A 0.5370 g sample of a pure soluble chloride compound is dissolved in water, and all of the chloride ion is precipitated as AgCl by the addition of an excess of silver nitrate. The mass of the resulting AgCl is found to be 1.2523 g.

What is the mass percentage of chlorine in the original compound?

I THINK you may have a typo in your problem. I THINK you want mass percent chloride and not mass percent chlorine.

mols AgCl = 1.2523/molar mass AgCl.
mols Cl^- = same as mols AgCl (because you have 1 mol Cl in 1 mol AgCl).
g Cl^- = mols Cl^- x atomic mass Cl^-
%Cl^- = (mass Cl^-/mass sample)*100 = ?

IF you want mass % chlorine, then go back and find mass Cl2 etc.

To calculate the mass percentage of chlorine in the original compound, we need to determine the amount of chlorine in the AgCl precipitate and the original mass of the compound.

Given:
Mass of AgCl = 1.2523 g
Mass of original compound = 0.5370 g

Step 1: Calculate the amount of chlorine in AgCl.
The molar mass of AgCl is the sum of the atomic masses of silver (Ag) and chlorine (Cl). The atomic masses of Ag and Cl are 107.87 g/mol and 35.45 g/mol, respectively.
Molar mass of AgCl = 107.87 g/mol + 35.45 g/mol = 143.32 g/mol

Next, we need to calculate the number of moles of AgCl using its molar mass and the given mass.
Number of moles of AgCl = Mass / Molar mass = 1.2523 g / 143.32 g/mol

Step 2: Calculate the amount of chlorine in the original compound.
Since AgCl is formed by the reaction of silver nitrate (AgNO3) with the chloride compound, the moles of AgCl will correspond to the moles of chloride ion in the compound.
Since AgCl contains one mole of chlorine ions (Cl-) for every mole of AgCl, the number of moles of chlorine in the original compound is also equal to the number of moles of AgCl.

Therefore, the number of moles of chlorine in the original compound = 1.2523 g / 143.32 g/mol

Step 3: Calculate the mass percentage of chlorine.
The mass percentage of an element is calculated by dividing the mass of the element by the total mass of the compound and then multiplying by 100.
Mass percentage of chlorine = (Moles of chlorine / Moles of the compound) * 100

Mass percentage of chlorine = [(1.2523 g / 143.32 g/mol) / (0.5370 g / Molar mass of the compound)] * 100

By substituting the values, you will find the mass percentage of chlorine in the original compound.