Question

What is the percentage by mass of $12M$ $HCl$ ?

Answer 1

Mass percentage is a method of expressing the concentration of any solution. Acids are usually dissolved in water and therefore the concentration terms indicate the amount of acid present in its aqueous form.

Complete answer:
Concentration on any solution is a way of measuring the amount of solute that is present in a fixed amount of solvent. There are different methods and formulas of calculating and expressing concentrations of the solutions.
The given concentration of hydrochloric acid is in terms of its molarity. Molarity is the form of expressing concentration that is calculated by dividing the number of moles of solute by the volume of solution in liters.
A concentration of $12M$ means that there are twelve moles of hydrochloric acid dissolved in a solution of one liter volume. This concentration can be expressed in terms of mass percentage by finding out the mass of hydrochloric acid present in twelve moles of it.
${\text{mass of }}HCl = {\text{molar mass}} \times moles$
${\text{mass of }}HCl = 36.5gmo{l^{ - 1}} \times 12mol = 438g$
The density of hydrochloric acid is normally taken to be $1.18gm{L^{ - 1}}$ which can be used to calculate the mass of the solution. The density multiplied by volume gives us the mass.
${\text{mass of solution }} = {\text{density}} \times {\text{volume}} = 1.18gm{L^{ - 1}} \times 1000mL = 1180g$
The mass percentage is determined by finding out the ratio of the mass of solute that is dissolved and the total mass of the solution obtained upon the dissolution of solute in solvent. The formula can be written as follows:
${\text{mass percentage}} = \dfrac{{{\text{mass of solute}}}}{{{\text{mass of solution}}}} \times 100$
${\text{mass percentage}} = \dfrac{{438g}}{{1180g}} \times 100 = 37.12\%$
$\Rightarrow$ Hence, the mass percentage of $12M$ hydrochloric acid is $37.12\%$ .

Note:
There are certain concentration standards that express mass percentages that are used on an industrial level. The reagent grade hydrochloric that is available in labs is $37\%$ but the commercial grade is considered to be $39\%$ .