Answer :

To determine the number of moles of hydrogen gas contained in 21.3 liters at STP (Standard Temperature and Pressure), follow these steps:

### Step-by-step Solution

1. Understand the relationship between volume and moles at STP:
At Standard Temperature and Pressure, 1 mole of any ideal gas occupies 22.4 liters.

2. Given Data:
The volume of hydrogen gas we have is 21.3 liters.

3. Formula:
To find the number of moles [tex]\( n \)[/tex], we use the formula:
[tex]\[ n = \frac{V}{V_m} \][/tex]
where [tex]\( V \)[/tex] is the volume of the gas, and [tex]\( V_m \)[/tex] is the molar volume of the gas at STP (22.4 liters/mole).

4. Substitute the values:
[tex]\[ n = \frac{21.3 \, \text{liters}}{22.4 \, \text{liters/mole}} \][/tex]

5. Calculate the number of moles:
[tex]\[ n \approx 0.9508928571428572 \, \text{moles} \][/tex]

### Final Answer:
Thus, the number of moles of hydrogen gas contained in 21.3 liters at STP is approximately [tex]\( 0.9509 \)[/tex] moles.