The reaction between Na2S and CuSO4 will give us the balanced chemical reaction of, Na2S + CUSO4 --> Na2SO4 + CuS This means that for every 78g of Na2S, there needs to be 159.6 g of CuSO4. The ratio is equal to 0.4887 of Na2S: 1 of CuSO4. Thus, for every 12.1g of CuSO4, we need only 5.91 g of Na2S. Thus, there is an excess of 9.58 g of Na2S. The answer is letter C.
Even though the average between the 4 measurements is 7.45, the measurements are not accurate. This is probably due to the pH meter being out of calibration or because there is some contamination in the buffer solution
