What is the actual mass of 5 protons?

Correct answer: b 8.36 × 10⁻²⁴ g

5 × 1.673 × 10⁻²⁴ g ≈ 8.36 × 10⁻²⁴ g.

Why the other options are wrong

• a 4.55 × 10⁻²⁷ g: This is close to the mass of 5 electrons (5 × 9.11 × 10⁻²⁸ g); for protons use 5 × 1.673 × 10⁻²⁴ g.
• c 8.36 × 10⁻²⁷ g: The digits are right but the power of ten is wrong: one proton is of order 10⁻²⁴ g, so five are 8.36 × 10⁻²⁴ g.
• d 4.37 × 10⁻²⁶ g: This is not the right product; 5 × 1.673 × 10⁻²⁴ g = 8.36 × 10⁻²⁴ g.