University:
California State University, San Marcos
Course:
CHEM 160 | General Chemistry II
Academic year:

2020
Views:

239

Pages:

4
Author:

Yemima C.

Molar mass The molar mass (mm) represents the mass possessed by 1 mole of substance. The mass of 1 mole of a substance is equal to the relative molecular mass (Mr ) of the substance in grams/mol. For elements whose particles are atoms, the molar mass is equal to Ar (relative atomic mass) in grams/mol. Example: • Molar mass of calcium (Ca) = mass of 1 mol of calcium (Ca) = Ar Ca = 40 grams/mol. • Molar mass of iron (Fe) = mass of 1 mole of iron (Fe) = Ar Fe = 56 gram/ mole. • Molar mass of aluminum (Al) = mass of 1 mol aluminum (Al) = Ar Al = 27 gram/mol For elements whose particles are molecules and compounds, then mass molar is equal to Mr (relative molecular mass) in grams/mol. with: Mr = relative molecular mass (gram/mol) Ar = relative atomic mass (gram/mol) Example 1. a. Molar mass of H2 = mass of 1 mol H2 = Mr H2 = 2 × Ar H = 2 × 1 gram/mol = 2 gram/mol b. Molar mass of O2 = mass of 1 mol O2 = Mr O2 = 2 × Ar O = 2 × 16 gram/mol = 32 gram/mol c. Molar mass of CO2 = mass of 1 mol CO2 = MrCO2 = Ar C + (2 × Ar O) = 12 + (2 × 16) = 12 + 32 = 44 gram/mol d. Molar mass of H2 O = (2 × Ar H) + Ar O = (2 × 1) + 16 = 2 + 16 = 18 grams/mol . e. Molar mass of H2 SO4 = (2 × Ar H) + Ar S + (4 × Ar O) = (2 × 1) + 32 + (4 × 16) = 2 + 32 + 64 = 98 grams/mol f. Molar mass of CH3 COOH = (2 × Ar C) + (4 × Ar H) + (2 × Ar O) = (2 × 12) + (4 × 1) + (2 × 16) = 24 + 4 + 32 = 60 grams/mol g. Molar mass of (NH4 )2 CO3 = (2 × Ar N) + (8 × Ar H) + Ar C + (3 × Ar O) = (2 × 14) + (8 × 1) + 12 + (3 × 16 ) = 28 + 8 + 12 + 48 = 96 grams/mol The relationship between the number of moles (n) and the mass of the substance (m) is: with: m = mass of substance (grams) n = number of moles (mol) mm = molar mass = Ar or Mr (grams/mol) So the number of moles becomes: Example 2. 1. Calculate the mass if you know the number of moles of substance. Calculate the mass of: a. 5 moles of iron (Ar Fe = 56) b. 0.75 mol urea CO[NH2 ]2 (Ar C = 12, O = 16, N = 14, and H = 1) c. 0.5 mol O2 (ArO = 16) Answer: a. mass of iron = n × Ar Fe = 5 mol × 56 mol/gram = 280 gram b. mass of urea = n × Mr CO[NH2 ]2 = 0.75 mol × 60 mol/gram = 45 gram c. mass of O2 = n × Mr O2 = 0.5 mol × 32 mol/gram = 16 grams 2. Calculating Moles If You Know the Mass of the Substance Calculate the number of moles of: a. 2.3 grams of sodium (Ar Na = 23) b. 45 grams of C6 H12O6 (Ar C = 12, H = 1, and O = 16) c. 35.1 grams of NaCl (Ar Na = 23 and Cl = 35.5) d. 196.5 grams of zinc (Ar Zn = 65.5) Answer : Exercise 1. 1. Calculate the mass of: a. 0.5 mol of barium (Ar Ba = 137) b. 5 moles of sulfur (Ar S = 32) c. 2.5 mol K2 SO4 (Ar K = 19, S = 32, and O = 16) d. 0.3 mol CO2 (Ar C = 12 and O = 16) e. 10 mol K2 Cr2 O7 (Ar K = 39, Cr = 52, and O = 16) 2. Calculate the number of moles of: a. 8 grams of H2 (Ar H = 1) b. 800 grams of CaCO3 (Ar Ca = 40, C = 12, and O = 16) c. 232 grams Mg(OH)2 (ArMg = 24, O = 16, and H = 1) d. 158 grams of KMnO4 (Ar K = 39, Mn = 55, and O = 16) e. 478 grams CHCl3 (Ar C = 12, H = 1, and Cl = 35.5) 3. Determine Ar silver if 5 moles of silver have a mass of 540 grams! 4. Determine Ar gold if 2 mol of gold has a mass of 394 grams!