A sample of hydrogen gas occupies 16.8 L at STP. How many moles of the gas are present?

PV = nRT

To find the number of moles of hydrogen gas present, we can use the Ideal Gas Law formula: PV = nRT, where P represents pressure, V represents volume, n represents the number of moles, R is the ideal gas constant, and T represents temperature.

At STP (Standard Temperature and Pressure), the pressure is 1 atmosphere (atm), and the temperature is 273.15 Kelvin (K). The ideal gas constant (R) is 0.0821 L·atm/(mol·K).

Given:
Volume (V) = 16.8 L
Pressure (P) = 1 atm
Temperature (T) = 273.15 K
Ideal gas constant (R) = 0.0821 L·atm/(mol·K)

We can rearrange the Ideal Gas Law formula to solve for n:
n = PV / RT

Substituting the given values:
n = (1 atm * 16.8 L) / (0.0821 L·atm/(mol·K) * 273.15 K)

Now, let's calculate it:

n = 16.8 / (0.0821 * 273.15)
n ≈ 0.627 mol

Therefore, there are approximately 0.627 moles of hydrogen gas present.