5.6 dm^3 of gaseous HCl (as measured in normal conditions) is dissolved in 100 cm^3 of water. What is the mass percent concentration of the obtained

5.6 dm^3 of gaseous HCl (as measured in normal conditions) is dissolved in 100 cm^3 of water. What is the mass percent concentration of the obtained acid?

The answer is 8.4%, but I do not understand how.

This is taken from a practice test, and on the test, densities and formulas will not be given, so I do not know how to approach the problem.