How Do You Calculate Molar Mass?

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Here, $n_i$ is the number of atoms of element $i$, and $A_{\text{r},i}$ is its relative atomic mass, obtained from the periodic table in the Chemistry data booklet.

Follow three steps. First, read the formula carefully and count the atoms of each element; a missing subscript means one. Second, multiply each atom count by the appropriate $A_{\text{r}}$ value. Third, add the contributions and attach the molar-mass unit. Keep extra decimal places during working and round only the final answer.

For sulfuric acid, $\text{H}_2\text{SO}_4$:

Therefore:

The result represents the mass of one mole of sulfuric acid molecules.

Brackets in a formula multiply every atom inside them. For calcium nitrate, $\text{Ca}(\text{NO}_3)_2$, there is one Ca atom, two N atoms, and six O atoms:

Subscripts apply only to the element or bracketed group immediately before them. A coefficient placed before a formula instead multiplies the entire formula but does not change the molar mass of the substance itself.

A common misconception is that relative formula mass, $M_{\text{r}}$, and molar mass have identical units. They have the same numerical value, but $M_{\text{r}}$ is dimensionless, whereas molar mass has units of $\text{g mol}^{-1}$. For ionic compounds, the calculation is based on the empirical formula because they contain formula units rather than discrete molecules.

In an IB exam, show each multiplication before adding, especially when brackets or several elements are present. Molar mass is commonly used in Structure 1.4 calculations through $n = \frac{m}{M}$, so include correct units and rearrange the equation carefully. Do not add atomic numbers or round individual contributions too early because either error changes the final answer.