Question

If we consider that 1/6, in place of 1/12, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will

Answer 1

The answer is,

Firstly, the mass of the other elements may not come out accurately because it comes in fractions, so the mass of the carbon will not be 1 unit.

Secondly, there is no need to make 1/6 as a reference of 1/2 because we have already a well defined reference like the C-12. 

Answer 2

The relative atomic mass of substances is the ratio of the mass of the atoms of a given element to that of 1/12th of carbon 12.

Therefore calculating the relative atomic mass of an element using 1/6th of carbon-12 give double the what its mass is. Calculate as follows:

if 1/12 gives x mass units
Then 1/6th mass would give x(1/6)/(1/12)

                                              x1/6(12)

                                             = 2x
Therefore the mass would be twice what its current mass is.