Concentrated hydrochloric acid is 12.0M and is 36.0% hydrogen chloride by mass. What is its density?

Mass for this case,

$\frac{12\times 36.5}{\text{ Mass }}\times 100=36$

As, density is the ratio of mass to the volume and volume of water is 1000. Thus,

$\rho =\frac{\text{ Mass }}{1000}=1.22\mathrm{g}/\mathrm{ml}$