Nitrogen is a gas because it is an airlike fluid substance which expands freely to fill any space available, irrespective of it's quantity
The buoyant force is the upward force or thrust...Simply, consider immersing your hand into a bucket of water. What happens to it? It raises (a little). The raising is due to the buoyant force. Basically it is the upward force/thrust that acts on any object immersed into a fluid. Hence why things float when out in water. Hope this helped!!
To determine the k for the second condition, we use the Arrhenius equation which relates the rates of reaction at different temperatures. We do as follows:
ln k1/k2 = E / R (1/T2 - 1/T1) where E is the activation energy and R universal gas constant.
ln 1.80x10^-2 / k2 = 80000 / 8.314 ( 1/723.15 - 1/593.15)
k2 = 0.3325 L / mol-s
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