Where does an element take its identity from?

It gets its identity from tiny particles: positively charged protons in the nucleus, balanced by negatively charged electrons all around, plus neutrons, which have no charge at all.

an element gets it's identity from the number of protons.

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a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.