# Question df6fa

Oct 27, 2015

Here's why that is.

#### Explanation:

Concentrated hydrochloric acid, $\text{HCl}$, 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 $\text{70 g/mL}$ at room temperature.

This means that you could dissolve $\text{70 g}$ of hydrogen chloride gas in $\text{100 g}$ of water to get

${m}_{\text{solution" = "70 g" + "100 g" = "170 g}}$

The percent concentration by mass for this solution would be

$\text{%m/m" = m_"HCl"/m_"solution} \times 100$

"% 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.