A certain substane A . Is mixed with an equal amount of a substance, B . At the end of 1.0 hr, A is 70% reacted . How much will it be left unreacted at the end of 2.5 hr, reaction with respect to A of first order ?

Options

A. `10%`
B. `5%`
C. `3%`
D. `1%`

Answer

B

Explanation

`k = (2.303)/t log [A]_(0)/[A]_(n) = ( 2.303)/(1.0) log (100)/30 `
Again , ` k = (2.303)/(2.5) log (100)/x`
Hence, ` (2.303)/(1.0) log (100)/30= (2.303)/(2.5) log 100/x`
` log 100/x = 2.5 log (100)/30`
` log 100/x = 2.5 ( log 100- log 30) `
` 2.5 xx 0.5220 = 1.307 `
` 100/x = " antilog 1.307 or 100/x = 20.28`
` or x= 100/(20.28) = 4.93% = 5%`
Density of FeO = 4 g/cc
mass of 1 unit cell = ` 1.25 xx 10^(-22) xx4 = 5 xx 10^(-22) g `
Mass of 1 molecule `= 72/ ( 6.023 xx 10^(23)) = 1.195 xx 10^(-22) g `
Hence, number of FeO molecules per unit cell
` (5xx 10^(-22))/(1.195 xx 10^(-22)) = 4.18 = 4 `
Hence, there are four ` Fe^(2+)` and four ` O^(2-)` ions in each unit cell.