A student has samples of two pure compounds, XClO3 and ZClO3, which contain unknown alkali metals X and Z . The student measures the mass of each sample and then strongly heats the samples to drive off all the oxygen, leaving solid residues of XCl and ZCl. The student measures the mass of the solid residue from each sample. Which of the following questions can be answered from the results of the experiment?
A: Which has the greater molar mass, X or Z ?
B: Which has the higher boiling point, X or Z ?
C: Which has the higher melting point, XCl or ZCl ?
D: Which has the greater density, XCl or ZCl ?

In this experiment reactions are undergoing decomposition reactions .
Since X and Y are alkali metals so they have + 1 charge on them.
2XClO₃ --> XCl  (s) + 3O₂ (g)
2ZClO₃  --> ZCl (s)  + 3O₂ (g)
Now in both decomposition reactions, one mole of compound is giving 3 moles of O2 which is leaving the reaction mixture as a gas. Residue in each reaction is metal chloride left as solid.
Now lets say we started with same mass of both samples (XClO3 and ZClO3). Now the compound with more molar mass will give less moles of compound and hence less moles of oxygen gas.This results in less decrease in mass of the sample and mass of the residue left.
It means just checking the difference in the mass of sample and mass of residue left we can compare the molar mass of X and Z. More decrease in mass means more moles of O₂ released means more moles of perchlorate salt which means molar mass of perchlorate salt is low which means that molar mass of alkali metal is lesser.