<u>Answer:</u> The number of moles of given amount of hydrogen gas is 
<u>Explanation:</u>
To calculate the number of moles, we use the equation:

We are given:
Let the number of moles of hydrogen gas be 'x'
Given mass of hydrogen gas = 900.0 g
Molar mass of hydrogen gas = 2 g/mol
Putting values in above equation, we get:

Hence, the number of moles of given amount of hydrogen gas is 