A sample of ascorbic acid is synthesized in the laboratory. It contains 1.50g of carbon and 2.00 grams of oxygen. Another sample of ascorbic acid isolated from citrus fruits contains 6.35 g of carbon.
How many grams of oxygen does it contain? Which law are you assuming in answering this question?
Ascorbic acid is a chemical compound with a chemical formula C6H8O6. This is a constant and is the same irrespective of how the acid is prepared or where it is obtained from. This follows from the law of definite proportions which states that in a chemical compound the proportion by mass of the constituent elements is the same.
The ascorbic acid sample created in the laboratory has 1.5 g of carbon and 2 g of oxygen. As the other sample isolated form fruits has 6.35 g of carbon it has (6.35/1.5)*2 = 8.466 g of oxygen.