First, you need grams but you have moles.
Find out the total molar mass of SiO₂
Si = 28.085 g/mol Si
O₂ = 32 g/mol O₂
Add them together to get the molar mass
28.085 g/mol Si + 32 g/mol O₂ = 60.085 g/mol SiO₂
Multiply the molar mass of SiO₂ by the number of moles you have of the same substance:
60.085 g/mol SiO₂ × 0.125 mol SiO₂ =
7.51 grams SiO₂
