Suppose a rocket at rest in space where there is a complete vacuum and no force to influence the rocket:
In the left we have the rocket with a metal ball in it. After the ball is thrown out of the rocket, according to Newton's first law of motion, because no force is influencing the system ($ \sum F = 0)$ and the system is initially at rest, the center of mass of the system will continue to be at rest, although the ball and the rocket will move in opposite directions. In this case we may reduce the Newton's first law of motion to this equation:
$$m_1 v_1 = m_2 v_2$$
(as it might be interpreted as the conservation of momentum)
Thus, dependent on the mass of the rocket $(m_1)$ and the mass of the metal ball $(m_2)$ and the velocity of the ball when moving out of the rocket $(v_2)$, we can calculate the velocity of the rocket $(v_1)$.
Now, in the right we have the same rocket but this time with a tank of liquefied gas (a real gas) instead of the metal ball. After opening the tank the gas will flow out of the rocket in the vacuum and we expect the rocket to move in the opposite direction the same way jet spacecrafts allegedly move in the vacuum of space. Suppose the mass of the liquefied gas initially is again $m_2$ and suppose after the gas flows out of the tank completely the rocket gets the velocity of $v_1$ and the mass of the rocket(including the empty tank) is again $m_1$. Applying the Newton's first law of motion again in this case we are (or should be) able to calculate the mean velocity $(v_2)$ of the total mass of the gas that have been flowed and scattered in the vacuum, HOWEVER obviously it is impossible to logically think of such a velocity! The molecules of the gas are moving in any direction in the vacuum of space; they won't move in the space the same way that a metal ball will move. In other words we can't THROW a mass of gas in the space and get some movement in the opposite direction out of it.
Conclusion: Movement of a jet spacecraft in the vacuum of space is impossible!
Question: What's wrong with the above reasoning?