Unformatted text preview: unit The combination of one Na and one Cl ion is the smallest collection of ions from which we can deduce the formula NaCl.It is a formula unit.
- + In fact,solid sodium chloride consists of enormous numbers + - of Na and Cl ions in a network called a crystal 3-2 The Mole Concept and Chemical Compounds Mole of a Compound Mole of an Element Basic concepts Formula mass is the mass of a formula unit in atomic mass units.It is always appropriate to use the term formula mass,but for a molecular compound,the formula unit is an actual molecule and we can speak of its molecular mass. Molecular mass is the mass of a molecule in atomic mass units. molecular mass H2O = 2(atomic mass H) + (atomic mass O) = 2(1.008u) + 15.999u
for molecular compound = 18.015u for ionic compound Formula mass MgCl2 = atomic mass Mg +2(atomic mass Cl) = 24.305u + 2(35.453u) = 95.211u Mole of a Compound
Specifically,a mole of compound is an amount of compound containing Avogadro`s number(6.022141023) of formula units or molecules. The mole mass is the mass of one mole compound--one mole of molecules of a molecular compound and one mole of formula units of an ionic compound. If we know the formula of a compound,we can equate the following terms 1 mol H2O = 18.015 g H2O = 6.022141023 H2O molecules 1 mol MgCl2 = 95.211 g MgCl2 = 6.02214102 MgCl2 formula units 1 mol Mg(NO3)2 = 148.313 g Mg(NO3)2 = 6.02214102 Mg(NO3)2 formula units Example 1.Relating molar mass, the Avogadro Constant, and Formula Units of an Ionic Compound.An analytical balance can detect a mass of 0.1 mg. What is the total number of ions present in this minimally detectable quantity of MgCl2 ? Solution After making the mass conversion,mg g,we can use the molar mass to convert from mass to number of moles of MgCl2.Then,with the Avogadro Constant as a conversion factor,we can convert from moles to number of formula units.The final factor we need is based on the fact that there are three ions per formula unit (fu) of MgCl2. As previously noted,it is often helpful to map out a conversion pathway that starts with the information given and proceeds through a series of conversion factors to the information sought.For this problem, we can begin with milligrams of MgCl2 and make the following conversions: mg g mol fu number of ions ? ions = 0.1 mg MgCl2 95 g MgCl 1000mg MgCl2 2 6.0 23 f.u.MgCl2 10 1 mol MgCl 2 = 2 18 ions 10 3 ions f.u.MgCl 1 2 1g MgCl2 1 mol MgCl2 2.Combining Several Factors in a Calculation Involving Mass. The volatile liquid ethyl mercaptan,C2H6S,is one of the most odoriferous substance know. It can be added to natural gas to make gas leaks detectable.How many C2H6S molecules are contained in a 1.0- L sample?(d = 0.84 g/mL.) Solution We convert from microliters, and then to milliters. At this point, we bring in Density as a conversion factor. The remaining conversions are fromm the Mass of the substance to the number of moles and ,finally,to the number of Molecules. L mL g mol number of molecules 0.84 g C2H6S 1000 mL ? molecules C2H6S = 1.0 L 10 1L...
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This note was uploaded on 05/07/2010 for the course FIN 401 taught by Professor Chemistry during the Spring '10 term at Uni Potsdam.
- Spring '10