According the van der Waal's theory of non-ideal gases, which of the following statement is/are correct?

Figure displays the plot of the compression factor Z verses p for a few gases Which of the following statements is//are correct for a van-der waals gas:

Regarding the van der Waals constant which of the following is/are correct?

Define van der Waal's radius.

Write the van der Waals equation for 5 mole of ideal gas.

Two van der waal's gases have same value of a but different value of b which of the following statement is correct ?

Two van der waal's gases have same value of a but different value of b which of the following statement is correct ?