Say you have 100 g of a solution. So, the % mass is equal to the mass of molecule/ total mass of solution, the latter which is 100 g in our hypothetical scenario. So, since each choice has the same % mass, then that means that we have the same mass of each molecule in consideration-
4 g KBr, 4 g KCl, 4 g NaBr, 4 g NaCl, since each of these values divided by the total mass of solution, which we have arbitrarily made 100g, gives us 4%.
Now, convert each mass value into the equivalent moles using their respective molar masses. This gives us:
.0336 mol KBr, .0536 mol KCl, .0389 mol NaBr, .0684 mol NaCl.
Right now, you can stop, since we know that because we have the most moles of NaCl, and since molarity is moles / L, NaCl will have the highest molarity. The reason we can do this is that the liter part of the molarity formula is the same for each of the four substances since we are using the same solvent for all of them (meaning same density), and the total amount of solution is 100g for each of them.
If you want to go further, then you know since you have 4 g of molecules in each case, and the total is 100g, the other 96 g must be the solvent. You can figure out the liters of solvent by using its density:
(96 g H2O)(1 cm^3 / 1 g H2O)(1 mL / 1 cm^3)(1 L/ 1000 mL) = .096 L solvent
Since you divide each of the moles by this same value, you can see this step is unecessary in determining which has the highest molarity.
