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Molar mass calculator

Molar mass in g/mol and mass percent composition of any chemical formula, with brackets and hydrates such as CuSO4·5H2O, from IUPAC 2021 atomic weights.

更新于 已验证的示例：6

Case matters: Co is cobalt, CO is carbon monoxide. Hydrates: CuSO4·5H2O, CuSO4.5H2O or CuSO4*5H2O
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摩尔质量
g/mol
摩尔质量: 180.156 g/mol
小数位数：3；取最近值，等距时远离零
Hill formula
C6H12O6
Mass percent
C 40.00%, H 6.71%, O 53.28%
Atoms per formula unit
24
Different elements
3
Mass of one formula unit
2.99156 × 10⁻²²g

One mole of C₆H₁₂O₆ weighs 180.156 g; oxygen contributes the most mass (53.28%).

Mass percent by element

180.156g/mol
C40.00%H6.71%O53.28%
Composition 行数：3
ElementAtomsAtomic weightMass (g/mol)Mass %
C (Carbon)612.01172.06640.00%
H (Hydrogen)121.008012.0966.71%
O (Oxygen)615.99995.99453.28%
计算方法 S

  1. Count the atoms

    C:6, H:12, O:6\mathrm{C}: 6,\ \mathrm{H}: 12,\ \mathrm{O}: 6

    Brackets multiply everything inside; a hydrate's leading number multiplies its whole part. Hill formula: C6H12O6.

  2. Add the atomic weights

    M=6×12.011+12×1.0080+6×15.999=180.156 g/molM = 6\times 12.011 + 12\times 1.0080 + 6\times 15.999 = 180.156\ \mathrm{g/mol}

    Shown to 3 decimals: the IUPAC abridged atomic weights carry about 5 significant figures, and some (Li, S, Cl, Pb…) fewer.

  3. Mass of one formula unit

    m=MNA=180.1566.02214076×1023=2.99156×10−22 gm = \frac{M}{N_A} = \frac{180.156}{6.02214076\times10^{23}} = 2.99156 \times 10^{-22}\ \mathrm{g}

关于Molar mass calculator

Molar mass is the mass of one mole of a substance, in grams per mole. The calculator reads the formula, counts the atoms of each element (multiplying through brackets and hydrate dots) and adds count × standard atomic weight, using the IUPAC 2021 abridged values. Each element's share of the total is its mass percent.

Chemists need molar mass to convert between grams and moles when weighing reagents. Glucose, C6H12O6, comes to 180.156 g/mol, of which oxygen supplies 53.28 %; a single molecule weighs 2.99 × 10⁻²² g.

Results are shown to three decimals because the abridged atomic weights carry about five significant figures. For elements with no standard atomic weight, such as technetium, the mass number of a long-lived isotope stands in, and the page says so.

计算示例

Water

Chemical formula
H2O
摩尔质量
18.015 g/mol
Mass percent
H 11.19%, O 88.81%
Atoms per formula unit
3

核验来源：Python 3.8 decimal with IUPAC 2021 abridged weights: 2 × 1.0080 + 15.999 = 18.015 (PubChem 18.015)

Glucose

Chemical formula
C6H12O6
摩尔质量
180.156 g/mol
Hill formula
C6H12O6

核验来源：Python 3.8 decimal: 6 × 12.011 + 12 × 1.0080 + 6 × 15.999 = 180.156 (PubChem 180.16)

Copper sulfate pentahydrate

Chemical formula
CuSO4·5H2O
摩尔质量
249.677 g/mol
Hill formula
CuH10O9S

核验来源：Python 3.8 decimal: 63.546 + 32.06 + 9 × 15.999 + 10 × 1.0080 = 249.677 (PubChem 249.69 with older S = 32.065)

Hydrate written with a plain dot

Chemical formula
CuSO4.5H2O
摩尔质量
249.677 g/mol

核验来源：Same compound as above; '.' is read as the hydrate dot

常见问题

Is molar mass the same as molecular weight?

Numerically, yes. Molar mass carries units of g/mol, while molecular weight (relative molecular mass, Mr) is the same number without units, measured against one twelfth of the mass of a carbon-12 atom. Water has a molar mass of 18.015 g/mol and a relative molecular mass of 18.015. For ionic solids such as NaCl, which have no discrete molecules, the usual term is formula mass.

How do you calculate molar mass by hand?

Multiply each element's atom count by its atomic weight and add the products. For water, H2O: 2 × 1.008 + 15.999 = 18.015 g/mol. Brackets multiply everything inside them, so Ca3(PO4)2 holds 2 phosphorus and 8 oxygen atoms. A hydrate's water counts in full: CuSO4·5H2O adds 10 hydrogen and 5 oxygen atoms to copper sulfate, for 249.677 g/mol.

Why does my textbook give a slightly different molar mass?

Atomic weights are revised as isotope measurements improve, and books round them differently. IUPAC's 2021 abridged table gives sulfur as 32.06 where older tables used 32.065, so copper sulfate pentahydrate comes to 249.677 g/mol here and 249.69 g/mol on PubChem. A difference in the second decimal place is normal; use whichever table your course or method specifies.

What does mass percent composition tell you?

It is the share of a compound's mass that each element contributes: the element's mass in one formula unit divided by the molar mass, times 100. Glucose is 40.00 % carbon, 6.71 % hydrogen and 53.28 % oxygen. Chemists compare these figures with percentages measured by combustion analysis to confirm a sample's identity or work out an empirical formula.

Why does capitalisation matter in a chemical formula?

Every element symbol starts with a capital letter and any second letter is lowercase, so case decides which atoms you mean. Co is cobalt, 58.933 g/mol, while CO is carbon monoxide, one carbon and one oxygen atom, 28.010 g/mol. Likewise NO is nitric oxide but No is nobelium. Type formulas with standard capitalisation to get the compound you intend.

“Molar mass calculator”有多准确？

准确性取决于输入值和方法的假设。十进制运算使用50位有效数字，但估算、数值方法和源数据的精度可能较低；显示时的舍入并不能消除这些限制。 已按独立来源核验的计算示例：6。 例如，“Water”根据Python 3.8 decimal with IUPAC 2021 abridged weights: 2 × 1.0080 + 15.999 = 18.015 (PubChem 18.015)进行核验。

这种方法出自哪里？

IUPAC CIAAW — Standard atomic weights of the elements 2021 (Prohaska et al., Pure Appl. Chem. 94, 573, 2022), abridged values; PubChem compound pages (molecular weight cross-check).

关于此计算器

M=∑ini Ar(i)  g/mol,wi=niAr(i)MM = \sum_i n_i\,A_r(i)\ \ \mathrm{g/mol},\qquad w_i = \frac{n_i A_r(i)}{M}

来源

  1. IUPAC CIAAW — Standard atomic weights of the elements 2021 (Prohaska et al., Pure Appl. Chem. 94, 573, 2022), abridged values
  2. PubChem compound pages (molecular weight cross-check)

已对照来源验证

此计算器包含 6 个已解示例，答案来自独立来源。这些示例会在测试套件中运行，你也可以在此运行验证。

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