To calculate the molar mass $m$ of a substance, you can use the formula:

m = $M\ast N$ / N_A

where:

M is the molar mass of the substance $142g/molinthiscase$N is the number of molecules or entities $3x10^{2}3inthiscase$N_A is Avogadro's number $6.022x10^{2}3mo{l}^{-}1$

Plugging in the values, we get:

m = $142\ast 3x10^{2}3$ / 6.022 x 10^23
m = $426x10^{2}3$ / 6.022 x 10^23
m = 70.81 g

Therefore, the mass of the substance is 70.81 g.