Here's why that is.
Concentrated hydrochloric acid,
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
This means that you could dissolve
#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