ANS:B - 50

To find the percentage of CaO in limestone, we need to consider the fact that limestone is primarily composed of calcium carbonate (CaCO3), not calcium oxide (CaO) like lime. The molar mass of CaO is approximately 56.08 g/mol, while the molar mass of CaCO3 is approximately 100.09 g/mol. Since the percentage of CaO in limestone is being asked, we need to consider the calcium oxide content in relation to the calcium carbonate content. From the information given, we know that lime (calcium oxide, CaO) contains about 90% CaO. So, if we were to convert limestone (calcium carbonate, CaCO3) to lime (CaO), we would lose one molecule of CO2, which has a molar mass of approximately 44.01 g/mol. So, for every 100.09 g of CaCO3, when converted to CaO, we would have 56.08 g of CaO. This is because 56.08 g is approximately 90% of 100.09 g. Now, to find the percentage of CaO in limestone, we need to calculate the percentage of CaO relative to the total mass of limestone: Percentage CaO in limestone = (Mass of CaO / Mass of limestone) * 100% Percentage CaO in limestone = (56.08 g / 100.09 g) * 100% Percentage CaO in limestone ≈ 55.99% Therefore, the closest option among the given choices is 50%.