Yield of carbon nanotubes (CNTs) depends on numerous process parameters such as temperature of synthesis, type of catalyst, type of precursor, time of precursor flow and partial pressure of precursor gas as well as carrier gas, etc. Experiments were performed in order to find the optimum temperature of synthesis for varying time of precursor flow. The yield was evaluated in terms of mass of crystalline CNTs per gram of substrate and/or catalyst. The CNTs were grown on a calcium carbonate (CaCO3) substrate, with iron-cobalt (Fe-Co) as a catalyst, using acetylene (C2H2) as a precursor gas and argon (Ar) as a carrier gas. A three-stage purification process, incorporating two acid treatment steps and one annealing step, was used for purification which ensures high grade purity of CNTs. The highest yield of 21.4 g of CNTs per g of catalyst was achieved at 700oC for 60 min of synthesis. The CNTs were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Raman, Thermo-gravimetric analysis (TGA), and Gas chromatography (GC).