Amount in moles substances

The concept of amount in moles is central to chemistry because it enables us to keep track of the number of molecules, atoms, or ions in a sample of a substance. Amount in moles is a counting unit (6.022 × 10²³mol^–1). Other more familiar counting units are a pair (2 pr^–1) and a dozen (12 dozen^–1).

A mole of water molecules has 6.022 × 10²³H₂O.
A mole of helium atoms has 6.022 × 10²³He.
A mole of sodium ions has 6.022 × 10²³Na⁺.

The amount in moles of substance in a

n =	m
	M

weighed sample can be calculated by dividing the mass (m) by the molar mass (M), the mass of 1 mole of substance.

Practice: Calculate the amount in moles of Na₂CO₃ in a 3.00 g Na₂CO₃. M(Na₂CO₃) is 106 g mol^–1.
Answer: 0.0283 mol (note the answer has three significant figures because all data has three significant figures)

n = cV

n = cV

solution of known concentration (c) in

can be calculated by multiplying by the volume (V) in

Practice: Calculate the amount in moles of NaNO₃ in 38.5

of a 0.215

solution.
Answer: 0.00828 mol (be sure to convert volume in

to volume in

)

Rearranged forms of the relationships can be used to calculate any variable in these equations provided the other two are known (m = nM and c=n/V).