Uranium-238 decays to Lead-206. Both Rock X and Rock Y initially had a ratio of 100%. Uranium-238 to 0% Lead-206. Today, Rock X has a ratio of 50%. Uranium-238 to 50% Lead-206. Rock Y has a ratio of 25% Uranium-238 to 75% Lead-206. Which rock is older?

Rock Y

Explanation:

The rock that is older shall be determined based on the number of half lives it has witnessed. A half life is simply the amount of time required for a sample to decay to half of its original mass.

Rock X

From 100% uranium, it decays to 50% uranium.  This means it went through just one half life.

Rock Y

From 100% uranium to 25% uranium

100 --> 50 (first half life)

50 --> 25 (second half life)

This means it wen through 2 half lives.

Rock Y is older than Rock X

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.