Molar Mass Calculator

Q: What is molar mass?

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). One mole contains 6.022 x 10^23 particles (Avogadro's number). For elements, molar mass equals the relative atomic mass in g/mol. For compounds, it is the sum of the molar masses of all atoms in the molecular formula. For example, water (H2O) has molar mass = 2(1.008) + 15.999 = 18.015 g/mol.

Q: How do you calculate molar mass from a chemical formula?

To calculate molar mass: (1) Write out the chemical formula. (2) Identify each element and how many atoms of it are present. (3) Multiply the number of atoms by the atomic mass of each element. (4) Sum all these values. Example: CaCO3 = Ca(40.078) + C(12.011) + 3 x O(15.999) = 40.078 + 12.011 + 47.997 = 100.086 g/mol.

Q: What is the molar mass of water (H2O)?

The molar mass of water (H2O) is 18.015 g/mol. This is calculated as: 2 hydrogen atoms x 1.008 g/mol + 1 oxygen atom x 15.999 g/mol = 2.016 + 15.999 = 18.015 g/mol. This means one mole of water (6.022 x 10^23 molecules) has a mass of 18.015 grams.

Q: What is the difference between molar mass and molecular weight?

Molar mass and molecular weight are often used interchangeably, but they differ slightly. Molecular weight (or relative molecular mass, Mr) is dimensionless — it is the ratio of the mass of one molecule to 1/12 the mass of a carbon-12 atom. Molar mass has units of g/mol. Numerically they are equal: the Mr of CO2 is 44.01, and its molar mass is 44.01 g/mol.

Q: How is molar mass used in calculations?

Molar mass is essential for converting between grams and moles: moles = mass (g) / molar mass (g/mol), and mass = moles x molar mass. It is used in stoichiometry to find reacting masses, in concentration calculations (mol/L), and in gas law problems. For example, to find how many moles in 36 g of water: 36 / 18.015 = 2 moles.

Q: What is Avogadro's number?

Avogadro's number (constant) is 6.022 x 10^23 per mole. It represents the number of atoms, molecules, or ions in one mole of a substance. It was named after Italian scientist Amedeo Avogadro. This constant links the macroscopic world (grams, litres) to the atomic scale (individual atoms and molecules), making it a cornerstone of quantitative chemistry.

Q: Can I calculate molar mass for ionic compounds?

Yes. For ionic compounds like NaCl, use the formula unit (not a molecule) and sum the atomic masses of all ions. NaCl = Na(22.990) + Cl(35.453) = 58.443 g/mol. For compounds with polyatomic ions in brackets, multiply everything inside the bracket by the subscript outside. Example: Ca(OH)2 = Ca + 2O + 2H = 40.078 + 2(15.999) + 2(1.008) = 74.092 g/mol.

Chemical Formula Molar Mass Calculator

Type a chemical formula. Use capital letters for element symbols, numbers for subscripts, and parentheses for groups (e.g. Ca(OH)2).

Chemical Formula

Molar Mass of :

g/mol (grams per mole)

Element Breakdown:

Element	Symbol	Count	Atomic Mass	Contribution

Atomic Mass Reference (Common Elements)

Symbol	Element	Atomic No.	Atomic Mass (g/mol)
H	Hydrogen	1	1.008
He	Helium	2	4.003
Li	Lithium	3	6.941
Be	Beryllium	4	9.012
B	Boron	5	10.811
C	Carbon	6	12.011
N	Nitrogen	7	14.007
O	Oxygen	8	15.999
F	Fluorine	9	18.998
Ne	Neon	10	20.180
Na	Sodium	11	22.990
Mg	Magnesium	12	24.305
Al	Aluminium	13	26.982
Si	Silicon	14	28.086
P	Phosphorus	15	30.974
S	Sulfur	16	32.065
Cl	Chlorine	17	35.453
Ar	Argon	18	39.948
K	Potassium	19	39.098
Ca	Calcium	20	40.078
Fe	Iron	26	55.845
Cu	Copper	29	63.546
Zn	Zinc	30	65.38
Br	Bromine	35	79.904
Ag	Silver	47	107.868
I	Iodine	53	126.904
Au	Gold	79	196.967
Pb	Lead	82	207.2

Understanding Molar Mass and the Mole

Molar mass is one of the most important quantities in quantitative chemistry. It connects the mass of a substance (something we can measure on a balance) to the number of particles it contains (something far too small to count directly). This connection is fundamental to stoichiometry, reaction calculations, and concentration work at both GCSE and A-Level chemistry.

What Is a Mole?

A mole is the SI unit of amount of substance. One mole contains exactly 6.022 × 10²³ particles (atoms, molecules, ions, or formula units) — this is Avogadro's number, N_A. The mole allows chemists to count enormous numbers of particles by simply weighing them.

One mole of any element equals its relative atomic mass in grams. One mole of carbon-12 atoms has a mass of exactly 12 g.

How to Calculate Molar Mass

For any chemical formula: (1) Identify each element and count the atoms of each. (2) Look up the relative atomic mass of each element. (3) Multiply each atomic mass by the count. (4) Sum all contributions.

Example: Glucose C₆H₁₂O₆

Carbon: 6 × 12.011 = 72.066 g/mol
Hydrogen: 12 × 1.008 = 12.096 g/mol
Oxygen: 6 × 15.999 = 95.994 g/mol
Total: 180.156 g/mol

Molar Mass with Parentheses

When a formula contains parentheses, the subscript outside multiplies everything inside. For example, Ca(OH)₂: the OH group appears twice, giving 2 oxygen and 2 hydrogen atoms in addition to 1 calcium. Ca(OH)₂ = Ca + 2O + 2H = 40.078 + 2(15.999) + 2(1.008) = 74.092 g/mol.

Using Molar Mass in Calculations

The key relationship is: n = m / M, where n is amount in moles, m is mass in grams, and M is molar mass in g/mol. Rearranging: m = n × M and M = m / n.

Examples of this in practice:

How many moles in 44 g of CO₂ (M = 44.01)? n = 44 / 44.01 = 1.00 mol
What mass of NaCl contains 0.5 mol? m = 0.5 × 58.44 = 29.22 g
How many molecules in 18 g H₂O? n = 18 / 18.02 = 1 mol = 6.02 × 10²³ molecules

Molar Mass in Concentration Calculations

Concentration is often expressed in mol/L (molarity). To make a 1 M solution of NaCl (M = 58.44 g/mol): dissolve 58.44 g in enough water to make 1 litre. To find concentration from mass: c = m / (M × V), where V is volume in litres. These calculations appear throughout A-Level chemistry, particularly in acid-base and redox titration problems.

Empirical vs Molecular Formula

The empirical formula gives the simplest whole-number ratio of atoms (e.g. CH₂O for glucose). The molecular formula gives the actual numbers (C₆H₁₂O₆ for glucose).

The ratio of molecular mass to empirical formula mass gives the multiplier: 180.156 / 30.026 = 6. This calculator works with molecular (and ionic) formulas as written.

Relative Atomic Mass and Isotopes

The atomic masses used in molar mass calculations are weighted averages of all naturally occurring isotopes of each element, taking into account their natural abundance. This is why the atomic mass of chlorine is 35.453 g/mol (not a whole number), reflecting the natural mixture of Cl-35 (75.77%) and Cl-37 (24.23%). For GCSE and A-Level, you use the values given on the periodic table provided in examinations.

Common Molar Masses for GCSE and A-Level

Knowing these saves time in exams: H₂O = 18 g/mol, CO₂ = 44 g/mol, NaCl = 58.5 g/mol, HCl = 36.5 g/mol, NaOH = 40 g/mol, CaCO₃ = 100 g/mol, H₂SO₄ = 98 g/mol, NH₃ = 17 g/mol, CH₄ = 16 g/mol. In UK exams (AQA, OCR, Edexcel), relative atomic masses are given on the data sheet, so precision is not required from memory.

Frequently Asked Questions

How the Molar Mass Calculator Works

This calculator uses established health formulas and UK-specific reference ranges to provide useful estimates. While online calculators are helpful for general guidance, they should not replace professional medical advice. Always consult your GP or a qualified health professional for personalised health assessments.

UK health guidelines are published by the NHS, Public Health England, and NICE (National Institute for Health and Care Excellence). This tool aligns with these official guidelines where applicable, providing results relevant to the UK population.

Key Information

The NHS recommends at least 150 minutes of moderate-intensity activity per week for adults, or 75 minutes of vigorous activity. A healthy BMI range for adults is 18.5 to 24.9. The UK Chief Medical Officers advise that both men and women should not regularly drink more than 14 units of alcohol per week. Calorie guidance suggests approximately 2,000 kcal per day for women and 2,500 kcal for men, though individual needs vary.

Example Calculation

A 30-year-old female who is 165cm tall and weighs 65kg would have a BMI of 23.9, which falls within the healthy range. Her estimated Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation would be approximately 1,387 kcal per day, rising to around 1,910 kcal with moderate activity.

Source: Based on NHS and Public Health England guidelines. Last updated March 2026.

What is molar mass? +

Molar mass is the mass of one mole of a substance (g/mol). One mole = 6.022 × 10²³ particles (Avogadro's number). For elements, molar mass equals the relative atomic mass in g/mol. For compounds, sum the molar masses of all atoms in the formula. Example: H₂O = 2(1.008) + 15.999 = 18.015 g/mol.

How do you calculate molar mass from a chemical formula? +

1. Identify each element and atom count. 2. Multiply atomic mass × count for each element. 3. Sum all values. Example: CaCO₃ = Ca(40.078) + C(12.011) + 3×O(15.999) = 40.078 + 12.011 + 47.997 = 100.086 g/mol.

What is the molar mass of water (H2O)? +

Molar mass of H₂O = 2(1.008) + 15.999 = 18.015 g/mol. This means one mole of water (6.022 × 10²³ molecules) has a mass of 18.015 grams, which is approximately 18 mL of liquid water.

What is the difference between molar mass and molecular weight? +

Molecular weight (relative molecular mass, M_r) is dimensionless; molar mass has units of g/mol. Numerically they are equal: M_r of CO₂ = 44.01; molar mass of CO₂ = 44.01 g/mol. In UK exams, "molar mass" and "M_r" are often used interchangeably.

How is molar mass used in calculations? +

Key formula: n = m / M (moles = mass / molar mass). Also: m = n × M and M = m / n. Example: 36 g of water = 36 / 18.015 = 2.00 mol. Essential for stoichiometry, titration, and gas law problems.

What is Avogadro's number? +

Avogadro's number is 6.022 × 10²³ mol^-1. It is the number of particles in exactly one mole of any substance. It links macroscopic mass measurements to atomic-scale particle counts, making quantitative chemistry practical.

Can I calculate molar mass for ionic compounds? +

Yes. Sum the atomic masses of all ions in the formula unit. NaCl = 22.990 + 35.453 = 58.443 g/mol. For bracketed groups, multiply inside by the subscript: Ca(OH)₂ = 40.078 + 2(15.999) + 2(1.008) = 74.092 g/mol.

Mustafa Bilgic — Chemistry & Science Content Author
Specialist in UK A-Level and GCSE Chemistry. Atomic mass data sourced from IUPAC 2021 standard atomic weights. All worked examples verified for AQA, OCR, and Edexcel syllabuses.