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How to Find Atoms of a Specific Element in a Compound?

Published in Chemistry Calculations 3 mins read

To find the number of atoms of a specific element within a compound, you generally need to use the chemical formula of the compound and Avogadro's number. Here's a step-by-step approach:

  1. Identify the Chemical Formula: Determine the chemical formula of the compound. This formula tells you the ratio of each element present in the compound. For example, in water (H₂O), there are two hydrogen atoms and one oxygen atom per molecule.

  2. Determine the Number of Moles of the Compound: If you're given the mass of the compound, convert it to moles using the compound's molar mass. The molar mass is the sum of the atomic masses of all the atoms in the formula. You can find atomic masses on the periodic table.

    • Moles = Mass (g) / Molar Mass (g/mol)

  3. Calculate the Number of Moles of the Specific Element: Using the chemical formula, determine the mole ratio between the compound and the element of interest. Multiply the number of moles of the compound by this ratio to find the number of moles of the specific element.

    • For example, if you have 1 mole of H₂O, you have 2 moles of H and 1 mole of O.

  4. Convert Moles of Element to Number of Atoms: Multiply the number of moles of the specific element by Avogadro's number (approximately 6.022 x 10²³ atoms/mol) to find the number of atoms of that element.

    • Number of Atoms = Moles of Element x Avogadro's Number

Example:

Let's say you have 10 grams of water (H₂O) and want to find the number of hydrogen atoms.

  1. Chemical Formula: H₂O
  2. Molar Mass of H₂O: (2 x 1.008 g/mol for H) + (1 x 16.00 g/mol for O) = 18.016 g/mol
  3. Moles of H₂O: 10 g / 18.016 g/mol = 0.555 moles
  4. Moles of H: 0.555 moles H₂O x (2 moles H / 1 mole H₂O) = 1.11 moles H
  5. Number of H Atoms: 1.11 moles x 6.022 x 10²³ atoms/mol = 6.68 x 10²³ atoms

Therefore, there are approximately 6.68 x 10²³ hydrogen atoms in 10 grams of water.

Summary:

Finding the number of atoms of a specific element in a compound involves using the chemical formula to determine mole ratios, converting mass to moles using molar mass, and finally, multiplying the number of moles of the element by Avogadro's number.

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