Dietz Chemistry Review-searchable version.pdf

Moles of water thus moles o molecular weight o mwo

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moles of water. Thus: . . (moles O,) (molecular weight O,) . . MWO, Mass fraction = 5 - mole fraction (moles H,0) (molecular weight H 2 0) MW H ; 0 = (5.237 x iO' 6 ) (32/18) = 9.3 1 x IV 6 (decimal) = 9.31 pprr 13
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2.4 Molar Concentrations Molar concentrations are the traditional units employed in chemistry applications. Molar units are indicated by use of brackets (for example, [FT] indicates the hydrogen ion concentration in units of moles per Liter). The molar concentration is defined below: ^ s moles mass (grams) /Molecular weight , . Concentration (M) = = (2.4 a) Volume (L) Volume (L) Determination of stoichiometric quantities is often facilitated by use of alternate "molar" units, such as lb-moles. These appropriate molar quantities are defined below: # gram-moles = mass (grams) / Molecular weight (grams/gram-mole) (2.4 b) # lb-moles - weight (lbs) / Molecular weight (lbs/lb-mole) (2.4 c) # kg-moles = mass (kg) / Molecular weight (kg/kg-mole) (2.4 d) In the preceding equations (liquations 2.4 a to 2.4 d), the molecular weights are numerically equal. By defai.lt, unless otherwise specified, designation of a quantity as a moie is presumed to mean a gram-mole (or gmoie). Gram-moiar concentrations must be used for all thermodynamic calculations (equilibrium chemistry). Example Problems I Determine the molarity cf a 2-Liter solution prepared by addition of 100 grams of NH4CI to water. MW = (Atomic weight N)(l) + (Atomic weight H)(4) + (Atomic weight Cl)(l) = (14)(1) + (1)(4) + (35.5)0) = 53.5 grams/mole Molarity = ('00 g™"s NH,CI)/(53.S graimNH.Cl/mole) = 0935 M 2. Determine the number of moles in 53 .5 grams of 'NHjCl. # moles = (53 5 grams) / (53.5 grams/mole) = 1.00 mole 3. Determine the number of moles in 53.5 pounds ofNTLCl. JJ , , (53.51bs)(453.6grams/lb) ... , . # moles = # gram-moles - —— 5 ~ 453.6 moles 53.5 grams/mole 4. Determine the number of lb-moles in 53.5 pounds ofNRjCi. # lb-moles = (53.5 lbs) / (53.5 lbs/lb-mole) = 1.00 lb-mole It is noted that a lb-mole does not equal a gram-mole. 14
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2.5 Mass Concentration Although molar units are typically used for chemistry applications, mass units of concentration are prevalent for engineering applications. In particular, environmental regulations are defined in most cases using mass concentrations. Liquid phase concentrations are commonly expressed with units of mg/L or gg/L Gas phase concentrations are routinely expressed with units of ng/m 3 mass of compound of interest , , Concentration = £ (2.5 a) volume of solution Example Problems 1. Determine the mass concentration for a 2-Liter solution prepared by addition of 100 grams of NH4CI to water. Concentration (g/L) = (100g)/(2L) = 50g/L 2 Determine the mass concentration of carbon monoxide (CO) in a room with the following dimensions: 20mby 10mby5m. The total number of moles of CO is 5. ^ / , 3 N (5 molesCO)(28g/mole)(10 6 i/g/g) ,, 3 Concentration (ug/m ) : = ——— •» 140,000 j^g/m (20 m)(10m)(5m) 15
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2.6 Mass Concentration - Aqueous Solutions For the special case of aqueous solutions (solutions in water) the density of the solution may be approximated as 1 kg/L. Although the actual density may vary slightly with changes in temperature, pressure, and/or concentration of dissolved or suspended materials, this variation is often ignored without noticeable loss
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