Calculations involving Avogadro's number

As chemists, we need to be able to answers questions of this type:

1. How many grams does 2.5 moles of NaCl weigh?
2. How many oxygen atoms are there in 5.0 grams of Na₂CO₃?
3. 5 moles of electrons is equivalent to how many electrons?

View these questions in terms of unit conversion. For example,

Question 1 involves unit conversion from moles to mass in grams.

Question 2 involves unit conversion from number of particles (atoms) to mass in grams.

Question 3 involves unit conversion from moles to number of particles (electrons).

Whenever there is a need to convert quantities from one unit to another, you need to use the appropriate conversion factor. Choose the conversion factor that relates the two units of quantities involved.

Conversion factors

Quantity	Possible units
Avogadro's number	6.022 x 1023 number of particles/mole	6.022 x 1023 amu/gram
Atomic mass	grams/mole	amu/atom amu/molecule amu/formula unit
Molar mass	grams/mole

Using the above conversion factors, we can solve questions involving conversion:

from moles to mass in grams	× (grams/mole)
from number of particles to mass in grams	÷ 6.022 x 1023 number of particles/mole × (grams/mole)
from moles to number of particles	× 6.022 x 1023 number of particles/mole

Converting between mass (in grams) and moles

Number of moles = (# of grams) ÷ (molar mass)

Number of grams = (# of moles) × (molar mass)

External link

mass to moles conversion

Content suitability

BCIT courses: CHEM 0011