What is the molarity of 3.5 wt % NaCl dissolved in 1000 mL of distilled water?

Molarity is defined as "Number of moles of solute per litre of solution".

To determine the number of moles of solute, you need to divide the mass of substance that has been dissolved by its molar mass.

But the question does not tell you what the name of the substance or its molar mass. So it is not possible to work out the number of moles, and therefore you cannot work out the molarity.

Assume you have 1000 g of the solution.

Step 1. Calculate the mass of each component

#"Mass of NaCl" = 1000 color(red)(cancel(color(black)("g solution"))) × "3.5 g NaCl"/(100 color(red)(cancel(color(black)("g solution")))) = "35.0 g NaCl"#

#"Mass of water = 1000 g - 35.0 g = 965.0 g"#

Step 2. Calculate the volume of the solution

I shall assume that the density of the solution 1.023 g/mL. Then

#"Volume of solution" = 1000 color(red)(cancel(color(black)("g solution"))) × "1 mL solution"/(1.023 color(red)(cancel(color(black)("g solution")))) = "977.5 mL solution"#

Step 3. Calculate the moles of #"NaCl"#

#"Moles of NaCl" = 35.0 color(red)(cancel(color(black)("g NaCl"))) × "1 mol NaCl"/(58.44 color(red)(cancel(color(black)("g NaCl")))) = "0.599 mol NaCl"#

Step 4. Calculate the molar concentration of #"NaCl"#

#c = "0.599 mol"/"0.9775 L" = "0.61 mol/L"#