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Q OAR 45% 8 10:29 AM, , , , So the molecular mass of water, which contains two atoms of hydrogen and one, atom of oxygen is, , =2x1+1x16, , =18u, , (b) The molecular mass of HNO3 = the atomic mass of H * the atomic mass of, N* 3 x the atomic mass of O, , =1+14+48 =63u, , 3.5.2 Formula unit mass, , The formula unit mass of a substance is a sum of the atomic masses of all, atoms in a formula unit of a compound. Formula unit mass is calculated in the, same manner as we calculate the molecular mass. The only difference is that we, use the word formula unit for those substances whose constituent particles are, ions. For example, sodium chloride as discussed above, has a formula unit NaCl., Its formula unit mass can be calculated as—, , 1x 23+1 x 35.5 =58.5u, , Example 3.2 Calculate the formula unit mass of CaCl 2., Solution:, , Atomic mass of Ca + (2 x atomic mass of Cl), , =40+2%x35.5=40+71=111u, , Questions, , 1. Calculate the molecular masses of Hyg, 09, Cla, CO2, CH4, C2H6, CoHa, NH3,, CH30H., , 2. Calculate the formula unit masses of ZnO, NagO, K2CO3,, , given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C=12u, and O = 16u.