No need for approximations., even no need for the exact formula of the xylocaine, as long as we get the number of carbons per molecule right. 0.80 moles with 14 carbon atoms per molecule means we have 0.80*14 moles of C. To convert to mass just multiply by 12 g/mol. 0.80*14*12 = 134.4 = 1.3×102 g.