaction
`O_3+2KI+H_2Oto2KOH+I_2+O_2`
Also: `I_2+2S_2O_3^(2-)toS_4O_6^(2-)+2I^(ɵ)`
1 mmol `I_2-=2mmolS_2O_3^(2-)`
`implies` m" mol of "`I_2` produced `=(1)/(2)((1)/(10)xx40)=2`
m mol `O_3=m mol I_2=2`
or moles of `O_3=2xx10^(-3)`
Also moles of `O_2+O_3=(1)/(22.4)=0.0446`
`implies` moles of `O_2=0.0446-0.002=0.0426`
`{:(Mass of O_2=0.0426xx32=1.363g),(Mass of O_3=0.002xx48=0.096g):}]`
Total mass `=1.459g`
`%O_3=(0.096)/(1.459)xx100=6.579`
Number of photons used for decomposing `O_3=` number of molecules of `O_3=(2xx10^(-3))(6xx10^(23))=1.2xx10^(21)`