Molar Mass Calculator

Tip: drag a photo here or paste a screenshot (Ctrl+V)

How are solve allowances used?

Math, physics, chemistry and all AI tools share your plan allowance.

Each successful standard AI submission uses one standard solve. Deep reasoning uses one deep solve instead. Both count toward the daily solve limit.

Photos and selected PDF pages also use one daily photo solve per submission. Uploading a file or selecting a page does not start a solve. PDF upload requires Plus or Pro.

Local calculations, worked examples, hints, copying answers and cached answers use no solves.

Technical failures return the reserved solve. An automatic retry is included in the same submission. A new successful AI submission counts again, even if it follows an earlier answer.

Step-by-step hints

Try:

Worked example

Step-by-step solution

Find the molar mass of H2O using H = 1.008 and O = 15.999.

Step 1 / 3

Count atoms in water

Water contains two hydrogen atoms and one oxygen atom per molecule.

\mathrm{H_2O}:\quad n_{\mathrm{H}}=2,\quad n_{\mathrm{O}}=1

How to use the Molar Mass Calculator

This Molar Mass Calculator finds molar mass by adding the atomic-weight contributions in a chemical formula. State the atomic weights required by your course if its table uses different rounding.

Enter the chemical formula with correct capitalization and subscripts.
Expand parentheses and count the atoms of each element.
Multiply each count by its stated atomic weight and add contributions.
Report grams per mole with rounding consistent with the supplied weights.

Adding atomic-weight contributions

M(\mathrm{H_2O})=2M(\mathrm{H})+M(\mathrm{O})

Molar mass is the mass per mole of a substance. For a given formula, add each element’s atomic weight multiplied by the number of its atoms.

Atomic weights depend on the reference table and can vary with isotopic composition. Hydrates and parentheses need careful atom counting; specify an isotope when isotope-specific mass is intended.

Using the Molar Mass Calculator to count every atom correctly

Separate the formula, atomic weights and sample amount

Molar mass describes mass per mole of a specified substance. It is obtained by adding each element’s atomic-weight contribution according to the formula’s atom counts. The calculation requires both the formula and the atomic weights being used. A table rounded for a school exercise may give a slightly different result from a table with more decimal places, without changing the counting method. State the supplied values rather than mixing tables. Use grams per mole when the mass contributions are expressed on that conventional molar basis.

Do not confuse molar mass with the mass of a particular sample. Water’s molar mass using H = 1.008 and O = 15.999 is 18.015 g/mol, but a sample could contain any stated amount of water. Multiplying 2 mol by 18.015 g/mol gives sample mass 36.030 g before significant-figure rounding. Dividing that sample mass by the molar mass recovers 2 mol. These conversions use the same specified substance; applying a different compound’s molar mass would change the result even if the numerical units still cancel.

Expand parentheses before adding contributions

Consider calcium hydroxide, Ca(OH)2, with Ca = 40.078, O = 15.999 and H = 1.008 g/mol. The outside 2 multiplies both oxygen and hydrogen inside the parentheses. The formula therefore contains one calcium atom, two oxygen atoms and two hydrogen atoms. Its molar mass is 40.078 + 2(15.999) + 2(1.008), which equals 74.092 g/mol. Counting only two hydrogen atoms while leaving one oxygen atom would omit 15.999 g/mol and produce the wrong compound total.

Nested groups should be expanded systematically, one multiplier at a time. A coefficient in a reaction is different from a formula subscript: writing 3 Ca(OH)2 does not change the substance’s molar mass from 74.092 g/mol. It describes three formula units or, on a molar interpretation, three moles relative to the other reaction coefficients. If the question instead asks for the mass of that amount, multiply the number of moles by the molar mass. Keep the identity of the compound separate from the amount participating in a reaction.

Handle hydrates and case-sensitive element symbols

A hydrate’s dot notation adds a stated number of water units to the complete formula. For CuSO4·5H2O, use Cu = 63.546, S = 32.06, O = 15.999 and H = 1.008 g/mol. The CuSO4 contribution is 63.546 + 32.06 + 4(15.999) = 159.602 g/mol. Five water units contribute 5(18.015) = 90.075 g/mol. Their sum is 249.677 g/mol before further rounding. The complete hydrate contains nine oxygen atoms and ten hydrogen atoms per copper and sulfur atom.

Capitalization is equally important even when a formula looks short. With Co = 58.933 g/mol, the symbol Co refers to cobalt and has that elemental molar mass. In contrast, CO contains one carbon and one oxygen atom; using C = 12.011 and O = 15.999 gives 28.010 g/mol. An OCR substitution that changes a lowercase letter to uppercase can therefore change both the substance and the answer. Read the original notation, including parentheses and the hydrate multiplier, before accepting an automatically transcribed formula.

Check rounding, isotope information and the requested quantity

To verify a result, list each distinct element, its total atom count and its contribution. Sum the contributions independently and check that the final unit is mass per mole. For a hydrate, compare a direct expanded-atom count with the compound-plus-water calculation; both should agree using the same weights. Carry enough precision through those sums, then round as required by the supplied table and the exercise. The extra digits in a worked arithmetic example do not establish an experimental measurement’s uncertainty or a guaranteed precision for every physical sample.

Standard atomic weights describe a stated reference composition and may differ from isotope-specific masses. A question about an identified isotope needs the relevant isotope data, rather than an unexplained substitution of a standard table value. Mixtures and solutions generally need composition information to obtain a meaningful average or component molar mass. A single pure-compound formula should not be used as though it describes every component in a sample. Identify the requested substance and conversion first, then review the counting and assumptions in the displayed explanation. AI tools share your existing account allowance; a new standard or deep solve counts against its corresponding solve limit.

Frequently asked questions

Why can two tables give different answers?

Atomic weights and rounding conventions can differ. Use the values requested by your course and state them alongside the result.

How are parentheses handled?

Multiply every atom count inside parentheses by the following subscript. Ca(OH)2 contains one Ca, two O and two H atoms.

Is molar mass the same as sample mass?

No. Molar mass has units g/mol. A sample mass is found by multiplying molar mass by the number of moles.

Can I include a hydrate?

Include the water count and dot notation clearly, such as CuSO4·5H2O, and supply the atomic weights if exact course rounding matters.

Chemical Equation Balancer

Have a different kind of math problem? Try the free math solver. Use the subject menu for physics and chemistry.