Which statement describes how to calculate the Mr of NaCl correctly?

Which statement describes how to calculate the Mr of NaCl correctly?

• A. Mr NaCl is equal to Ar Na plus Ar Cl = 58.5.
• B. Mr NaCl is equal to Ar Na multiplied by Ar Cl.
• C. Mr NaCl is the product of Ar values divided by the sum.
• D. Mr NaCl is the number of moles.

Answer: (A)

The key idea is that the relative formula mass (Mr) is found by adding the relative atomic masses of every atom in the formula unit. For sodium chloride there is one sodium atom and one chlorine atom, so you add Ar of sodium and Ar of chlorine. Using the standard values, Ar Na = 23 and Ar Cl = 35.5, giving 23 + 35.5 = 58.5. This is why the statement describing Mr NaCl as Ar Na plus Ar Cl is the correct approach. It isn’t about multiplying the masses, dividing them, or counting moles—the Mr is a sum of the constituent atoms’ Ar values.

The key idea is that the relative formula mass (Mr) is found by adding the relative atomic masses of every atom in the formula unit. For sodium chloride there is one sodium atom and one chlorine atom, so you add Ar of sodium and Ar of chlorine. Using the standard values, Ar Na = 23 and Ar Cl = 35.5, giving 23 + 35.5 = 58.5. This is why the statement describing Mr NaCl as Ar Na plus Ar Cl is the correct approach. It isn’t about multiplying the masses, dividing them, or counting moles—the Mr is a sum of the constituent atoms’ Ar values.