Since atoms are too light to weigh individually, we compare them to a standard: Carbon-12.
Relative Atomic Mass (\(A_r\))
The weighted average mass of an atom of an element compared to 1/12 of the mass of an atom of Carbon-12.
Found on the Periodic Table (e.g. Cl = 35.45)
Relative Formula Mass (\(M_r\))
The sum of the relative atomic masses of all atoms in the formula.
e.g. H₂O = (1.01×2) + 16.00 = 18.02
The Golden Rule of Stoichiometry
\( n = \frac{m}{M} \)
n = moles (mol)m = mass (g)M = Molar Mass (g mol⁻¹)
Calculating Reacting Quantities
Paper 2 StyleHow many moles are in 5.0 grams of Calcium Carbonate (\(CaCO_3\))?
1. Calculate Mr:
Ca = 40.08
C = 12.01
O = 16.00 × 3 = 48.00
Total Mr = 100.09 g/mol
2. Apply Formula: \( n = \frac{m}{M} \)
3. Solve: \( n = \frac{5.0}{100.09} = 0.050 \text{ mol} \)
Student Practice Set
1.
What is the Molar Mass of Sulfuric Acid (\(H_2SO_4\))?