How are isotopos defined?
forms of different elements that have the same number of neutrons in each atom
forms of different elements that have the same number of protons in each atom
forms of the same element that differ in the number of neutrons in each atom
forms of the same element that differ in the number of protons in each atom

isotopes are atoms that have the same number of protons and electrons but different numbers of neutrons and therefore have different physical properties.

"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.

therefore, you had 15.67 - 7.58 = 8.09g h2o  

moles of each :

7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles  

8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles  

molar ratio between the two by dividing both numbers by the smallest number :

0.09 / 0.09 = 1  

0.44 / 0.09 = 4.8  

mgco3-5h2o"

the answer is b. dilution