Final answer:
In a 1:1 atomic ratio mixture of bromine and chlorine, each element contributes equally, resulting in a 50% Br and 50% Cl mixture. The percent abundance of chlorine isotopes is used for calculating their respective contributions to the mixture's mass, not the percentage in a mixture by count.
Step-by-step explanation:
The question is about determining the percentage composition of bromine and chlorine in a given sample. Chlorine naturally occurs as two isotopes, chlorine-35 and chlorine-37. The percent abundance of these isotopes is 75.77% and 24.23% respectively, and they have atomic masses of 34.969 amu for chlorine-35 and 36.966 amu for chlorine-37. To calculate the percentage of chlorine in a compound, you would divide the molar mass of chlorine by the molar mass of the compound and multiply by 100.
To answer the student's question, we must assume that the equal mixture of bromine and chlorine refers to the atomic count, resulting in each element contributing 50% to the mixture. Thus, in a 1:1 mixture of bromine and chlorine, option B) 50% Br, 50% Cl would be correct. However, it is important to note that without additional context regarding the nature of the mixture (e.g., atomic ratio, mass ratio, or volume ratio), this answer is an assumption based on the typical interpretation of such mixtures.