22.4L of hydrogen gas at STP contains how many molecules

1 mole

Explanation:

At Standard Temperature and Pressure (273.15 Kelvin and 1 atmosphere), 22.4 L of gas is always equal to 1 mole.

We can use the Ideal Gas Equation to explain this:

At STP, like noted above, the Pressure is 1 atmosphere (or 101,325 Pa) and 273.15 Kelvin. Using this information, we can determine that the Ideal Gas Constant, R, is 0.08206. We can now solve this equation.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

it means there are 3 atoms of oxygen