How do I calculate molecular weight?

For each element in the formula, multiply its atomic weight by the number of atoms, then add them all up. For glucose C6H12O6, rounded values give 12 × 6 + 1.0 × 12 + 16 × 6 = 180, and precise values give 180.156. With parentheses, multiply the atoms inside by the number after the bracket: Ca(OH)2 has one Ca and two each of O and H, so 40 + (16 + 1.0) × 2 = 74.

What is the formula weight of copper(II) sulfate pentahydrate, CuSO4·5H2O?

With rounded values (Cu=63.5, S=32, O=16, H=1.0) it is 63.5 + 32 + 16 × 4 + 18 × 5 = 249.5, and with IUPAC atomic weights 249.68. The five waters of crystallization (about 90) make up about 36% of the total. Typing “CuSO4.5H2O” or “CuSO4*5H2O” is also read as the hydrate.

How many grams do I need to make 500 mL of a 0.1 mol/L solution?

Mass needed = molarity × volume (L) × molar mass. For sodium chloride (58.44 g/mol), weigh 0.1 × 0.5 × 58.44 = 2.92 g, dissolve it in water and make it up to 500 mL in a volumetric flask. For 250 mL of 0.1 mol/L copper(II) sulfate pentahydrate, it is 0.1 × 0.25 × 249.68 = 6.24 g. For a reagent that is 95% pure, divide by 0.95.

What is the molarity of 98% concentrated sulfuric acid?

Taking the density as 1.84 g/cm³, 1 L of solution (1840 g) contains 1840 × 0.98 = 1803 g of sulfuric acid; dividing by the molar mass 98.07 gives about 18.4 mol/L. To make 100 mL of 1 mol/L sulfuric acid, take about 5.4 mL of the concentrated acid, add it slowly to water, and make it up to 100 mL. 37% hydrochloric acid with density 1.19 is about 12.1 mol/L.

How do I find an empirical formula?

Divide each element’s mass percent (or mass) by its atomic weight, then divide by the smallest result to get a whole-number ratio. For C 40.0%, H 6.7%, O 53.3%: 40.0 ÷ 12 : 6.7 ÷ 1.0 : 53.3 ÷ 16 ≈ 3.33 : 6.7 : 3.33 = 1 : 2 : 1, so the empirical formula is CH2O; with a molar mass of 180, 180 ÷ 30 = 6, so the molecular formula is C6H12O6.

How do I get the empirical formula from the masses of CO2 and H2O in combustion analysis?

Multiply the mass of CO2 by 12/44 for the mass of carbon and the mass of H2O by 2/18 for the mass of hydrogen; what is left of the sample mass after subtracting both is oxygen. If burning 23.0 mg of a sample gives 44.0 mg of CO2 and 27.0 mg of H2O, that is C 12.0 mg, H 3.0 mg and O 8.0 mg, a mole ratio of 1 : 3 : 0.5 = 2 : 6 : 1, so the empirical formula is C2H6O.

Molar Mass Calculator | Molecular Weight, Molar Mass and Mass Percent of Each Element from a Chemical Formula, Grams–Moles–Particles–Gas Volume Conversion, How to Make a Molar Solution and Convert Mass Percent, Empirical and Molecular Formula, With an Atomic Weight Table

The Molar Mass Calculator takes a chemical formula and shows the molecular weight (formula weight), the molar mass in g/mol, and for each element the number of atoms, atomic weight, subtotal and mass percent (composition). Besides simple formulas such as H2O, NaCl and C6H12O6, it reads parentheses as in Ca(OH)2 and Al2(SO4)3, complex ions such as [Cu(NH3)4]SO4, hydrates such as CuSO4·5H2O and Na2CO3·10H2O (written with “·”, “.” or “*”) and ions such as SO4^2-, and it also accepts full-width characters and subscript digits. You can switch between IUPAC standard atomic weights (abridged values) and the rounded values used in Japanese high-school textbooks and entrance exams (H=1.0, C=12, N=14, O=16, Na=23, S=32, Cl=35.5, Cu=63.5), and for a hydrate it shows the share of water and the formula weight of the anhydrous salt. It converts between grams, moles, number of particles (Avogadro constant 6.02 × 10²³) and gas volume (22.4 L at 0 °C and 1 atm, 24.5 L at 25 °C), works out how many grams to weigh for, say, 500 mL of a 0.1 mol/L solution (with reagent purity), converts mass percent and density such as 98% sulfuric acid with density 1.84 into molarity and molality, and calculates how much stock solution to dilute. It finds the empirical formula from the mass percent of each element or from combustion analysis (masses of CO2 and H2O), and the molecular formula from the molar mass. Includes an atomic weight table (periodic table) of all 118 elements.

Calculate molecular weight (molar mass) from a chemical formula

Enter a chemical formula to get the molecular weight (formula weight), molar mass in g/mol, and the breakdown and mass percent of each element. Hydrates, brackets and complex ions are supported.

Chemical formula

Examples: H2O, Ca(OH)2, [Cu(NH3)4]SO4, CuSO4·5H2O (write hydrates with “·”, “.” or “*”), SO4^2-. Full-width characters and subscript digits also work.

Common substances

Atomic weights to use

Molecular weight (formula weight)

249.677

No unit (relative mass)

Molar mass

249.677 g/mol

1 mol weighs 249.677 g

Atoms per formula unit

Composition CuH10O9S

Element	Atoms	Atomic weight	Subtotal	Mass percent
Cu	1	63.546	63.546	25.45%
S	1	32.06	32.06	12.84%
O	9	15.999	143.991	57.67%
H	10	1.008	10.08	4.04%

Calculation: 63.546×1 + 32.06×1 + 15.999×9 + 1.008×10 = 249.677

The 5 waters of crystallization (90.075) are 36.08% of the total, and the anhydrous salt’s formula weight is 159.602. 1 g of anhydrous salt corresponds to 1.5644 g of the hydrate.

Convert grams, moles, particles and gas volume

Enter any one to convert it to the others. 1 mol = molecular weight in g = 6.02 × 10²³ particles = 22.4 L for a gas (0 °C, 1 atm).

Amount

Unit

Amount of substance

0.04005 mol

= 40.0517 mmol

Mass

10 g

= 10,000 mg

Number of particles (molecules or ions)

2.412 × 10²²

5.065 × 10²³ atoms

Gas volume (0 °C, 1 atm (STP))

0.8977 L

Taking 1 mol of gas = 22.414 L. Does not apply to solids or liquids

Everything is calculated on your device. For lab work, check the reagent purity and label values and the safety data sheet (SDS).

ABOUT

About Molar Mass Calculator | Molecular Weight, Molar Mass and Mass Percent of Each Element from a Chemical Formula, Grams–Moles–Particles–Gas Volume Conversion, How to Make a Molar Solution and Convert Mass Percent, Empirical and Molecular Formula, With an Atomic Weight Table

The Molar Mass Calculator is a free tool that shows the molecular weight (formula weight), molar mass in g/mol and mass percent of each element as soon as you type a chemical formula. It reads hydrates such as CuSO4·5H2O, parentheses and complex ions, and switches between IUPAC atomic weights and rounded school values. It also converts grams, moles, particles and gas volume, works out how to make a molar solution, converts mass percent concentration, and finds empirical and molecular formulas.

How to use Molar Mass Calculator | Molecular Weight, Molar Mass and Mass Percent of Each Element from a Chemical Formula, Grams–Moles–Particles–Gas Volume Conversion, How to Make a Molar Solution and Convert Mass Percent, Empirical and Molecular Formula, With an Atomic Weight Table

1On the “Molar mass & moles” tab, enter a chemical formula such as “H2O”, “Ca(OH)2” or “CuSO4·5H2O” (or pick a common substance from the buttons)
2Choose “Rounded school values” to match a textbook or problem set, or “Precise (IUPAC)” for lab work, and check the molecular weight, molar mass and the breakdown by element
3Enter any one of grams, moles, particles or gas volume to convert it into the others
4To make a solution, open the “Solution prep & concentration” tab and get the mass to weigh from the molarity and final volume (and the reagent purity)
5If you have mass percents or combustion analysis results, open the “Empirical & molecular formula” tab to find the empirical formula, then enter the molar mass to get the molecular formula

Great for

Checking molecular weight or formula weight calculations in chemistry homework and problem sets
Finding the formula weight of a hydrate such as copper(II) sulfate pentahydrate and its share of water
Knowing how many grams to weigh to make 500 mL of a 0.1 mol/L solution
Working out the molarity of concentrated sulfuric or hydrochloric acid from its mass percent and density, and how much to dilute
Finding the empirical and molecular formula from combustion analysis (masses of CO2 and H2O) or from mass percents
Checking conversions between moles and grams, number of molecules, and gas volume (22.4 L)
Finding the mass percent of an element in a fertilizer, supplement or chemical

Good to know

The molecular weight is the sum of the atomic weights of all the atoms in the formula. H2O is 1.008 × 2 + 15.999 = 18.015, or 18 with rounded school values (H=1.0, O=16). Ionic compounds such as NaCl have no molecules, so the term is “formula weight”, but it is calculated the same way. Molar mass is the mass of 1 mol; it has the same number as the molecular weight, with the unit g/mol.
For a hydrate, the water of crystallization is added too. CuSO4·5H2O is 249.68 with precise values (249.5 with rounded values), and about 36.1% of it is water. Anhydrous CuSO4 is 159.60, so the same amount in moles weighs very differently for the anhydrous salt and the hydrate.
You can switch between IUPAC standard atomic weights (abridged values) and the rounded values often given in Japanese high-school textbooks and exam questions (H=1.0, C=12, N=14, O=16, Na=23, Mg=24, Al=27, S=32, Cl=35.5, K=39, Ca=40, Fe=56, Cu=63.5, Zn=65.4, Ag=108). When solving a problem, use the atomic weights given in the question.
Molarity (mol/L) is the amount of solute in moles per litre of solution. To make a solution, weigh the solute, dissolve it in a little water, and make it up to the mark in a volumetric flask (it is the final volume, not the volume of water). From a mass percent w% and density d g/cm³, the molarity is 1000 × d × w/100 ÷ molar mass, so 98% sulfuric acid with density 1.84 is about 18.4 mol/L.
The empirical formula is the simplest whole-number ratio of the amounts in moles, found by dividing each element’s mass by its atomic weight. When a ratio comes out like 1.5 or 1.33, multiply by 2 or 3 to make it whole. The molecular formula is a whole-number multiple of the empirical formula, and molar mass ÷ empirical formula weight tells you which multiple.

Questions this tool answers

Use Molar Mass Calculator | Molecular Weight, Molar Mass and Mass Percent of Each Element from a Chemical Formula, Grams–Moles–Particles–Gas Volume Conversion, How to Make a Molar Solution and Convert Mass Percent, Empirical and Molecular Formula, With an Atomic Weight Table for searches like these:

Calculate molecular weight, formula weight or molar mass from a chemical formula
Know the formula weight of a hydrate such as CuSO4·5H2O
Convert between grams and moles, number of molecules and gas volume
Know how many grams to dissolve to make a molar solution
Convert between mass percent concentration and molarity
Find the empirical and molecular formula from mass percents or combustion analysis

FAQ

How do I calculate molecular weight?: For each element in the formula, multiply its atomic weight by the number of atoms, then add them all up. For glucose C6H12O6, rounded values give 12 × 6 + 1.0 × 12 + 16 × 6 = 180, and precise values give 180.156. With parentheses, multiply the atoms inside by the number after the bracket: Ca(OH)2 has one Ca and two each of O and H, so 40 + (16 + 1.0) × 2 = 74.
What is the formula weight of copper(II) sulfate pentahydrate, CuSO4·5H2O?: With rounded values (Cu=63.5, S=32, O=16, H=1.0) it is 63.5 + 32 + 16 × 4 + 18 × 5 = 249.5, and with IUPAC atomic weights 249.68. The five waters of crystallization (about 90) make up about 36% of the total. Typing “CuSO4.5H2O” or “CuSO4*5H2O” is also read as the hydrate.
How many grams do I need to make 500 mL of a 0.1 mol/L solution?: Mass needed = molarity × volume (L) × molar mass. For sodium chloride (58.44 g/mol), weigh 0.1 × 0.5 × 58.44 = 2.92 g, dissolve it in water and make it up to 500 mL in a volumetric flask. For 250 mL of 0.1 mol/L copper(II) sulfate pentahydrate, it is 0.1 × 0.25 × 249.68 = 6.24 g. For a reagent that is 95% pure, divide by 0.95.
What is the molarity of 98% concentrated sulfuric acid?: Taking the density as 1.84 g/cm³, 1 L of solution (1840 g) contains 1840 × 0.98 = 1803 g of sulfuric acid; dividing by the molar mass 98.07 gives about 18.4 mol/L. To make 100 mL of 1 mol/L sulfuric acid, take about 5.4 mL of the concentrated acid, add it slowly to water, and make it up to 100 mL. 37% hydrochloric acid with density 1.19 is about 12.1 mol/L.
How do I find an empirical formula?: Divide each element’s mass percent (or mass) by its atomic weight, then divide by the smallest result to get a whole-number ratio. For C 40.0%, H 6.7%, O 53.3%: 40.0 ÷ 12 : 6.7 ÷ 1.0 : 53.3 ÷ 16 ≈ 3.33 : 6.7 : 3.33 = 1 : 2 : 1, so the empirical formula is CH2O; with a molar mass of 180, 180 ÷ 30 = 6, so the molecular formula is C6H12O6.
How do I get the empirical formula from the masses of CO2 and H2O in combustion analysis?: Multiply the mass of CO2 by 12/44 for the mass of carbon and the mass of H2O by 2/18 for the mass of hydrogen; what is left of the sample mass after subtracting both is oxygen. If burning 23.0 mg of a sample gives 44.0 mg of CO2 and 27.0 mg of H2O, that is C 12.0 mg, H 3.0 mg and O 8.0 mg, a mole ratio of 1 : 3 : 0.5 = 2 : 6 : 1, so the empirical formula is C2H6O.

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