At which temperature and pressure would a sample of helium behave most like an ideal gas?

Here we have to get the temperature and pressure at which helium gas mostly behaves as ideal gas.

Helium (He) behave most like an ideal gas upto 24K temperature and 0 atm pressure.

The deviation of a real gas to ideal gas occurs at high temperature and low pressure.

The deviation of ideal gas to real gas occurs on taking into account the van der waals' force of attraction between the gas molecules. Now, the van der waals' interaction depends upon the polarisibility of the gas molecule.

As helium (He) is non-polarisable and very small (atomic number 2) it mostly behaves as ideal gas upto 24K temperature and 0 atm pressure.

Although the deviation from the ideal gas behavior to real gas is not so prominent at little high temperature also. Upto 50K it mostly behaves like an ideal gas.    

The Helium gas tends to behave as an ideal gas at 0temperature and 1 atm pressure.

By applying the real gas law to the gases at the high temperature,m the gases start to behave as the ideal gas.

The gas behaves at the ideal gas when the van der wall force between the gases has been the lowest and they tend to react less with the other atoms.

The Helium gas has the maximum tendency to behave as an ideal gas, as it has been the single atomic gas. Being single atomic, the gas tends to exhibit less forces, and the complete octet helps the gas to react less with the other atoms.

Because of the characteristics, Helium acts as an ideal gas at STP.

Thus the Helium gas tends to behave as an ideal gas at 0temperature and 1 atm pressure.

For more information about the ideal gas, refer to the link:

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the answer is 75.0%.