Question

What is the molar concentration of `"78% w/w"` `HCl(aq)`?

Answer 1

Are you sure there is a 78% (w/w) hydrochloric acid?

Explanation:

The concentrated `HCl` solution supplied to laboratories is 38% (w/w), with `rho` `=` `1.189*g*mL^-1`. This corresponds to a molarity of approx. `12` `mol*L^-1`. I imagine stronger concentrations than this would have to be made under `HCl(g)` atmospheres.

At any rate, the procedure for diluting strong acids IS ALWAYS ACID TO WATER. Why? Because if you spit in acid, it spits back at you!