The volume (V) of ideal gases is related to the temperature (T) of the gas and the pressure (P) in which it is placed by the formula: P*V = n*R*T where n is the number of moles and R is a constant equal to 0.0821 L*atm*K^-1*mol^-1.
STP or standard temperature and pressure is defined by IUPAC as a condition where the temperature is 273.15 K and pressure is 0.986 atm.
Taking hydrogen as an ideal gas and substituting these values in the formula given earlier:
1.5*0.986 = n*0.0821*273.15
=> n = (1.5*0.986)/(0.0821*273.15)
=> n = 0.0659
There are 0.0659 moles of hydrogen in a 1.5 L balloon at STP.