A molecular weight calculator is a powerful tool used to determine the mass of a molecule or compound based on its chemical formula.
For example, consider water (H2O):
- Hydrogen (H) has an atomic mass of 1.008 g/mol
- Oxygen (O) has an atomic mass of 15.999 g/mol
Using a molar mass calculator, we can swiftly determine that water’s molecular weight is approximately 18.015 g/mol (2 * 1.008 + 15.999).
Molecular Weight Calculator
| Compound | Formula | Calculation | Molecular Weight (g/mol) |
|---|---|---|---|
| Methane | CH4 | (12.0107 × 1) + (1.008 × 4) | 16.0425 |
| Ethanol | C2H6O | (12.0107 × 2) + (1.008 × 6) + (15.999 × 1) | 46.0684 |
| Sodium Chloride | NaCl | 22.98977 + 35.453 | 58.44277 |
| Calcium Carbonate | CaCO3 | 40.078 + 12.0107 + (15.999 × 3) | 100.0869 |
| Aspirin | C9H8O4 | (12.0107 × 9) + (1.008 × 8) + (15.999 × 4) | 180.1574 |
| Glucose | C6H12O6 | (12.0107 × 6) + (1.008 × 12) + (15.999 × 6) | 180.15588 |
| Fructose | C6H12O6 | (12.0107 × 6) + (1.008 × 12) + (15.999 × 6) | 180.15588 |
| Sucrose | C12H22O11 | (12.0107 × 12) + (1.008 × 22) + (15.999 × 11) | 342.296 |
| Acetic Acid | C2H4O2 | (12.0107 × 2) + (1.008 × 4) + (15.999 × 2) | 60.052 |
| Citric Acid | C6H8O7 | (12.0107 × 6) + (1.008 × 8) + (15.999 × 7) | 192.124 |
| Lactic Acid | C3H6O3 | (12.0107 × 3) + (1.008 × 6) + (15.999 × 3) | 90.078 |
| Urea | CH4N2O | (12.0107 × 1) + (1.008 × 4) + (14.00674 × 2) + (15.9994 × 1) | 60.06 |
| Caffeine | C8H10N4O2 | (12.0107 × 8) + (1.008 × 10) + (14.00674 × 4) + (15.9994×2) | 194.19 |
| Benzene | C6H6 | (12.0107 × 6) + (1.008 × 6) | 78.111 |
| Cholesterol | C27H46O | (12.0107 × 27) + (1.008×46)+ (15.999×1) | 386.65 |
| Vitamin C | C6H8O6 | (12.0107×6)+(1.008×8)+(15.999×6) | 176.124 |
Molecular Weight Formula
The formula for calculating molecular weight is:
Molecular Weight = Σ (Atomic Weight of each element × Number of atoms of that element)Let’s break this down with glucose (C6H12O6) as an example:
- Carbon (C): 12.0107 g/mol × 6 = 72.0642 g/mol
- Hydrogen (H): 1.00794 g/mol × 12 = 12.09528 g/mol
- Oxygen (O): 15.9994 g/mol × 6 = 95.9964 g/mol
Sum these values: 72.0642 + 12.09528 + 95.9964 = 180.15588 g/mol
The molecular weight of glucose is approximately 180.16 g/mol.
How do you calculate molecular weight?
To calculate molecular weight:
- Identify the elements in the compound
- Determine the number of atoms for each element
- Look up the atomic weight of each element
- Multiply the atomic weight by the number of atoms for each element
- Sum all the results
Let’s calculate the molecular weight of sulfuric acid (H2SO4):
- Elements: Hydrogen (H), Sulfur (S), Oxygen (O)
- Number of atoms: 2 H, 1 S, 4 O
- Atomic weights: H = 1.008 g/mol, S = 32.065 g/mol, O = 15.999 g/mol
- Multiply:
- H: 1.008 × 2 = 2.016 g/mol
- S: 32.065 × 1 = 32.065 g/mol
- O: 15.999 × 4 = 63.996 g/mol
- Sum: 2.016 + 32.065 + 63.996 = 98.077 g/mol
The molecular weight of sulfuric acid is approximately 98.08 g/mol.
What is the unit of molecular weight?
The standard unit for molecular weight is grams per mole (g/mol). This unit represents the mass of one mole of a substance, where a mole is defined as 6.022 × 10^23 particles (Avogadro’s number).
The molecular weight of carbon dioxide (CO2) is 44.009 g/mol. This means that one mole of CO2 molecules weighs 44.009 grams and contains 6.022 × 10^23 CO2 molecules.
What is 1 molecular weight?
The concept of “1 molecular weight” is not typically used in chemistry. Instead, we refer to the unified atomic mass unit (u) or Dalton (Da). One unified atomic mass unit is defined as 1/12 of the mass of a carbon-12 atom.
1 u ≈ 1.660539 × 10^-24 gThis unit is useful when discussing the mass of individual atoms or molecules. For example, a single water molecule has a mass of about 18.015 u.
To convert from atomic mass units to grams per mole:
1 g/mol = 1 u × NA (Avogadro's number)So, 1 g/mol is equivalent to 6.022 × 10^23 u.
References
- National Institute of Standards and Technology. “Atomic Weights and Isotopic Compositions.” https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl
- Royal Society of Chemistry. “Molecular Weight Calculator.” https://www.rsc.org/periodic-table/molcalc
- International Union of Pure and Applied Chemistry. “Atomic Weights of the Elements 2021.” https://iupac.org/what-we-do/periodic-table-of-elements/
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