density of hydrogen at 87.5MPa equals 45.96 g/L [3]
122.4L at 45.96 g/L equals equals 5.63kg [4]
So the amount is somewhere between 4.5kg and 5.63kg, depending upon the final pressure after filling. Which seems to line up squarely with "Approx. 5.0kg".
Hydrogen storage is "Approx. 5.0 kg"
122.4L of volume (60L front, 62.4L rear)
pressure is between 70 MPa and 87.5MPa
Doing the math shows:
density of hydrogen at 70MPa equals 36.69 g/L [1]
122.4L at 36.69 g/L equals 4.49kg [2]
density of hydrogen at 87.5MPa equals 45.96 g/L [3]
122.4L at 45.96 g/L equals equals 5.63kg [4]
So the amount is somewhere between 4.5kg and 5.63kg, depending upon the final pressure after filling. Which seems to line up squarely with "Approx. 5.0kg".
[0] - https://pressroom.toyota.com/releases/2016+toyota+mirai+fuel...
[1] - http://www.wolframalpha.com/input/?i=density+of+hydrogen+at+...
[2] - http://www.wolframalpha.com/input/?i=122.4L+at+36.69+grams%2...
[3] - http://www.wolframalpha.com/input/?i=density+of+hydrogen+at+...
[4] - http://www.wolframalpha.com/input/?i=122.4L+at+45.96+g%2FL