\textbf{219--} If in a 5-liter closed flask at a certain temperature, 4 mol of hydrogen gas and 3 mol of nitrogen gas are mixed and heated, and at equilibrium 2 mol of nitrogen gas exists in the mixture, the equilibrium constant of this reaction is:
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(1) $50$ \hfill (2) $100$ \hfill (3) $80.75$ \hfill (4) $40.25$
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