Question

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

Answer 1

Knowing the concentration is about `"12 M"` (it's specifically about `"12.06 M"`), we can assume a volume of `"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.

`("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")`

`= 0.374`

Or, about `color(blue)("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.