Mass and energy can not be created or destroyed, they may be able to just be converted, and neither one seems without the opposite. For this reason in closed systems, both mass and energy are conserved individually. " I hope this helps "
Answer : The final pressure of the gas will be, 26.8 kPa
Explanation :
According to the Boyle's law, the pressure of the gas is inversely proportional to the volume of the gas at constant temperature of the gas and the number of moles of gas.

or,

or,

where,
= initial pressure of the gas = 209 kPa
= final pressure of the gas = ?
= initial volume of the gas = 10.0 L
= final volume of the gas = 78.0 L
Now put all the given values in this formula, we get the final pressure of the gas.


Therefore, the final pressure of the gas will be, 26.8 kPa
Isotopes of elements have same atomic numbers and different mass number(atomic masses).
The answer for question 14 would be C and question 15 is B