Question #df6fa

Concentrated hydrochloric acid, `"HCl"`, is usually prepared in percent concentrations by mass that do not exceed `38%`.

The reason for why that is the case lies with the solubility of hydrogen chloride.

Remember, hydrochloric acid is solution that contains dissolved hydrogen chloride, a gas. This means that, at room temperature, for example, you have a limited amount of hydrogen chloride that can be dissolved per 100 mL of water.

Now, hydrogen chloride has a solubility of approximately `"70 g/mL"` at room temperature.

This means that you could dissolve `"70 g"` of hydrogen chloride gas in `"100 g"` of water to get

`m_"solution" = "70 g" + "100 g" = "170 g"`

The percent concentration by mass for this solution would be

`"%m/m" = m_"HCl"/m_"solution" xx 100`

`"% m/m" = (70color(red)(cancel(color(black)("g"))))/(170color(red)(cancel(color(black)("g")))) xx 100 ~~ 41%`

However, a very important thing to take into account here is that this solution is close to saturation, and it will have a significant rate of evaporation.

This implies that special storage conditions must be used in order to make sure that the hydrogen chloride, which is a poisonos gas, vapors are contained properly.

So, to sum this up, hydrochloric acid comes at a purity of about `37-38%` because that is the maxiumum concentration by mass that can be obtained without having to worry about significant evaporation rates.