Relative Atomic Mass (Ar)
The weighted mean mass of an atom of an element, relative to ¹⁄₁₂ the mass of a carbon-12 atom.
Symbol: Ar (dimensionless ratio, no units).
Relative Formula Mass (Mr)
The sum of all relative atomic masses of the atoms in a formula unit. It is a ratio with no units.
e.g. Mr(NaCl) = 22.99 + 35.45 = 58.44
Molar Mass (\(M\))
The mass of one mole of a substance. Its value is numerically equal to \(A_r\) or \(M_r\), but has units of g mol⁻¹.
\( n = \frac{m}{M} \)
Calculate Amount in Moles from Mass
Problem: How many moles are in \(10.0\text{ g}\) of Calcium Carbonate (\(\text{CaCO}_3\))?
1. Calculate Molar Mass (\(M\)): \(40.08 + 12.01 + (3 \times 16.00) = 100.09\text{ g mol}^{-1}\)
2. Apply Formula: \(n = \dfrac{m}{M} = \dfrac{10.0\text{ g}}{100.09\text{ g mol}^{-1}} = \mathbf{0.0999\text{ mol}}\)
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