![]() It is different from its molecular formula (C 2H 4O 2)). Since the molar ratio of C, H, and O in acetic acid is 1:2:1 (the simplest ratio), so you can express this relationship as CH 2O. For C 2H 4O 2, its molar mass is 60.052 g/mol, which means if you have 60.052 g acetic acid (i.e. Molecular formula can be used to determine the mass percent of each atom in the compound. ![]() (6) Mass percent of each element in a molecule. If someone claimed a pure sample is an acetic acid but you found the molar ratio of C:H:O is not 1:2:1, then that sample will not be acetic acid. This in fact, is the Proust Law of Constant Proportions. This ratio will be the same in any acetic acid molecule no matter where and how you get it. Therefore, 1:2:1 is the molar ratio of C, H, and O atoms in CH 3COOH molecule. (5) (It tells you the molar ratio of atoms in a molecule.įor every one mole of C 2H 4O 2 molecules, there are two moles of C atoms, four moles of H atoms and two moles of O atoms, therefore, the molar ratio of C:H:O = 2:4:2 = 1:2:1 (simplest ratio). Because electrons are very light, so the molar mass of an ion is the same as its neutral form, so when you do the calculation, just ignore the charge and calculate the molar mass from the fomula you have. For example, what is the molar mass of CH 3COO -? Since ions are formed by losing electrons. Sometimes, you will need to determine the molar mass of an ion. We frequently use it to convert grams to moles or moles to grams. The molar mass is very useful in chemistry. I would suggest you memorize those for atoms from 1 to 20. You can find the atomic mass for each element on the Periodic Table. For C 2H 4O 2, if you add the atomic mass of two C, four H, and two O together, it will give you its molar mass as 60.052 g/mol.Ģ × 12.011 + 4 × 1.008 + 2 × 15.999 = 60.052 g/mol It is the sum of atomic mass of all atoms. The molar mass is the mass of one mole of molecules (ions, atoms), which can be calculated from the molecular formula. (4) You can determine the molar mass from it. This information can be used for some related calculations, such as if you have 0.5 moles of C 2H 4O 2, then you will have 1 mole C atoms, 2 mol H atoms, and 1.0 mol O atoms.Ġ.5 mol C 2H 4O 2 × (2 mol C)/( 1 mol C 2H 4O 2)=1.0 mol CĠ.5 mol C 2H 4O 2 × (4 mol H)/( 1 mol C 2H 4O 2) = 2.0 mol HĠ.5 mol C 2H 4O 2 × (2 mol O)/( 1 mol C 2H 4O 2) = 1.0 mol O For example, if you have one mole of C 2H 4O 2 molecules, then you will have two moles of C atoms, four moles of H atoms and two moles of O atoms. You can get this information from the molecular formula directly. (3) It tells you the moles of each atom in one mole of molecules. In this case, there are 8 atoms in total. If you add all atoms together, that will give you the total number of atoms in an acetic acid molecule. In each acetic acid molecule, you will have 2 carbon atoms, 4 hydrogen atoms, and 2 oxygen atoms. (2) It shows the number of each atom in a compound. Another example is NaNO 3, which indicates that this compound has Na, N, and O atoms. (1) It tells you what types of atoms are in a compound. the functional group -COOH to indicate it is a carboxylic acid. The reason why we write the formula as CH 3COOH is because it shows another piece of information, i.e. ![]() For example, we can write this formula as C 2H 4O 2. Keep in mind that sometimes we combine the same atoms and use a subscript to indicate its quantity. I will use it as an example to show all information you can learn from this simple formula. It is indeed the molecular formula of acetic acid, which is the major component of vinegar. Here is a typical molecular formula: CH 3COOH. Home Publications Group Member Research Tools Teaching Learning Resources Contact General Chemistry (The contents will be updated periodically) Q1: What should you know about molecular formulas?
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