Question

15.3 of an unknown substance was found to contain 0.15 moles. Calculate the relative formula mass of the substance.

Answer 1

M_r = 100

Molar mass = grams/moles = 15.3 g/0.15 mol = 100 g/mol

∴ M_r = 100

Note: The answer can have only two significant figures because that is all you gave for the number of moles.