This question involves the Ideal Gas Law.

**Ideal gas law**

#PV=nRT#,

where P is pressure, V is volume, n is moles, R is the gas constant, and T is temperature in Kelvins.

The current STP temperature is #0^@"C"# or #"273.15 K"# (used for gas laws), and the pressure is #"10^5 Pa#, which most people write as #"100 kPa"# (both pressures are equal).

**Organize the data:**

**Known**

#P="100 kPa"#

#n="3.4 mol"#

#R="8.3144598 L kPa K"^(-1) "mol"^(-1)"#

https://en.wikipedia.org/wiki/Gas_constant

#T="273.15 K"#

**Unknown:** #V#

**Solution**

Rearrange the equation to isolate #V#. Insert the data and solve.

#V=(nRT)/P#

#V=(3.4color(red)cancel(color(black)("mol"))xx8.3144598color(white)(.)"L" color(red)cancel(color(black)("kPa")) color(red)cancel(color(black)("K"))^(-1) color(red)cancel(color(black)("mol"))^(-1)xx273.15color(red)cancel(color(black)("K")))/(100color(red)cancel(color(black)("kPa")))="77 L HCl(g)"# rounded to two sig figs