CHEMISTRY 2013-2014
In this unit, we shall begin the study of stoichiometry (from the Greek words "stoicheion" which means element and "metron" which means to measure. Stoichiometry problems are those problems which involve calculations. There are 2 types of stoichiometry problems – those requiring a balanced chemical equation (which we do not know how to write yet) and those problems which do not require a balanced equation, but do required a correctly-written formula. Thus, we have COMPOUND STOICHIOMETRY which we are just about to learn and REACTION STOICHIOMETRY which we will learn in about 3 weeks.
There are several different types of problems that you need to be able to recognize and calculate.
1. FORMULA or MOLECULAR MASS – these are two different phrases that really mean the same thing mathematically. We use the term formula mass for ionic compounds and the term molecular mass for molecular compounds (bonded covalently).
Step 1: Write a correct formula for the compound
Step 2: List the elements separately
Step 3: Beside the element’s symbol, write the number of atoms of each
Step 4: Multiple by the average atomic mass given on the Periodic Table. Round each mass to two decimal places.
Step 5: Calculate the answer for each
Step 6: Add the answers
Example 1: Find the formula mass of calcium phosphate.
Example 2: Find the formula mass of ammonium sulfate
Example 3: Find the molecular mass of dichlorine heptoxide.
Hydrates are ionic compounds that trap water molecules in their crystalline structures. You do not need to worry about which ionic compounds will do this and which will not – you just need to know how to name and recognize hydrates. A hydrate will be written as a correct ionic formula followed by a dot (that looks like a multiplication dot) and then some number of molecules of water: CuSO4 5H2O
To name a hydrate, simply name the ionic part as usual. For example if you are asked to name the compound CuSO4 5H2O, you will name the ionic part normally – copper(II) sulfate. Then you will add another word that consists of a prefix that indicates how many water molecules are trapped and the word "hydrate". Thus, the above-mentioned compound is copper(II) sulfate pentahydrate. We will use the same prefixes as you learned for the molecular compounds.
Prefixes for hydrates:
1 – mono 2 – di 3 – tri 4 – tetra
5 – penta 6 – hexa 7 – hepta 8 – octa
9 – nona 10 – deca
If you need to find the formula mass of a hydrate, add all of the average atomic masses of the ionic compound together and then add the mass of "x" number of water molecules to it. The dot does not mean multiplication.
Example 4: Find the formula mass of barium chlorate hexahydrate.
5A - FORMULA MASS AND MOLECULAR MASS WORKSHEET
Find the formula mass for each of these compounds which are ionic; find the molecular mass for each of the compounds which are molecular:
1. dichlorine heptoxide
2. cesium sulfite
3. tin (IV) hydroxide
4. acetic acid
5. lead (IV) oxalate
6. magnesium nitrite octahydrate
7. potassium phosphide
8. mercury (II) chloride
9. barium bicarbonate
10. ammonium chromate tetrahydrate
11. oxygen gas
12. strontium cyanide
13. silver perchlorate
14. iron (II) phosphate
15. nitric acid
16. copper (I) nitrate
17. cadmium dichromate decahydrate
18. zinc nitride
19. diphosphorus pentoxide
20. copper (II) bromide
2. MOLE PROBLEMS – The term mole is a unit used in chemistry that indicates a definite number of particles. In the American system of measures, we know that a "dozen" is a measurement that always means 12, no matter what we are measuring: eggs, pencils, or doughnuts.
The mole is a unit that means the number of representative particles found in the
formula/molecular mass of a substance expressed in grams.
The representative particle is important. For elements (except diatomics), it is the atom; for compounds and diatomics, it is the molecule (molecular compounds or diatomics) or the formula unit (ionic)
1 mole of sodium chloride is a different number of grams than 1 mole of water or 1 mole of calcium carbonate, but one mole of each of the compounds contains the same number of representative particles.
Sodium chloride weighs 58.5 amu; therefore 58.5 grams = 1 mole of NaCl
Water weighs 18 amu; therefore, there are 18 grams of water in 1 mole of water.
Calcium carbonate has a formula mass of 100 amu. This means that it takes 100 grams of calcium carbonate to make 1 mole of calcium carbonate.
Another label for formula or molecular mass can be g/mol – grams per mole (a good conversion factor since it is a double label).
Example 5: What if I have 200 grams of calcium carbonate, how many moles is this?
You calculate mole problems using dimensional analysis and the formula mass as a conversion factor.
Example 6: How many moles are there in 28.7 grams of lithium nitrate
Example 7: How many moles are there in 100 grams of nitrogen gas?
Be careful of the molecular mass of diatomics!!!!!!
Example 8: If 47.4 grams of an ionic compound are known to be 1.185 moles of that compound, what is the formula mass of the compound?
Example 9: If 56 grams of a pure monatomic element are known to be 8.26 moles of that element, what element are we talking about?
Example 10: If I tell you to measure out 1.34 moles of water, how many grams of water is this?
5B - MOLES WORKSHEET
1. How many moles are there in 768 grams of iron(III) sulfate?
2. How many moles are there in 1.2 kg of potassium dichromate?
3. I have 13.2 moles of lead(II) nitrate. How many grams is this?
4. How many pounds would 5.75 moles of magnesium hydroxide weigh?
5. How many moles are there in 78 grams of nitrogen?
6. How many grams would 12.30 moles of sulfur hexafluoride weigh?
7. I have 0.51101 pounds of an unknown compound, and it is known this amount represents 4.50 moles of this compound. What is the molecular mass of the compound?
8. I have 7.60 moles of chlorine gas. Would this amount weigh more than a pound? Prove your answer.
9. What is the molecular mass of a compound if it is known that 658 grams of this compound represent 6.89 moles?
10. If it is known that 41.6 grams of pure element that is not diatomic represent 1.30 moles of this element, what is the atomic mass of the element? What element is it?
11. If the above element were known to be one of the diatomic elements, what element would you predict it to be?
12. How many moles are there in 34.5 grams of strontium chloride?
13. If the molecular mass of a substance is 203 g/mol, how many moles are there in 4567 milligrams of this substance?
14. What is the molecular mass of a compound if 0.544 moles weighs 3.4509 grams?
15. What is the molecular mass of a compound if it is known that 8.00 x 108 micrograms of it represent 2.55 moles?
16. Which would weigh more: 3.40 moles of aluminum nitrite or 3.40 moles of cadmium phosphate?
3. AVOGADRO'S NUMBER PROBLEMS – Avogadro's number (6.022 x 1023) is the number of representative particles in one mole of a substance. For elements (except diatomics), there are 6.022 x 1023 atoms in one mole of that element. For molecules, ionic compounds and diatomics, there are 6.022 x 1023 molecules or formula units in one mole of that compound.
There are 65 grams in 1 mole of pure zinc metal. Therefore, there are 6.022 x 1023 atoms of zinc in 65 grams (or 1 mole) of zinc.
There are 18 grams of water in 1 mole of water. Therefore, there are 6.022 x 1023 molecules of water in 18 grams or 1 mole of water.
There are 32 grams of oxygen in 1 mole of oxygen gas. Therefore, there are 6.022 x 1023 diatomic oxygen molecules in 32 grams of oxygen gas or 1 mole of oxygen gas. However, there are 2(6.022 x 1023) or 1.2044 x 1024 oxygen atoms in 32 grams of oxygen gas.
There are 58.5 grams of NaCl in 1 mole of this ionic compound. Therefore, there are 6.022 x 1023 formula units of NaCl in 58.5 grams of NaCl.
Formula unit : ionic compound : : molecule : molecular compound
The most basic or the representative particle of an element (except diatomics) that will retain the properties of that element is the atom.
The most basic or the representative particle of a molecular (2 non-metals) compound is the molecule.
The most basic or the representative particle of an ionic compound is called the formula unit.
Example 11: How many atoms are present in a pure iron nail that weighs 13.2 grams?
Example 12: If there are known to be 8.24 x 1026 molecules of water present, would it weigh more than a pound?
Example 13: How many moles of carbon dioxide are present in 4.55 x 1024 molecules of CO2?
WHEN IN DOUBT, GO TO MOLES!!
Example 14: How many formula units are there in 7.55 grams of sodium sulfate?
Example 15: If I know that a sample of mercury(II) chloride contains 7.32 x 1024 formula units, how much does the sample weigh in pounds?
Example 16: Which would contain more representative particles: 46 grams of sodium hydroxide or 46 grams of carbon dioxide?
Example 17: Which weighs more: 6.24 x 10 23 formula units of sodium bromide or 6.24 x 1024 molecules of phosphorus tribromide?
Being aware of what the representative particle of any substance is very important! The following problems sound exactly like the ones we have just worked, but they are worked with an extra step that is essential due to the representative particle being different than the particle asked for or given in the problem.
Example 18: How many carbon atoms are there in 16.0 grams of glucose?
Example 19: If a sample of aluminum sulfite is known to contain 4.55 x 1023 atoms of aluminum, how much does the entire sample of aluminum sulfite weigh in pounds?
Example 20: How many nitrogen atoms are there in 45.8 grams of ammonium nitride?
5C - AVOGADRO'S NUMBER WORKSHEET
1. How many formula units would there be in a 54.3 gram sample of sodium nitrate?
2. How many atoms of copper are there in 1.00 kg of pure copper?
3. How much would 3.42 x 1023 formula units of mercury(II) oxide weigh in grams?
4. Which would weigh more: 6.54 x 1022 formula units of aluminum chloride or 7.34x1022 formula units of tin(II) fluoride?
5. How many molecules are present in 0.00115 grams of carbon dioxide?
6. How many atoms of zinc are required to weigh 5.40 ng?
7. I have 3.45 x 1024 molecules of water. How much would this weigh in pounds? How many moles are there in this amount of water?
8. How many moles are represented by 8.27x1023 molecules of any molecular substance?
9. Which would be more moles of a molecular compound: 6.57 x 1023 molecules or 6.59x 1023 molecules. How do you know?
10. How many diatomic molecules of chlorine were there be in 28.0 grams of chlorine gas? How many atoms of chlorine would be present in this same amount?
11. How many carbon atoms would there be in 200 grams of aluminum carbonate?
12. If there are known to be 7.93 x 1024 atoms of oxygen present in a sample of magnesium sulfate, how much does the sample of magnesium sulfate weigh in pounds?
13. How many oxygen atoms are there in 2.4 pounds of calcium acetate?
14. How many chlorine atoms are there in 45 grams of mercury(I) chlorate?
15. If there are known to be 1.06 x 1024 oxygen atoms in a sample of barium permanganate, what is the mass of the entire sample of barium permanganate in grams?
16. If there are known to be 9.76 x 1025 chlorine atoms in a sample of aluminum chloride, would the entire sample weigh more than 15 pounds?
17. How many oxygen atoms are there in 170 grams of cesium phosphite?
4. PERCENTAGE COMPOSITION PROBLEMS – just like any percentage problems you are comparing the part to the whole. In chemistry, percentage composition is based on mass, not on numbers of atoms present. For example, if I have 5 pieces of fruit in a dish and 2 of the pieces of fruit are bananas, what percentage of the fruit in the dish is bananas?
We are going to find the percentage by mass of certain elements in a compound by comparing the mass of the element to the mass of the entire compound.
How to find percentage composition:
Step 1: Write a correct formula for the compound in the question
Step2: Find the formula or molecular mass of the entire compound
Step 3: Divide the total mass of the element by the total mass of the entire compound
Step 4: Multiply the result by 100 to convert it to a percentage.
Step 5: Since there are no significant digits given in the problem except masses that have already been rounded, express your answer to 2 decimal places and place a % sign as the label.
Example 21: What is the complete percentage composition (by mass) of potassium dichromate?
Example 22: What is the percentage of calcium in calcium phosphide?
Example 23: What is the percentage of oxygen in barium chlorate hexahydrate?
Remember that a percentage is an excellent conversion factor since it represents a comparison of units to 100 units. The following problems can be worked using dimensional analysis or by finding the percentage of a particular element present and then multiplying it times the mass of the compound given.
Example 24: How many grams of pure magnesium could be recovered from the decomposition of 49.4 grams of magnesium fluoride?