# What is the percent by mass of "12 M HCl"?

May 3, 2017

Knowing the concentration is about $\text{12 M}$ (it's specifically about $\text{12.06 M}$), we can assume a volume of $\text{1 L}$ to calculate the mass density.

(12 cancel"mols HCl")/"L solution" xx "36.4609 g"/cancel"mol HCl" = "437.5308 g HCl"/"L solution"

Based on the mass density, we can calculate the mass percent by converting the solution volume to its mass.

$\left(\text{437.5308 g HCl")/cancel"L solution" xx cancel("1 cm"^(3))/("1.17 g solution") xx cancel"1 mL"/cancel("1 cm"^(3)) xx cancel"1 L"/(1000 cancel("mL}\right)$

$= 0.374$

Or, about $\textcolor{b l u e}{\text{37.4% w/w}}$. Note that 37% is actually reagent-grade, not commercial-grade, which is about 39%.

See if you can calculate the concentration in 39% commercial-grade HCl! HINT: Use a ratio of the molar concentrations of the 37% to the 39%, and assume the density is similar enough.