How many grams of ice at \(-20^\circ\mathrm{C}\) will be needed to cool \(1.1\) litres of water from \(30^\circ\mathrm{C}\) to \(20^\circ\mathrm{C}\)?
\[
c_{\text{ice}}=0.5\ \text{cal g}^{-1}\,{}^{\circ}\mathrm{C}^{-1}
\]
\[
L_f=80\ \text{cal g}^{-1}
\]