Here's how to calculate the pH of the HCl solution:

1. Calculate the moles of HCl

* Convert pressure to atmospheres: 747 mmHg * (1 atm / 760 mmHg) = 0.983 atm

* Convert temperature to Kelvin: 26.5 °C + 273.15 = 299.65 K

* Use the ideal gas law: PV = nRT

* n = (PV) / (RT)

* n = (0.983 atm * 0.535 L) / (0.0821 L*atm/mol*K * 299.65 K) = 0.0214 moles HCl

2. Calculate the molarity of the HCl solution

* Molarity (M) = moles of solute / liters of solution

* M = 0.0214 moles / 0.625 L = 0.0342 M

3. Calculate the pH

* HCl is a strong acid, so it completely dissociates in water. This means the concentration of H+ ions is equal to the concentration of HCl.

* pH = -log[H+]

* pH = -log(0.0342) = 1.47

Therefore, the pH of the HCl solution is approximately 1.47.