The molarity of a solution is defined as the number of moles of solute per liter of solution. To calculate the molarity of the HCl solution, we need to know the number of moles of HCl in 500 cm3 of solution.

To calculate the number of moles of HCl, we need to use the formula:

```

Moles of HCl = Mass of HCl / Molar mass of HCl

```

The molar mass of HCl is 36.46 g/mol. Therefore, the number of moles of HCl in 31.0 grams is:

```

Moles of HCl = 31.0 g / 36.46 g/mol = 0.850 moles

```

The volume of the solution is 500 cm3, which is equal to 0.5 liters. Therefore, the molarity of the HCl solution is:

```

Molarity = Moles of HCl / Volume of solution (in liters)

```

```

Molarity = 0.850 moles / 0.5 liters = 1.70 M

```

Therefore, the molarity of the HCl solution is 1.70 M.