Does Stoichiometry Suck Your Life?

Oh God if I knew what the easiest way to teach stoichiometry for my students, I would run that way! I do don't know other way to explain stoichiometry after trying all ways I'd known. For me personally stoichiometry is the best part of chemistry. The most I loved in chemistry when I was at school was stoichiometry. I'd take the highest score in this part but not in the characteristic of Periodic System because this part might ask you to memorize too much thighed statements as I didn't like to memorize. For me at that time when a kinda natural lesson asked you much to memorize, the lesson should go to a social science. No wonder then why I loved stoichiometry more than other parts of chemistry.

But, oh my, I still need to face the worst world of mine after years I've struggled it. With no good mood of the weakest heart I put my students' papers on the floor as I was sitting around and checked all points of each paper. I hoped I'd find something wrong with my check point on some papers, hoped I was blank when I checked one or more papers and I needed to recheck the score. But I couldn't find something to increase even one score of the papers.

It's the second exam of stoichiometry. I needed to rerun the same field because nobody had taken a good score in last stoichiometric exam. Every single person should take minimally 70 as her/his score to pass the exam. In the last exam, the highest score was 42 and the lowest one was 14. And now I'm rechecking results of the second exam. The highest score is 48 and the lowest is 8. There are some students take 8 and some others take 16.

If I'd be a uncared teacher or I'd leave this part after leaving the chemistry class, I'd love it but how I could because I'd accept students as my sons and daughters. Will a mommy leave her kids alone when they need her? So I'm here with my students' papers and just face their papers on as my paled face going much paler. I don't think my students too stupid to answer such stoichiometry questions. Yes they all were A-level questions but we'd discussed such questions in the class. When I asked them why they couldn't answer the questions and many didn't answer essay questions, they said they didn't understand the questions because they were in English and I used new vocabularies in many questions. Of course I won't give same questions with what we'd studied in the class. All vocabularies I used in those questions were just common vocabularies and as I used Cambridge tutorial to check the British English style on the questions.

I'm curious what's wrong with my students. Do I use wrong ways when I'm teaching stoichiometry? Should I change the way I teach them? Can someone out there tell me what better way i should run on?

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General Chemistry (Part 2): Questions for X Level

The following questions are about:

• All mater
• Elements, compounds, and mixtures
• Atomic structure and Periodic Table
• Bonding and structure
• Chemical calculation
  

1) Which one is a compound?

A. A leaf

B. A paper

C. A pocket of dry ice

D. A tube of toothpaste

2) Which of the following electronic configurations represents an element that will form possible one simple ion with a charge of –3?

A. 1s² 2s² 2p⁶ 3s² 3p¹

B. 1s² 2s² 2p⁶ 3s² 3p³

C. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹

D. 1s² 2s² 2p⁶ 3s² 3p¹ 4s² 3d³

3) Which of the following ions contains unpaired electrons?

A. Fe²⁺ (Z = 26)

B. Mg²⁺ (Z = 12)

C. Ti⁴⁺ (Z = 22)

D. Zn²⁺ (Z = 30)

4) The successive ionization energies, in kJ.mol^–1, of an element X are 870; 1800; 3000; 3600; 5900; 7000; 13200. What is X?

A. ₃₃As

B. ₃₅Br

C. ₅₃I

D. ₅₂Te

5) The atoms X and Y have the electronic configurations shown below:

X: 1s² 2s² sp⁶ 3s² 3p¹ 4s² 3d¹⁰

Y: 1s² 2s² 2p⁴

Which one of the following compounds are they likely to form?

A. XY

B. XY₂

C. X₂Y

D. X₂Y₃

6) The electronic configuration of A and B are shown below:

A: 1s² 2s² sp⁶ 3s² 3p¹ 4s²

B: 1s² 2s² sp⁶ 3s² 3p⁵

What is the bonding type of the compound formed?

A. Ionic bonding

B. Covalent bonding

C. Covalent double bonding

D. Covalent coordinate bonding

7) In absence of catalyst, ammonia burns in oxygen to produce steam and nitrogen. What is gas and its volume remaining when 100 cm³ of ammonia is burned in 125 cm³ of oxygen? (All volumes are being measured at the same temperature and pressure).

A. 33.3 cm³ of ammonia

B. 66.7 cm³ of ammonia

C. 50 cm³ of oxygen

D. 75 cm³ of oxygen

8) How many grams of sodium hydroxide do you need to add into 0.2 liters of magnesium chloride 1 M to produce 5.8 grams of settled magnesium hydroxide? (Ar H = 1, O = 16, Na = 23, Mg = 24)

A. 4 grams

B. 8 grams

C. 16 grams

D. 32 grams

9) Which solution will have the highest electricity conduction hence they all are 1 M?

A. HCl

B. HNO₃

C. H₂SO₄

D. H₃PO₄

Write your answer in dark blue or black pen.

Do not use highlighters, glue, or correction fluid on your answer sheet.

10) There are 3 elements: A, B, C. They three are in same period of period system.

A and B are metals.

B is an explosive metal when it is burned with some water.

C is a polyatomic element which is in right side-neighbor of A.

a) Determine reactivity of those elements with oxygen from the highest to the lowest.

b) Suggest a real atomic symbol for each element.

11) A number of aluminum is added into aqueous sulfuric acid. A half of volume of gas produced is passed through a nitrogen tube and heated to produce ammonia. The ammonia is measured and it shows 2.24 liters of volume. How many grams of initial aluminum should be added? (Ar Al = 27)

Good Luck!

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General Chemistry (Part 1): Questions of Basic Stoichiometry

The following questions are about the basic stoichiometry. You may think why I wrote chemistry field again? It ain't a fun part of chemistry. Because I know many students get hard with stoichiometry so I put this field once again. But here are questions only. You need to read 'Let's Try to Learn Stoichiometry' at first and continue reading this part then. Here we go....

1. Calculate the empirical formula for a compound with the following percent composition: 23.29% of Mg, 30.72% of S and 45.99% of O.
   
   
2. A saturated hydrocarbon contains 82.66% of carbon. what is its empirical formula and molecular formula?
   
   
3. Calculate of the mass of chloride ion in 1.00 liter of each of the following solutions: a) 10% of NaCl (1.07 g/ml of density); b) 1.00 M NaCl; c) Calculate the molarity of the first solution.
   
   
4. Express the following gas pressures in atmospheres: a) 300 cmHg; b) 380 torr.
   
   
5. Calculate the molarity of each type of ion remaining in solution after 20.0 ml of 6.00 M hydrochloric acid is mixed with 50.0 ml of 2.00 M barium hydroxide.
   
   
6. How would one prepare exactly 300 ml of 5.00 M HCl solution by diluting sufficient 12.00 M stock solution.
   
   
7. Calculate the mass of CuS produced and the concentration of hydrogen ion produced by bubbling excess sulfuric acid into 1.00 liter of 0.10 M copper (II) chloride solution.
   
   
8. A refrigeration tank is holding 5.00 liters of freon gas (C₂C_l2F₂) at room temperature and 3.00 atm of pressure develop a leak. When the leak was discovered and repaired, the tank had lost 76.0 grams of the gas. What was the pressure of the gas remaining in the tank at the room temperature above?
   
   
9. In an auto engine with no pollution controls, about 5% of the fuel (assume 100% octane, C₈H₁₈) is unburnt. Calculate the relative masses of CO and C₈H₁₈ emitted in the exhaust gas. Calculate the relative volumes of these gases.
   
   
10. When 0.75 mole of a solid "C₈" and 2 moles of gaseous oxygen are heated in a seated vessel (bomb), completely using up the reactants and producing only one compound, it is found that when the temperature is reduced to the initial temperature, the contents of the vessel exhibit a pressure equal to half the original pressure . What conclusions can be drawn from these data about the product of the reaction?
    

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Let's Try to Learn Stoichiometry

You may think it's funny for you when you check the tittle of this post. 

'Is it a view? Why don't you leave this post and let other blogs to create this?' you may ask me. Let's me explain. As I'm a chemistry teacher and I've created this blog for my students especially so it's no wonder why I create an explanation of lessons as a view. I hope such posts will be used by other students of other senior high schools especially in Indonesia. Indonesia has a specific curriculum called national curriculum which adopts O-level curriculum and A-level curriculum and adds some new fields for X, XI, and XII grades. As I've been teaching in a senior high school with plus curriculum for about 11 years, I'm experienced enough to know what kinda chemistry problems for Indonesian students. I hope they may find some solutions for their chemistry problems here. This post will explain the stoichiometry concept only because it's commonly the hardest part of chemistry for Indonesian students but it will relate with many parts of chemistry and it's the basic of chemistry calculations. What's the basic of Stoichiometry? The basic concept is mathematics. Once you understand math well you can learn stoichiometry easier for sure. To make this field being narrower, here are the equations you need to get closer with. General equations Molar mass of an atom is equal to the molecular weight of the given atom. Molar mass of a molecule is equal to the molecular weight of the given molecule.

For all conditions

• mass = the number of moles x molar mass

m = n . mm

• the number of molecules = the number of moles X Avogadro number

X = n . L

• the number of moles = Molarity x Volume

n = M. V For specific conditions

• At standard temperature and pressure

Volume = the number of moles x Molar Volume V = n . Vm V = n x 22.4 liter/moles

• At specific temperature and pressure

P V = n R T where P = pressure (atm) n = the number of moles R = the ideal gas constant (0.082 liter.atm/mol.K) V1 . n2 = V2 . n1 where V1 and V2 = volume of gases n1 and n2 = the number of moles of gases What are steps to solve a stoichiometry problem? Here are the steps and do them in the manner:

• Read and understand the problem and list the known stuffs. Don't mention the earlier word of a given number but the later one of the given number.

Here's an example of a part of an unfinished question: What is the number of mass of 2.24 liters of ammonia.... So the known stuff is 'volume of ammonia or V' not 'the mass of ammonia or m'

• Determine if the problem needed a chemical equation or not. Write a balanced chemical equation if it needed it. Go to the next point if it didn't.

Example:

200 mL of nitrogen gas is flown into a tube of 200 mL of hydrogen gas and they are heated. In this case, you need to write a chemical equation and then balance it. Just use a balanced equation to solve any chemistry problems.

• List the equation(s) which you need to solve by using the known stuffs. Circle the known numbers and leave the unknown ones. Solve equations as many as possible.

• Add the new known number(s) into the equation(s) which you must solve.
• Convert the equation into the asked stuff in the problem and run it.
• You just solve a chemistry problem!

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