Remembering the quadratic equation

11/3 in-class activity

A cylinder of molecular hydrogen is separated by a metal plate from a cylinder of molecular oxygen.  The plate is removed, a spark starts the reaction, and gaseous water is formed.  Assume the product has had time to cool to ambient temperatures of 400K.  24 g of molecular hydrogen tracts with 76 g of molecular oxygen.  The volume of each cylinder is 1.00 L.  (Use the ideal gas law, not Van der Waal’s equation of state.)

1.) Write the balanced equation.

                                                   

2 H₂(g) + O₂ (g) à 2 H₂O (g)

2.) How many moles of molecular hydrogen and molecular oxygen are there originally?  What is the limiting reagent?

12 mol H₂, 2.375 mol O₂

O₂  is the limiting reagent.

3.) What is the initial pressure in each cylinder in atm (before mixing the reagents)?  Under these estimated pressures, would you expect the gases to behave ideally?

Use PV=nRT.  Use R = 0.08206 L atm/mol K so that the units cancel.

P for H₂ = 394 atm

P for O₂ = 77.96 atm

For pressures above 10 atm, you would not expect the gases to behave ideally.  These pressures are far higher than 10 atm, so you would not expect the gases to behave ideally.  However, the question indicates that you should use the ideal gas law, although it is likely a poor approximation in this situation.

4.) How many moles of water are formed?  How many moles of excess reagent are there?

4.75 mol H₂O formed

7.25 mol excess O₂

5.) What are the resulting partial pressures of water and the excess reagent in the combined cylinders? What is the total pressure?

Partial pressures: 78.0 atm H₂O, 119 atm H₂

Total pressure: 197 atm

*Note that each resulting number should have two significant figures.  Rounding should be done that the end here, but extra digits were included for comparison.

A fun way to remember the molecular formulas for polyatmics!

10/27 in-class activity

1. Fill in the table:

Symbol	#Protons	#Neutrons	#Electrons	Charge
­­­­3216S2-	16	16	18	-2
13756Ba2+	56	81	54	+2
3717 Cl-	17	20	18	-1
5224Cr7+	24	28	17	+7

2. Name the family to which each of the following elements belong:

a.) Transition metal b.) Noble Gas c.) Alkaline earth metal  d.) Alkali metal 
e.) Actinides   f.) Lanthanides  g.) Halogens

3. Fill in the table:

NH4Cl	ammonium chloride
MgSO4	magnesium sulfate
UF4	uranium (IV) fluoride
NaCN	sodium cyanide
FeCl3	iron (III) chloride
RbNO3	rubidium nitrate
(NH4)2Cr2O7	ammonium dichromate
HBr	hydrobromic acid

4. An unknown sample of mystery element “T” is injected into the mass spectrometer.  According to the mass spectrum, 7.42% of the element is present as ⁶T, and 92.58% is present as ⁷T.  The mass values are 6.02 amu and 7.02 amu, respectively.  Calculate the average atomic mass and identify the mystery element.

The average atomic mass is 6.95 amu.  The element is Lithium.

5. How many moles of each element are present of 1.0 mol Ba₃(AsO₄)₂ (barium aresenate)?

3.0 mol Ba, 2.0 mol As, 8.0 mol O

6. Balance the following equation:

2 Fe(s) + 6 HC₂H₃O₂(aq) à 2 Fe(C₂H₃O₂)₃ (aq) + 3 H₂(g)

7. How many grams of water vapor can be generated from the combustion of 18.74 g ethanol?

21.99 g

8. Balance the following redox reaction in acid solution.  What is the reducing agent?  What is the oxidizing agent?

H₃AsO₄ + 4 Zn + 8 H⁺ à AsH₃ + 4 Zn²⁺ + 4 H₂O

As is reduced: it is the oxidizing agent.

Zn is oxidized: it is the reducing agent.

9. Balance the following redox reaction in basic solution.  What is the reducing agent?  What is the oxidizing agent?

HXeO₄^- + 3 Pb + 2 OH^- à Xe + 3 HPbO₂^-

Pb is oxidized: it is the reducing agent.

Xe is reduced: it is the oxidizing agent.

 Original Worksheet:

1. Fill in the table:

Symbol	#Protons	#Neutrons	#Electrons	Charge
­­­­3216S2-
	56	81	54
____ Cl-		20
5224____7+

2. Name the family to which each of the following elements belong:

a.) Rh b.) Xe  c.) Ca   d.) Rb  e.) Pu  f.) Dy   g.) I

3. Fill in the table:

NH4Cl
	magnesium sulfate
UF4
NaCN
	iron (III) chloride
RbNO3
	ammonium dichromate
	hydrobromic acid

4. An unknown sample of mystery element “T” is injected into the mass spectrometer.  According to the mass spectrum, 7.42% of the element is present as ⁶T, and 92.58% is present as ⁷T.  The mass values are 6.02 amu and 7.02 amu, respectively.  Calculate the average atomic mass and identify the mystery element.

5. How many moles of each element are present of 1.0 mol Ba₃(AsO₄)₂ (barium aresenate)?

6. Balance the following equation:

___Fe(s) + ___HC₂H₃O₂(aq) à ___Fe(C₂H₃O₂)₃ (aq) + ___H₂(g)

7. How many grams of water vapor can be generated from the combustion of 18.74 g ethanol?

8. Balance the following redox reaction in acid solution.  What is the reducing agent?  What is the oxidizing agent?

H₃AsO₄ + Zn à AsH₃ + Zn²⁺

9. Balance the following redox reaction in basic solution.  What is the reducing agent?  What is the oxidizing agent?

HXeO₄^- + Pb à Xe + HPbO₂^-