The concentration of a stock reagent is 10 × mass percent × density ÷ molar mass: 37 percent hydrochloric acid is 12.0 mol/L, 98 percent sulfuric acid is 18.4 and 70 percent nitric acid is 15.7.
A litre of the stock weighs 1,000 mL × the density; the solute is the mass percent of that, and dividing by the molar mass gives moles per litre. For hydrochloric acid that is 1,000 × 1.18 × 0.37 / 36.46 = 12.0 mol/L. The factor of ten is 1,000 mL a litre over 100 for the percent.
Bottles are labelled with a range (hydrochloric acid is 36 to 38 percent) and the density varies with it, so the true strength is a percent or two either way. The label’s figure is for the stock as bought, and an old bottle of a volatile acid (hydrochloric acid, ammonia) loses strength.
Estimate with the rule, then check it against the calculator that models it properly.
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