If carbon-14 has a half-life or 5,730 years, how much of a 50g sample of carbon-14 would be left after 2 half-lives?

After 1st half life the remaining carbon is 25 g as 50/2= 25g showing that 25g decay and 25g will left.

After second half life it is 25/2=12.5 shows that

After two half lives there will be 12.5g of C-14 is left from 50g of sample

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.