Question #df6fa

1 Answer
Oct 27, 2015

Here's why that is.


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.