How do you calculate the Mr of NaCl?

How do you calculate the Mr of NaCl?

• A. Mr NaCl is equal to Ar Na multiplied by Ar Cl.
• B. Mr NaCl is equal to Ar Na plus Ar Cl, but in grams.
• C. Mr NaCl equals the product of Ar values divided by the sum.
• D. Mr NaCl equals Ar(Na) plus Ar(Cl) = 58.5.

Answer: (D)

Relative formula mass is found by adding the relative atomic masses of all atoms in a formula, taking into account how many of each atom are present. For NaCl there is one sodium atom and one chlorine atom. Ar for sodium is about 23 and Ar for chlorine is about 35.5. Adding them gives 23 + 35.5 = 58.5. So the Mr of NaCl is 58.5. This value is a dimensionless relative mass; if you were calculating molar mass, you’d express it in g/mol. The other ideas try to multiply or otherwise combine the Ar values or treat the result as grams, which isn’t how Mr is defined.

Relative formula mass is found by adding the relative atomic masses of all atoms in a formula, taking into account how many of each atom are present. For NaCl there is one sodium atom and one chlorine atom. Ar for sodium is about 23 and Ar for chlorine is about 35.5. Adding them gives 23 + 35.5 = 58.5. So the Mr of NaCl is 58.5. This value is a dimensionless relative mass; if you were calculating molar mass, you’d express it in g/mol. The other ideas try to multiply or otherwise combine the Ar values or treat the result as grams, which isn’t how Mr is defined.