When phosphorus burns in chlorine gas, a mixture of two chloride compounds are produced: phosphorus trichloride and phosphorus pentachloride. A 7.45 g of phosphorus reacts with 25.6 g of chlorine gas to form phosphorus trichloride. On the other hand, 7.45 g of phosphorus reacts with 42.7 g of chlorine gas to form phosphorus pentachloride. Which fundamental law is demonstrated by this observation?