The piece of ice was heated from -40 to -10. In this case, 21 kJ of heat was expended. Determine the mass of ice.

The amount of heat that was spent on heating the ice:
Q = C * m * (tк – tн), where Q is the consumed amount of heat (Q = 21 kJ = 21 * 10³ J), C is the specific heat capacity of ice (C = 2100 J / (kg * K)), m – sought ice mass (kg), tc – final ice temperature (tc = – 10 ºС), tн – initial ice temperature (tн = -40 ºС).
Let us express and calculate the mass of heated ice:
m = Q / (С * (tк – tн)) = 21 * 10³ / (2100 * (-10 – (-40))) = 0.333 kg.
Answer: The ice mass will be 0.333 kg.