The idea here is that you need to use the molar volume of a gas at STP to determine how many moles of hydrogen chloride,
It's worth mentioning that hydrochloric acid is the term used for aqueous solutions of hydrogen chloride. Simply put,
#overbrace("HCl"_ ((g)))^(color(blue)("hydrogen chloride")) -> overbrace("HCl"_ ((aq)))^(color(red)("hydrochloric acid"))#
Once you know how many moles of hydrochloric acid are present in solution, use the compound's molar mass to convert it to grams.
So, the old definition of STP conditions imply a pressure of
This means that your sample, which is kept at STP conditions, will contain
#22.4 color(red)(cancel(color(black)("L"))) * overbrace(("1 mole HCl"_ ((g)))/(22.4color(red)(cancel(color(black)("L")))))^(color(purple)("molar volume of a gas at STP")) = "1 mole HCl" _((g))#
As you know, the molar mass tells you the mass of one mole of a given compound. In this case, hydrogen chloride's molar mass of
Since you're dealing with a
The total mass of the sample will now be
#m_"total" = m_"water" + m_"HCl"#
#m_"total" = "263.5 g" + "36.46 g" = "299.96 g"#
This means that the percent concentration by mass of hydrochloric acid will be
#"% HCl" = (36.46 color(red)(cancel(color(black)("g"))))/(299.96color(red)(cancel(color(black)("g")))) xx 100 = color(green)(|bar(ul(color(white)(a/a)12.15%color(white)(a/a)|)))#
I'll leave the answer rounded to four sig figs.