Question 1

What is molecular weight?

Accepted Answer

Molecular weight (also called molar mass) is the total mass of one mole of a compound, measured in grams per mole (g/mol). It's calculated by adding up the atomic masses of every atom in the compound's chemical formula. Water (H2O) has a molecular weight of about 18.015 g/mol because two hydrogen atoms (1.008 each) and one oxygen atom (15.999) add up to that total. Molecular weight is a fixed property of the compound's formula — it doesn't change with how much of the substance you have.

Question 2

How do you calculate molecular weight from a formula?

Accepted Answer

Break the formula into its individual elements and their subscripts, look up each element's standard atomic mass, multiply mass by subscript count for every element, then sum the results. For glucose (C6H12O6): carbon contributes 6 × 12.011 = 72.066, hydrogen contributes 12 × 1.008 = 12.096, and oxygen contributes 6 × 15.999 = 95.994 — added together, glucose's molecular weight is 180.156 g/mol. This calculator automates that arithmetic and shows the per-element breakdown as you type.

Question 3

What is the difference between molecular weight and molar mass?

Accepted Answer

In practice the two terms are used interchangeably and this calculator treats them the same way, but there's a technical distinction: molecular weight (relative molecular mass, Mr) is a dimensionless ratio compared to 1/12 the mass of carbon-12, while molar mass expresses that same number with units of grams per mole. Because both use the same numeric scale of atomic masses, a compound's molecular weight and molar mass share the same number — water's Mr is 18.015 and its molar mass is 18.015 g/mol — so most chemists use the two terms as synonyms outside strict metrology contexts.

Question 4

What is percent composition in chemistry?

Accepted Answer

Percent composition tells you what fraction of a compound's total mass comes from each element, calculated as (mass contributed by that element ÷ total molecular weight) × 100. In table salt (NaCl, MW 58.443 g/mol), sodium makes up 22.990 ÷ 58.443 × 100 = 39.34% of the mass and chlorine makes up the remaining 60.66%. Percent composition is used to verify a compound's identity in the lab, calculate how much of a target element a fertilizer or supplement delivers, and check yields in synthesis reactions.

Question 5

How do you convert mass to moles?

Accepted Answer

Divide the mass of your sample (in grams) by the compound's molecular weight (in g/mol): moles = mass ÷ MW. If you have 90 grams of glucose (MW 180.16 g/mol), that's 90 ÷ 180.16 ≈ 0.4996 moles. To go the other direction, multiply moles by molecular weight to get grams. The moles converter built into this calculator does this instantly for whatever formula you've entered — type a mass and it reports moles and total molecules.

Question 6

What is Avogadro's number?

Accepted Answer

Avogadro's number, 6.02214076 × 10²³, is the number of particles — atoms, molecules, or ions — contained in exactly one mole of a substance. It's the bridge between the atomic-scale world (individual atoms and molecules, far too small to weigh directly) and the macroscopic world you can measure on a lab scale. One mole of any compound, by definition, weighs exactly its molecular weight in grams and contains precisely Avogadro's number of molecules, regardless of which compound it is.

Question 7

What is an empirical formula vs molecular formula?

Accepted Answer

An empirical formula shows the simplest whole-number ratio of atoms in a compound, while a molecular formula shows the actual number of atoms in one molecule. Glucose's molecular formula is C6H12O6, but its empirical formula reduces to CH2O (dividing every subscript by 6). Both formulas describe the same ratio of elements, but only the molecular formula gives the true molecular weight — CH2O's formula weight of 30.03 g/mol is exactly one-sixth of glucose's actual 180.16 g/mol molecular weight.

Question 8

How do you handle parentheses in chemical formulas?

Accepted Answer

Everything inside a set of parentheses gets multiplied by the number immediately following the closing parenthesis, and if no number follows, the multiplier is 1. Calcium hydroxide, Ca(OH)2, expands to one calcium atom plus two oxygen atoms plus two hydrogen atoms, because the 2 outside the parentheses applies to both O and H inside. For nested groups like aluminum sulfate, Al2(SO4)3, the 3 multiplies everything inside — one sulfur and four oxygens — giving 3 sulfur atoms and 12 oxygen atoms total. This calculator's parser expands parentheses and brackets automatically, including nested groups.

Question 9

What is a hydrate in chemistry?

Accepted Answer

A hydrate is a crystalline compound that has water molecules locked into its structure in a fixed ratio, written with a raised dot between the anhydrous formula and the water count — for example, copper(II) sulfate pentahydrate is CuSO4·5H2O. Those five water molecules count toward the compound's total molecular weight: anhydrous CuSO4 weighs 159.61 g/mol, but adding 5 × 18.015 g/mol of water brings the hydrate's molecular weight to 249.69 g/mol. This calculator recognizes the dot (·) or period (.) notation and includes the water molecules in its breakdown.

Question 10

How does isotope affect molecular weight?

Accepted Answer

Standard atomic masses — the values this calculator uses — are already weighted averages across an element's naturally occurring isotopes, so an ordinary molecular weight calculation automatically reflects natural isotopic abundance; chlorine's atomic mass of 35.453 already accounts for the mix of chlorine-35 and chlorine-37 found in nature. If you're working with an isotopically enriched or labeled compound — like deuterated water, D2O, using hydrogen-2 instead of hydrogen-1 — you'd substitute that isotope's specific mass (2.014 for deuterium) instead of the natural-abundance average, which is why D2O's molecular weight (20.03 g/mol) is noticeably higher than ordinary water's 18.015 g/mol.

Symbol	Name	Atomic Mass (g/mol)	Atomic #	Common In
H	Hydrogen	1.008	1	Water, acids, organic compounds
C	Carbon	12.011	6	All organic molecules
N	Nitrogen	14.007	7	Proteins, DNA, fertilizers
O	Oxygen	15.999	8	Water, oxides, organic compounds
Na	Sodium	22.99	11	Table salt, sodium hydroxide
Mg	Magnesium	24.305	12	Chlorophyll, Epsom salt
Al	Aluminum	26.982	13	Alum, aluminum oxide
P	Phosphorus	30.974	15	DNA, ATP, fertilizers
S	Sulfur	32.06	16	Sulfuric acid, amino acids
Cl	Chlorine	35.453	17	Table salt, bleach, PVC
K	Potassium	39.098	19	Potassium chloride, fertilizers
Ca	Calcium	40.078	20	Limestone, gypsum, bone mineral
Fe	Iron	55.845	26	Rust, hemoglobin, steel
Cu	Copper	63.546	29	Copper sulfate, wiring
Zn	Zinc	65.38	30	Zinc oxide, galvanizing
Br	Bromine	79.904	35	Hydrobromic acid, flame retardants
Ag	Silver	107.868	47	Silver nitrate, photography
I	Iodine	126.904	53	Potassium iodide, tincture
Ba	Barium	137.327	56	Barium sulfate, fireworks
Pb	Lead	207.2	82	Lead-acid batteries, old paint

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