ChemBench

Mass Percentage Calculator

What fraction of the mass is this element?

Chemical formula

Element symbol

Mass % of O

88.809%

What you entered

Formula: H2OElement: O

1
Mass of O in formula
15.999 × 1= 15.999 g/mol
2
Total molar mass
H2O= 18.015 g/mol
3
Mass percentage
15.999 ÷ 18.015 × 100= 88.8093%

Result

Mass % of O: 88.809%

O makes up 88.809% of the total mass of H2O.

Mass percent of an element in common compounds

Mass percent is the element's total mass in one mole of the compound, divided by the compound's molar mass.

Compound	Element	Element mass	Molar mass	Mass percent
H2O	O	15.999	18.015	88.81%
H2O	H	2.016	18.015	11.19%
CO2	C	12.011	44.009	27.29%
NaCl	Na	22.990	58.440	39.34%
C6H12O6	C	72.066	180.156	40.00%
H2SO4	S	32.060	98.072	32.69%
CaCO3	Ca	40.078	100.086	40.04%

Glucose is exactly 40.00% carbon by mass, which is a useful check: its empirical formula CH2O gives the same percentage at any molecular size.

Element mass divided by total mass

Mass percentage is simply (mass of the element in the formula ÷ total molar mass) × 100 — for water, oxygen makes up about 88.8% of the mass despite being just 1 of the 3 atoms.

Where this matters

Percent composition checks whether a proposed formula matches lab measurements, and is a required step before finding a compound's empirical formula from experimental mass data.

Frequently asked questions

What percentage of sulfuric acid (H2SO4) is oxygen by mass?

H2SO4 has a molar mass of about 98.08 g/mol. The 4 oxygens contribute 4 × 16.00 = 64.00 g/mol, so oxygen is 64.00/98.08 × 100 = 65.3% of the mass.

How is mass percentage different from the molar mass calculator?

The molar mass calculator gives you the total molecular weight. Mass percentage goes one step further and tells you what fraction of that total comes from a specific element — useful for checking experimental data against a proposed formula.

Can mass percentage exceed 100%?

No. Each element's mass percentage is its atomic mass contribution divided by the total molar mass, and all elements in a formula must sum to exactly 100%.