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# Mole and Molar Mass ## Mole - A mole is the amount of a substance that contains as many entities or particles as there are atoms in exactly 12 g (or 0.012 kg) of Carbon-12 isotope. - One mole contains *6.0221367 × 1023* particles/entities. - 6.0221367 × 1023 is called Avogadro's constant. ##...

# Mole and Molar Mass ## Mole - A mole is the amount of a substance that contains as many entities or particles as there are atoms in exactly 12 g (or 0.012 kg) of Carbon-12 isotope. - One mole contains *6.0221367 × 1023* particles/entities. - 6.0221367 × 1023 is called Avogadro's constant. ## Molar Mass - The mass of one mole of a substance (element/compound) in grams is called its molar mass. - The molar mass of an element in grams is numerically equal to the atomic mass of that element in u. | Element | Atomic mass (u) | Molar mass (g mol-1) | |---|---|---| | H | 1.0 u | 1.0 g mol-1 | | C | 12.0 u | 12.0 g mol-1 | | O | 16.0 u | 16.0 g mol-1 | - Similarly the molar mass of any substance, existing as a polyatomic molecule, is numerically equal to its molecular/formula mass in u. | Polatomic substance | Molecular/formula mass (u) | Molar mass (g mol-1) | |---|---|---| | O2 | 32.0 u | 32.0 g mol-1 | | H2O | 18.0 u | 18.0 g mol-1 | | NaCl | 58.5 u | 58.5 g mol-1 | ## Problem 1.5 Calculate the number of moles and molecules of urea present in 5.6 g of urea. **Solution:** - Mass of urea = 5.6 g - Molecular mass of urea, NH2CONH2 = 2 (average atomic mass of N) + 4 (average atomic mass of H) + 1(average atomic mass of C) + 1(average atomic mass of O) = 2 x 14 + 1 x 12 + 4 x 1 + 1 x 16 u = 60 u - Molar mass of urea = 60 g mol-1 **Number of moles:** mass of urea in g / molar mass of urea in g mol-1 = 5.6 g / 60 g mol-1 = 0.0933 mol **Number of molecules:** Avogadro's constant x Number of moles = Number of molecules of urea = 0.0933 x 6.022 x 1023 molecules/mol = 0.5618 x 1023 molecules = 5.618 x 1022 molecules **Answer:** - Number of moles = 0.0933 mol - Number of molecules of urea = 5.618 x 1022 molecules