# At $90^{\circ}C$ the concentration of $H_3O^+$ in pure water is $10^{-6}\; mol\;L^{-1}$. What is the value of $K_w$ at this temperature ?
( A ) $10^{-8}$
( B ) $10^{-6}$
( C ) $10^{-12}$
( D ) $10^{-14}$