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So, you’ve been tasked with finding the molar mass of a compound or a single element. Luckily, you’ve come to the right place! Calculating the molar mass in chemistry might seem hard, but it’s actually easier than you may think. All it takes is a quick look at the periodic table and some multiplication and addition.
Molar Mass Equation
The molar mass equation is M = m/n . M represents the molar mass, m the mass of a substance (in grams), and n is the number of moles of a substance. Molar mass is measured in g/mol and is found by multiplying the atomic mass by the number of atoms of an element.
Steps
Calculating the Molar Mass of a Compound
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Use the chemical formula of the compound to determine the number of atoms. The chemical formula represents the number of atoms in each element that make up the compound. For example, the formula for hydrogen chloride is HCl; for glucose, it’s C6H12O6. The number of atoms within each element of the compound is represented by the number at the bottom right of the element. [1] X Research source
- For HCl, there is 1 atom of hydrogen and 1 atom of chlorine.
- When there is no atomic number, the number of atoms is automatically 1. [2] X Research source
- For C6H12O6, there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.
- For HCl, there is 1 atom of hydrogen and 1 atom of chlorine.
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Find the relative atomic mass of each element in the compound. Using the periodic table, locate the relative atomic mass for each element (it’s the number located beneath the symbol for the element). [3] X Research source Then, multiply each relative atomic mass by 1 gram/mole.
- The relative atomic masses of the elements in HCI are:
- Hydrogen: 1.007 g/mol
- Chlorine: 35.453 g/mol
- The relative atomic masses of the elements in C6H12O6 are:
- Carbon: 12.0107 g/mol
- Hydrogen: 1.007 g/mol
- Oxygen: 15.9994 g/mol
Advertisement - The relative atomic masses of the elements in HCI are:
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Multiply the element's atomic mass by the number of atoms of that element in the compound. To determine the molar mass of each element in the compound, multiply the relative atomic mass by the number of atoms in the compound. This gives you the relative amount that each element contributes to the compound. [4] X Research source
- For HCl, the molar mass of each element is 1.007 g/mol for hydrogen and 35.453 g/mol for chlorine.
- For C6H12O6, the molar mass of each element is: carbon, 12.0107 x 6 = 72.0642 g/mol; hydrogen, 1.007 x 12 = 12.084 g/mol; and oxygen, 15.9994 x 6 = 95.9964 g/mol.
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Add the molar masses of each element in the compound to find the compound’s molar mass. To determine the molar mass for the entire compound, add all of the molar masses together. The final result will be measured in g/mol. [5] X Research source
- For hydrogen chloride, the molar mass is 1.007 + 35.453 = 36.460 g/mol. 36.46 grams is the mass of one mole of hydrogen chloride.
- For glucose, the molar mass is 72.0642 + 12.084 + 95.9964 = 180.1446 g/mol. 180.14 grams is the mass of one mole of glucose.
Calculating the Molar Mass of an Element
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Find the relative atomic mass of the element on the periodic table. An element's relative atomic mass is the average mass, in atomic units, of a sample of all its isotopes. [6] X Research source This information can be found on the periodic table of elements. Locate the element and find the number underneath the symbol for the element. This number will be a decimal.
- For example, for hydrogen, the relative atomic mass is 1.007; for carbon, it is 12.0107; for oxygen, it is 15.9994; and for chlorine, it is 35.453.
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Multiply the relative atomic mass by the molar mass constant (1 g/mol). Molar mass is the mass (in grams) of one mole of a substance, or g/mol. [7] X Research source This is defined as 1 gram per mole. Multiplying the atomic mass by the molar mass converts atomic units to grams per mole , determining the molar mass of the element. [8] X Research source
- For example, for sodium (Na), the relative atomic mass is 22.989. Multiplying that by 1 g/mol gives you 22.989 g/mol. The number stays the same; only the measurement changes.
- As another example:
- H2: 1.007 x 2 = 2.014 grams per mole
- O2: 15.9994 x 2 = 31.9988 grams per mole
- Cl2: 35.453 x 2 = 70.096 grams per mole
- Know that some elements are only found in molecules of 2 atoms or more. If you want to find the molar mass of elements that are composed of 2 atoms, such as hydrogen, oxygen, and chlorine, then you'll have to find their relative atomic masses. Multiply them by the molar mass constant, and then multiply the result by 2.
- One way to remember diatomic elements (molecules of 2 atoms) is: Have No Fear Of Ice Cold Beverages (Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, Bromine).
Calculator & Practice Problems with Answers
Expert Q&A
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QuestionWhat is the molar mass of Helium?Bess Ruff is a Scientist based in Sydney, Australia. Her research interests and previous scientific experience include environmental science, geography, biotechnology, mariculture, marine spatial planning, stakeholder engagement, and spatial ecology. She is a Postdoctoral Researcher at University of Sydney and a Project Manager at Offshore Biotechnologies. Prior to her work in Sydney, Bess was a Postdoctoral Researcher for over 2 years at Florida State University. She received a PhD in Geography from Florida State University, with a doctoral dissertation entitled "Culturing a Sustainable Seafood Future: How Governance, Economics, and Society Are Driving the Global Marine Aquaculture Industry”. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group.4.002602 g/mol; you might find that some periodic tables will round this number to 4.00
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QuestionWhat is the molecular mass of oxygen and hydrogen as a mixture?Community AnswerOxygen with Hydrogen gives you WATER (H2O). The relative atomic mass for Oxygen is 16, while Hydrogen is 1 since Hydrogen (H2) have two molecules you multiply by 2 1*2=2. Oxygen (O) being 16*1=16. Add up your answers to get 18, and the molecular mass of Oxygen and Hydrogen which is water gives you 18.
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QuestionHow do you calculate the molar mass of water?Community AnswerWater's chemical formula is H2O, this means it is composed of 2 Hydrogen atoms and 1 Oxygen atom. To find the molar mass, find the atomic mass of all the components of a chemical. You can either memorize it, or find all of the atomic masses located on the periodic table of elements. In this case, hydrogen has an atomic mass of 1, and oxygen has an atomic mass of 16. The equation is therefore: 1(2) + 16(1) = 18. Therefore, the molar mass of water, is 18.
Video
Tips
- While most relative atomic masses are known to a precision of 1 part in 10 thousand (4 decimal places), in most laboratory work, molar masses are normally quoted to 2 decimal places and fewer for particularly large masses.Thanks
References
- ↑ https://chem.libretexts.org/Courses/University_of_British_Columbia/CHEM_100:_Foundations_of_Chemistry/06:_Chemical_Composition/6.9:_Calculating_Molecular_Formulas_for_Compounds
- ↑ https://www.nde-ed.org/Physics/AtomElements/chemicalformula.xhtml
- ↑ https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas
- ↑ https://www.chemteam.info/Mole/MolarMass.html
- ↑ https://chem.libretexts.org/Courses/University_of_British_Columbia/CHEM_100:_Foundations_of_Chemistry/06:_Chemical_Composition/6.9:_Calculating_Molecular_Formulas_for_Compounds
- ↑ https://www.chemteam.info/Mole/AverageAtomicWeight.html
- ↑ https://uen.pressbooks.pub/introductorychemistry/chapter/molar-mass/
- ↑ https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Atomic_Theory/The_Mole_and_Avogadro's_Constant
About This Article
When you’re measuring extremely tiny amounts of a substance, it can be helpful to use molar mass. According to the International System of Units, a mole is the amount of any substance that contains the same number of elementary entities—typically atoms or molecules—as there are atoms in 12 grams of the isotope carbon-12. Molar mass is the mass in grams of 1 mole of any given substance. To find an element’s molar mass, start by checking the periodic table for the relative atomic mass of the element. This is usually located under the symbol and name of the element. For instance, the relative atomic mass of zirconium (Zr) is 91.22. Next, multiply the atomic mass by the molar mass constant, which is equal to 1 gram per mole. For most elements, this means that the relative atomic mass is equal to the molar mass. For instance, zirconium has a molar mass of 91.22 x 1 g/mol, or 91.22 g/mol. However, some elements, such as hydrogen, nitrogen, and oxygen, only occur naturally in molecules of 2 or more atoms. In these cases, you’ll need to multiply the relative atomic mass of the element by the number of atoms in the molecule, then multiply the result by the molar mass constant. For example, the molar mass of H2 is 1.007 x 2 x 1 g/mol, or 2.014 g/mol. Finding the molar mass of a compound is a little more complicated. First, you’ll need to find the chemical formula for the compound. For example, if you’re calculating the molar mass of water, you’d start with the formula H2O. Then, calculate the molar mass of each element in the compound. Don’t forget to take into account the number of atoms of each element when you make your calculation. In this case, the molar mass of the two hydrogen atoms is 2.014 g/mol, while the single oxygen atom is 15.999 g/mol. Add them together to get the total molar mass of 18.013 g/mol. To see specific examples for how to calculate molar mass, read on!
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