The molecular formula CH4 has a relative formula mass of 16. If the mass of carbon is 12, what is the mass contributed by hydrogen in CH4?

The molecular formula CH4 has a relative formula mass of 16. If the mass of carbon is 12, what is the mass contributed by hydrogen in CH4?

• A. 4
• B. 12
• C. 16
• D. 28

Answer: (A)

Think of the relative formula mass as the total mass contributed by all atoms in a molecule. For CH4, the total is 16. The carbon part is 12, so the hydrogen part must be 16 − 12 = 4. There are four hydrogens, each with a relative atomic mass of 1, which indeed adds up to 4. So the mass contributed by hydrogen is 4, and hydrogen makes up 4/16 = 25% of the molecule’s mass. The other numbers don’t fit because they’d imply incorrect total masses for the hydrogen portion (for example, 12 would push the total beyond 16, 16 would mean hydrogen contributes all the mass, and 28 is larger than the given total).

Think of the relative formula mass as the total mass contributed by all atoms in a molecule. For CH4, the total is 16. The carbon part is 12, so the hydrogen part must be 16 − 12 = 4. There are four hydrogens, each with a relative atomic mass of 1, which indeed adds up to 4. So the mass contributed by hydrogen is 4, and hydrogen makes up 4/16 = 25% of the molecule’s mass. The other numbers don’t fit because they’d imply incorrect total masses for the hydrogen portion (for example, 12 would push the total beyond 16, 16 would mean hydrogen contributes all the mass, and 28 is larger than the given total).