If sodium sulfate is considered to be completely dissociated into cations and anions in aqueous solution, the change in freezing point of water $\left(\mathrm{\Delta }{T}_f\right)$, when 0.01 mole of sodium sulfate is dissociated in 1 kg of water, is : $(\mathrm{Kr}= 1.86 \mathrm{K} \mathrm{kg} {\mathrm{mol}}^{-1})$