The one relationship
Every problem in this section is solved by two relationships. Here is the first.
Amount of substance
amount of substance = mass ÷ molar mass
n = m ÷ M
n = number of moles, m = mass (g), M = molar mass (g mol-1)
You already know the second
To turn a number of moles into a number of particles, multiply by the Avogadro constant.
number of particles = n × 6.022 × 1023
The two of them together
| Given | Wanted | Do this |
|---|---|---|
| Mass | Moles | Divide by the molar mass |
| Moles | Mass | Multiply by the molar mass |
| Moles | Number of particles | Multiply by 6.022 × 1023 |
| Mass | Number of particles | Divide, then multiply — go through moles |
Always go through moles
There is no direct route from a mass to a number of particles. Find the number of moles first, every time; it is the step in the middle of every calculation in this chapter.