Rules: This take-home exam is open book and note anybody except Dr. Pratt until

Question

Rules: This take-home exam is open book and note anybody except Dr. Pratt until after it is handed in. The exam is due at the beginning of class on Monday, October 30. You may hand in the exam early if you wish but late exams will accepted. Use extra p penalized 5 points. You must show your work to receive credit. No work or incorrect work aper as needed and STAPLE to the exam. Unstapled sheets will be no credit 1. A 21.496-g sample of magnesium is burned in air to form magnesium oxide and magnesium nitride. When the products are treated with water, 2.813 g of gaseous ammonia are generated. Calculate the amounts of magnesium nitride and magnesium oxide formed. (20 points) 2. Sulfuric acid is produced by oxidization of sulfur dioxide with oxygen to sulfur trioxide, and the sulfur trioxide is dissolved in water If 640.7 lb-mass of sulfur dioxide are oxidized per hour, mow many pound-moles of O are consumed in an 8 hour shift? (20 points) 64.01 Lb-mole per 3. A compound X contains 63.3% manganese and 36.7%)oxygen by mass. When X is heated, oxygen gas is evolved and a new compound Y containing 72.0% manganese and 28.0% oxygen is formed. Determine the empirical formulas of X and Y. (20 points)

Explanation / Answer

let x= Grams of Mg reacted to form Mg3N2 and then 21.496-x= gms of Mg react to form Mg3N2

moles of Mg reacted to form MgO= mass/molar mass = x/24=0.042x, moles of Mg reacted to form Mg3N2= (21.496-x)/24= 0.042x*(21.496-x)= 0.90-0.042x

the reactions are Mg+0.5O2---->MgO (1), 3Mg+ N2-------->Mg3N2 (2)

moles of MgO fomed per mole of Mg= 1, moles of Mg used= 0.042x, moles of MgO formed =0.042x

moles of Mg3N2 formed = (1/3) moles of Mg= (0.90-0.042x/3 =0.3-0.014x

Mg3N2 reacts with water to produce NH3 as per the reaction

Mg3N2+6 H2O ----->3Mg(OH)2+ 2NH3 . 1 mole of Mg3N2 geenrated to 2 mole of NH3

moles of NH3 generated= mass/molar mass = 2.813/17=0.165

moles of Mg3N2 utilized= 0.165/2=0.0825 = (0.3-0.014x)

0.014x= 0.3-0.0825

x= 15.53,   moles of Mg3N2 formed = 0.0825 , mass of Mg3N2 generated= moles* molar mass = 0.0825*101=8.33 gm

moles of MgO formed =0.042*15.53= 0.65 moles, mass of MgO formed =moles* molar mass = 0.65*40.3=26.195 gm

2. The overall reaction is SO2+0.5O2+ H2O------>H2SO4

1 lb moles of SO2 gives rise to 1 lb moles of H2SO4

mass of SO2 used =no of hours*( mass oxidized/hr)= 640.7*8 = 5126 lb

moles of SO2 = mass of SO2/ molar mass =5126/64= 80.1 lb moles

moles of Oxygen used= 0.5 times moles of SO2= 0.5*80.1= 40.05 lb moles

3. atomic weight of Mn= 55 and that of Oxygen= 16

moles = mass/molar mass

Basis: 100 gm of Mixture of Mn andO

moles= mass/molar mass : moles of Mn= 63.3/55 = 1.15 , O= 36.7/16= 2.3

molar ratio of Mn :O= 1.15: 2.3= 1:2

so empirical formula is MnO

when heated some oxygen is removed. nothing happens to Mn.

let y= mass of Mixture of Mn and O after heating

since Mn remains constant, y*0.72= 63.3, y= 87.92

after heating, there is 72% Mn and 28% Oxygen

mass of Mn= 87.92*0.72= 63.3, O= 87.92*0.28= 24.62

moles =   mass/molar mass : Mn= 63.3/55= 1.15 and O= 24.62/16= 1.54

mole rati of Mn and O is Mn:O= 1.15 :1.54

dividing by 1.15, the smallest no, the ratio becoome 1: 1.54/1.15= 1:1.33= 1: 4/3 =3:4

the empirical formula is Mn3O4.

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