Wait. I thot the Q said that AFT 16.3g of ice is melted the heater is turned off.. so why r we subtracting 2.1 from 16.3?? Thts wht i dont get!! Thx a lot!
Yeah, well, the heater gave it the activation energy to BEGIN melting!
When the heater stopped, the melting that took place for the next two minutes, was by the heat energy provided by the surroundings.
That heat energy from the surroundings was also provided WITH the heater, so the 16.3 g was a combination of melting due to heater, AND melting due to heat from surroundings.
Therefore, to calculate the mass melted by THE HEATER ITSELF, we subtracting the reading WITHOUT HEATER, from the reading for both.