Final answer:
To calculate the percent by mass of chlorine in PNCl₂, you must consider the molar mass of chlorine using its natural isotopic abundance, then calculate the entire molar mass of PNCl₂, and compute the ratio of chlorine's mass to the compound's total mass.
Step-by-step explanation:
The percent by mass of chlorine in PNCl₂ can be calculated by determining the molar mass of chlorine in the compound and then dividing by the total molar mass of PNCl₂. To find the molar mass, we will use the atomic masses of phosphorus (P), nitrogen (N), and chlorine (Cl). Given that naturally occurring chlorine consists of 75.77% 35Cl and 24.23% 37Cl, its average atomic mass is 35.453 amu. We multiply the atomic mass of chlorine by 2 because there are two chlorine atoms in PNCl₂. Then, we would add the atomic masses of phosphorus and nitrogen to this total to get the molar mass of PNCl₂. Using these numbers, we can set up a ratio to calculate the percent composition of chlorine in the compound.