$3.$ Producing urea involves a CO$$ to NH$$ molar ratio of $1$:$2.$ If $1,000$ kg of CO$$ is available, how much urea $($CO$($NH$))$ can be produced, assuming $100\%$ conversion efficiency?

$4.$ In the manufacturing of nitric acid, oxygen gas $($O$)$ reacts with ammonia. Calculate the mass of oxygen needed to react with $500$ kg of ammonia to produce nitric acid, assuming $100\%$ efficiency.