The magnetic susceptibility of a paramagnetic substance at $-173^o$C is $1.5\times 10^{-2}$ then its value at $-73^o$C will be.
• A
$7.5\times 10^{-1}$
• B
$7.5\times 10^{-2}$
• C
$7.5\times 10^{-3}$
• D
$7.5\times 10^{-4}$

#### Solution

In paramagnetic substance the magnetic susceptibility
$\chi =\dfrac{C}{T}\Rightarrow\dfrac{\chi_1}{\chi_2}=\dfrac{T_2}{T_1}$; Then $\chi_2=\chi_1\dfrac{T_1}{T_2}$
Here, $\chi_1=1.5\times 10^{-2}$
$T_1=273-173=100$K, $T_2=273-73=200$K
$\therefore \chi_2=1.5\times 10^{-2}\times \dfrac{100}{200}=7.5\times 10^{-3}$