Molar mass of magnesium = 24g

Given mass of magnesium = 12g

Number of moles = given mass of the element/ atomic weight of element

Number of moles of magnesium = 12/24 = 0.5 moles

Molar mass of carbon = 12g

Given mass of carbon = 6g

Number of moles = given mass of the element/ atomic weight of element

Number of moles of carbon = 6/12 = 0.5 moles

Hence, 12 g of magnesium and 6 g of carbon represents a mole ratio of 1 : 1