For hydrogen gas, Z is ......unity at all pressure.

Explanation

To solve the question "For hydrogen gas, Z is ......unity at all pressure," we need to understand the concept of the compressibility factor (Z) and how it behaves for different gases, particularly hydrogen.

### Step-by-Step Solution:

1. **Understanding the Compressibility Factor (Z)**:
- The compressibility factor (Z) is defined as \( Z = \frac{PV}{nRT} \), where:
- \( P \) = pressure,
- \( V \) = volume,
...