Answer
a. FM of $SO_{2}$= 64.1 amu
b. FM of $PCl_{3}$ = 137 amu
Work Step by Step
a. There are 2 atoms O and 1 atom S in the figure, therefore we have to find formula mass for $SO_{2}$.
FM of $SO_{2}$ is the sum of atomic masses of all atoms in formula:
1 x AM S = 1 x 32.06 amu = 32.06 amu
2 x Am O = 2 x 15.999 = 31.99 amu
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FM of $SO_{2}$= 64.05 amu
FM of $SO_{2}$= 64.1 amu ( to three significant figures)
b. There are 3 atoms Cl and 1 atom P in the figure, therefore we have to find formula mass for $PCl_{3}$.
FM of $PCl_{3}$ is the sum of atomic masses of all atoms in formula:
1 x AM P = 1 x 30.97 = 30.97 amu
3 x AM Cl = 3 x 35.45 = 106.35 amu
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FM of $PCl_{3}$= 30.97 + 106.35 amu
FM of $PCl_{3}$ = 137.32 amu
FM of $PCl_{3}$ = 137 amu ( to three significant figures)
