To convert a mass number to mass, you must understand that the mass number refers to the total number of protons and neutrons in an atom's nucleus, and it's not a direct measure of mass in grams. Instead, you need to use the element's relative atomic mass and Avogadro's number. Here's how:
Converting Relative Atomic Mass to Grams
The process involves these steps:
- Identify the element: Determine the element you are working with.
- Find the relative atomic mass: Obtain the relative atomic mass of the element from the periodic table. This value represents the mass of the element's atom relative to the mass of a carbon-12 atom, and it is usually found below the element symbol on the periodic table.
- Divide by Avogadro's number: Divide the relative atomic mass of the element by Avogadro's number (approximately 6.022 x 1023). This converts atomic mass units (amu) to grams.
Example
Let's calculate the mass in grams of a single hydrogen atom:
- Hydrogen (H) atomic mass: approximately 1.008 amu
- Avogadro's number: 6.022 x 1023 atoms/mol
- Calculation: 1.008 amu / 6.022 x 1023 = approximately 1.67 x 10-24 grams
Therefore, one hydrogen atom weighs approximately 1.67 x 10-24 grams.
Converting Molecular Mass to Grams
The same logic applies to converting a molecule's mass to grams:
- Determine the molecular formula: Identify the atoms and their quantities in the molecule.
- Calculate the total relative molecular mass: Add the relative atomic masses of all the atoms in the molecule.
- Divide by Avogadro's number: Divide the total molecular mass by Avogadro's number to find the mass of one molecule in grams.
Example
Let's calculate the mass in grams of a single water (H2O) molecule:
- Hydrogen (H) atomic mass: approximately 1.008 amu
- Oxygen (O) atomic mass: approximately 16.00 amu
- Total Molecular mass: (2 x 1.008) + 16.00 = 18.016 amu
- Avogadro's number: 6.022 x 1023 molecules/mol
- Calculation: 18.016 amu / 6.022 x 1023 = approximately 2.99 x 10-23 grams
Thus, one water molecule weighs approximately 2.99 x 10-23 grams.
Summary
| Step | Process |
|---|---|
| 1. Identify element or molecule | Determine what element or molecule you are working with. |
| 2. Determine relative mass | Find the relative atomic mass from the periodic table, or calculate the relative molecular mass from the combined atomic masses of all atoms in the molecule. |
| 3. Divide by Avogadro's number | Divide the relative atomic or molecular mass by Avogadro's number to get the mass in grams. |
In conclusion, the conversion of relative atomic or molecular mass (amu) to mass (grams) is done by dividing the relative atomic mass of the element or molecular mass of a molecule by Avogadro's number, as clearly stated in the reference.
