Silicon carbide, commonly known as carborundum, is a very hard and abrasive substance. The compound is prepared by heating silicon dioxide in the presence of graphitic carbon:

Question

SiO2(s) + 2C(s) --> SiC(s) + CO2(g)

How many grams of silicon carbide can be formed from 20.0 g of graphite and 65.0 g of silicon dioxide?

Answer 1

A limiting reagent (LR) problem. You know that when you are given amounts for BOTH reactants.
mols C = 20/12 = approx 1.7 but you need a more accurate answer for this and all of the estimates that follow.

mols Si = g/atomic mass = 65/60 = about 1

mols SiC formed if you had 1.7 mols C and all of the SiO2 needed is 1.7 x (1 mol SiC/2mols C) = 1.7 x 1/2 = about 0.85

mols SiC formed if you had 1 mol SiO2 and all of C needed. That's 1 x (1 mol SiC/1 mol SiO2) = 1 x 1/1 = about 1 mol SiC.
You note that the values for mols SiC don't agree which means one of them is not right. The correct value in LR problems is ALWAYS the smaller value ad the reagent producing that value is the LR. So C is the limiting reagent and you could produce approx 0.8 mols SiC.

Now convert 0.8g SiC to grams. g = mols x molar mass.