Molar Mass Calculator
Calculate molecular weight of any compound
How to Calculate Molar Mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in the molecular formula.
Enter any chemical formula such as H2O, NaCl, H2SO4, C6H12O6, or Ca(OH)2. The calculator supports parenthetical grouping and any element from the periodic table.
Why Molar Mass Matters
Molar mass is a fundamental concept in chemistry that bridges the gap between the atomic scale and the laboratory scale. It allows chemists to convert between the number of moles of a substance and its mass in grams, making it essential for preparing solutions, performing stoichiometric calculations, and analyzing experimental data.
For example, knowing that water (H2O) has a molar mass of 18.015 g/mol means that 18.015 grams of water contains exactly one mole, or approximately 6.022 × 10²³ molecules. This conversion factor is used constantly in quantitative chemistry.
Common Examples
- Water (H2O): 18.015 g/mol — two hydrogen atoms (1.008 each) plus one oxygen atom (15.999).
- Sodium chloride (NaCl): 58.44 g/mol — common table salt.
- Glucose (C6H12O6): 180.16 g/mol — a simple sugar used in cellular respiration.
- Sulfuric acid (H2SO4): 98.08 g/mol — a strong acid widely used in industry.
- Calcium carbonate (CaCO3): 100.09 g/mol — found in limestone and seashells.
Element-by-Element Breakdown
This calculator provides a detailed breakdown showing each element's atomic mass, the number of atoms, total mass contribution, and percentage composition. This information is valuable for understanding the composition of compounds and for gravimetric analysis calculations.
Worked example: calcium hydroxide, Ca(OH)2
Parentheses are where most molar mass errors start, so this example uses them deliberately. The subscript outside the bracket multiplies everything inside it.
- Expand the formula. Ca(OH)2 contains one calcium atom, two oxygen atoms, and two hydrogen atoms. The 2 applies to both the O and the H, not to the O alone.
- Look up each atomic mass. Calcium is 40.078, oxygen is 15.999, and hydrogen is 1.008, all in grams per mole.
- Multiply each by its atom count. Calcium gives 40.078. Oxygen gives 2 times 15.999, which is 31.998. Hydrogen gives 2 times 1.008, which is 2.016.
- Add the contributions together. 40.078 plus 31.998 plus 2.016 gives 74.092.
Answer. The molar mass of Ca(OH)2 is 74.09 g/mol. Calcium alone accounts for about 54 percent of that mass, which is the kind of figure a gravimetric analysis depends on.
Common mistakes
These are the errors that come up most often, and each one changes the answer rather than merely looking untidy.
Applying a bracket subscript to only one element
In Ca(OH)2 the subscript multiplies every atom inside the parentheses. Reading it as one oxygen and two hydrogens gives 58.09 g/mol instead of 74.09, an error of more than 20 percent that then propagates through every calculation built on it.
Confusing a subscript with a coefficient
The 2 in H2O is a subscript and is part of the compound. The 2 in 2H2O is a coefficient and means two separate molecules. Molar mass uses the subscripts only. A coefficient belongs to the balanced equation, not to the formula weight.
Rounding atomic masses too early
Rounding oxygen to 16 rather than 15.999 seems harmless, but in a large molecule with many oxygens the error accumulates. Carry the full atomic masses through the sum and round only the final answer to the precision your data justifies.
Forgetting the water in a hydrate
Copper sulfate pentahydrate is written CuSO4 times 5H2O and its molar mass includes all five water molecules. Using the anhydrous value of 159.6 g/mol instead of 249.7 g/mol will make every solution you prepare from it too concentrated.
Frequently asked questions
Is molar mass the same as molecular weight?
They are numerically identical, which is why the terms are often used interchangeably, but they are not the same quantity. Molecular weight is a ratio of masses and is dimensionless. Molar mass is the mass of one mole and carries units of grams per mole. For calculations that convert between grams and moles you want molar mass.
How do I calculate the molar mass of a hydrate?
Treat the water of crystallisation as part of the formula. Calculate the molar mass of the anhydrous compound, calculate the molar mass of water as 18.015 g/mol, multiply that by the number of water molecules, then add the two together.
What is percentage composition and how is it found?
Percentage composition is the fraction of a compound's total mass contributed by each element. Divide the total mass of one element in the formula by the compound's molar mass and multiply by 100. This calculator shows that breakdown for every element automatically.
Does the calculator use average atomic masses or isotopes?
It uses standard IUPAC average atomic weights, which reflect the natural isotopic abundance of each element. That is what a periodic table in a textbook gives you and what nearly all laboratory work assumes. Isotopically enriched material would need a different value.