How many SO_3^{2-} ions are present in 99.6 mg of Na_2SO_3? (Molar mass of Na_2SO_3 = 126.05 g/mol)

• A. 1.90 × 10^{21} SO_3^{2-} ions
• B. 2.38 × 10^{20} SO_3^{2-} ions
• C. 4.77 × 10^{20} SO_3^{2-} ions
• D. 7.58 × 10^{21} SO_3^{2-} ions

Answer: (C)

Verified step by step guidance

1. Convert the given mass of sodium sulfite (Na_2SO_3) from milligrams to grams by dividing by 1000, since molar mass is given in grams per mole.
2. Calculate the number of moles of Na_2SO_3 using the formula: \(\text{moles} = \frac{\text{mass in grams}}{\text{molar mass}}\), where the molar mass of Na_2SO_3 is 126.05 g/mol.
3. Recognize that each formula unit of Na_2SO_3 contains exactly one sulfite ion (SO_3^{2-}), so the number of moles of Na_2SO_3 equals the number of moles of SO_3^{2-} ions.
4. Use Avogadro's number, \$6.022 \times 10^{23}\( entities per mole, to convert moles of SO_3^{2-} ions to the actual number of ions: \)\text{number of ions} = \text{moles} \times 6.022 \times 10^{23}$.
5. Perform the multiplication to find the total number of SO_3^{2-} ions present in the given sample mass.