How do you solve for relative formula mass?

The relative formula mass (Mr) of a compound is calculated by adding up the relative atomic masses of all the atoms present in the chemical formula of that compound.

Understanding Relative Formula Mass

Relative formula mass (Mr), sometimes referred to as formula weight, indicates the mass of one molecule (or formula unit for ionic compounds) relative to the mass of a carbon-12 atom. Here’s how to calculate it:

Steps to Calculate Relative Formula Mass:

1. Identify the Chemical Formula: Start with the correct chemical formula of the compound (e.g., CaCO3, H2O).
2. Find Relative Atomic Masses: Use the periodic table to find the relative atomic mass (Ar) of each element in the formula.
3. Multiply by Subscripts: For each element, multiply its Ar by the subscript (the number after the element symbol) in the chemical formula, which represents the number of atoms of that element present.
4. Sum the Results: Add up the results from the multiplication steps for all elements in the formula. This total is the Mr.

Example Calculation: CaCO3

Let's calculate the relative formula mass (Mr) of calcium carbonate (CaCO3), as provided in the reference:

• Calcium (Ca): Ar = 40
• Carbon (C): Ar = 12
• Oxygen (O): Ar = 16

Using the steps above:

1. There is one atom of calcium, so its mass contribution is 1 x 40 = 40.
2. There is one atom of carbon, so its mass contribution is 1 x 12 = 12.
3. There are three atoms of oxygen, so their combined mass contribution is 3 x 16 = 48.
4. Add all these together: 40 + 12 + 48 = 100.

Therefore, the relative formula mass (Mr) of CaCO3 is 100.

Example Calculation: Water (H2O)

Another example for practice:

• Hydrogen (H): Ar = 1
• Oxygen (O): Ar = 16

Using the steps:

1. There are two atoms of hydrogen, so its mass contribution is 2 x 1 = 2.
2. There is one atom of oxygen, so its mass contribution is 1 x 16 = 16.
3. Add them together: 2 + 16 = 18.

Therefore, the relative formula mass (Mr) of H2O is 18.

Practical Insights:

• The Mr is a dimensionless quantity, often followed by the unit g/mol when referring to molar mass.
• The relative formula mass is a fundamental concept for stoichiometry calculations in chemistry, such as those used to convert from grams to moles and vice versa.
• Ensure that you use the correct subscripts from the chemical formula and find the relative atomic masses from the periodic table.