Question

what is the formula to find the %concentration of Hcl in w/v, If a bottle of Hcl labeled as : Hydrochloric Acid, 6 M, Laboratory Grade, 500 mL,

Answer 1

I never used a formula. If you have 500 mL of a 6 M solution you must have 500 mL that has 6 mols/L of solution.
Percent w/v is defined as grams solute/100 mL of solution; therefore, you must have 3 mols HCl/500 mL. Three (3) mols is 3(36.5 g/mol) = 109.5 g/500 mL or 109.5 x (100/500) = 2.19/100 mL or 2.19%
The formula is (grams solute/100 mL solution) = % solute w/v or
(grams solute/1 mL solution)*100 = % solute w/v
There is an easier way to do this. Remember a 6 M solution is 6 mols/L so % w/v = (6*36.5)/10 so the formula is M*molar mass/10 = % w/v

Answer 2

Hold up on that answer above. I'm not satisfied with it. It will give you an answer that is close. The definitions I've found on the web disagree. I believe the correct definition of % w/v is grams solute/100 grams SOLVENT as opposed to 100 g SOLUTION.