5.3 – Molar Mass Ms. Munir

Molar mass The mass of one mole of a substance is called its molar mass (symbol M). Molar mass of an element is equivalent to the atomic mass unit of that element. E.g. C – 12 atom has 12.01 a.m.u. and 1 mole of C – 12 atoms weigh 12.01 g/mol.

Finding Molar Mass of Compounds Example – Find molar mass of CO2. MCO2 = MC + 2MO = 12.0107 g/mol + 2(15.9994 g/mol) = 44.0095 g/mol One mol of CO2 = 6.022 x 1023 = 44.0095g/mol

Converting from moles to mass Mass = # of moles x Molar mass g = mol x g/mol m = n x M multiply by Moles ------------------------------> Mass(g) molar mass(g/mol) m M n

Example A flask contains 0.750 mol of CO2 gas. What mass of CO2 is in this flask? Given: n = 0.750 mol m = ? g Find M from periodic table { calculated above } = 44.0095 g/mol m = 0.750 mol x 44.0095 g/mol = 33.0 g The mass of CO2 is 33.0g

Converting from Mass to moles Divide by M (g/mol) Moles (mol) < ------------------------------- Mass (g) n = m / M E.g. How many moles of acetic acid, CH4COO, are in a 23.6g sample? Given: m = 23.6 g n = ? mol

Solution MCH4OO = 2MC + 4MH + 2MO = 2(12.01) + 4(1.01) + 2(16.00) = 60.06 g/mol n (mol CH3COOH)=23.6g/60.06g/mol = 0.393mol Therefore, there are 0.393mol of acetic acid in 23.6 g of acetic acid.

Converting between moles, mass and number of particles Moles (mol) Mass (g) NA M(g/mol)

Example What is the mass of 5.67 x 1024 molecules of cobalt (II) chloride, CoCl2? Solution: mass = ? g N = 5.67 x1024 molecules NA = 6.022 x 1023 molecules/ mol MCoCl2 = 58.9332 + 2(35.4527) = 129.8380 g/mol

Solve by units  

Example 2 Chlorine gas, Cl2, can react with iodine, I2, to form iodine chloride, ICl, how many molecules of ICl are contained in a 2.74x10-1 g sample? m = 2.74 x 10-1 g MICl = 126.90447 + 35.453 = 162.357 g/mol NA = 6.022 x 1023 molecules/ mol N = ? molecules

Solve by units  

Practice P 192 # 1 – 6 McGraw hill P 277# 1 – 12 Nelson