Duncan UNIT 7 - CHEMICAL FORMULAS WRITING FORMULAS NOTES. Does the second word end with -ide?

Transcription

1 WRITING FORMULAS NOTES Does the second word end with -ide? yes no / \ Is the second word "hydroxide"? \ no yes \ \ \ Is the first word "ammonium"? \ no yes > TERTIARY COMPOUND BINARY COMPOUND Is the first word the name of a metal or "ammonium"? Is the first word the name of a / nonmetal or a metal? > General Rules for Writing Formulas 1. Write "NH4 +1 " 1. Write the symbol for the metal with 2. Write symbol for the second 1. Write symbol for first its oxidation number. element with its oxidation element. 2. Write the symbol for the second number or formula for poly- 2. Prefix of first word tells element with its oxidation number atomic ion with its oxidation subscript of first symbol. or formula for polyatomic ion with number 3. Write symbol for second its oxidation number 3. Cross out the "+" and "-". element. 3. Cross out the "+" and "-". 4. Reduce (simplify) the 4. Prefix of second word 4. Reduce (simplify) the numbers if numbers if possible. tells subscript of second possible. 5. "Criss-cross" the numbers. symbol. 5. "Criss-cross" the numbers. (The oxidation number of (The oxidation number of the first ammonium becomes the part becomes the subscript of the subscript of the second part. second part. The oxidation number The oxidation number of the of the second part becomes the second part becomes the subscript of the first part.) subscript of the first part.) EXAMPLES: 1.) calcium oxide 2.) carbon tetrachloride 3.) ammonium chromate 4.) lead (II) nitrate 5.) aluminum hydroxide 6.) iron (III) bromide 1

2 Alternate notes for Writing Formulas: 1.) Does the name end with -ide? If yes, go to # 2 If not, go to # 4 2.) Is the second word hydroxide? If yes, go to # 4 If no, go to # 3 3.) Is the first word ammonium? If yes, go to # 4 If no, go to # 5 4.) The compound is tertiary (or ternary compound has more than 2 elements). Is the first word the name of a metal or ammonium? If first word is name of a metal, go to # 7 If first word is ammonium, go to # 8 5.) The compound is binary (has only two elements). Is the first word the name of a nonmetal or a metal? If first word is name of a nonmetal, go to # 6 If first word is name of a metal, go to # 7 6.) To write formula for compound: ~ Write the symbol for the first element. ~ Prefix of first word tells subscript of first element. ~ Write symbol for second element. ~ Prefix of second word tells subscript of second element. 7.) To write formula for compound: ~ Write the symbol for the element with its oxidation number. ~ Write the symbol for the second element with its oxidation number (if binary cmpd) OR ~ Write the formula for the polyatomic ion with its oxidation number (if tertiary cmpd) ~ Cross out the + and -. ~ Reduce (simplify) the numbers if possible. ~ Criss-cross the numbers. (The oxidation number of the 1 st part becomes subscript of the 2 nd part. The oxidation number of 2 nd part becomes the subscript of the 1 st part.) 8.) To write formula for compound: ~ Write NH4 +1 ~ Write the symbol for the second element with its oxidation number. OR ~ Write the formula for the polyatomic ion with its oxidation number. ~ Cross out the + and -. ~ Reduce (simplify) the numbers if possible. ~ Criss-cross the numbers. (The oxidation number of ammonium becomes the subscript of the 2 nd part. The oxidation number of the 2 nd part becomes the subscript of ammonium.) EXAMPLES: 1.) calcium oxide 2.) carbon tetrachloride 3.) ammonium chromate 4.) lead (II) nitrate 5.) aluminum hydroxide 6.) iron (III) bromide 2

3 Writing Formulas Worksheet 1. sodium nitrate 16. aluminum sulfide 2. iron (III) sulfate 17. strontium hydroxide 3. copper (I) phosphate 18. cesium carbonate 4. nickel (II) oxide 19. silicon dioxide 5. potassium bromide 20. zinc chlorate 6. barium acetate 21. iron (II) phosphide 7. magnesium chromate 22. calcium nitride 8. silver sulfite 23. sodium bicarbonate 9. copper (II) chloride 24. ammonium dichromate 10. dinitrogen tetroxide 25. potassium iodide 11. rubidium sulfate 26. lead (II) acetate 12. beryllium nitrate 27. ammonium sulfide 13. phosphorus trichloride 28. silver carbonate 14. magnesium fluoride 29. ammonium phosphate 15. lead (II) bromide 30. nickel (II) iodide ===================================================================== NAMING COMPOUNDS NOTES How many elements does the compound contain? BINARY CMPD. < only 2 more than > TERTIARY CMPD. Is the first element a nonmetal or a metal? > Is the metal in Group 1, 2, or Is the first element a Al, Zn, or Ag? < metal or a nonmetal? V yes no 1. name of first \ V element with V V 1. first word = prefix telling 1. name of first 1. name of first "ammonium" subscript of element element 2. second word = first element* 2. name of second 2. (Roman numeral name of second 2. prefix telling element with -ide telling oxidation element with subscript of ending or name of number of first -ide ending or second element polyatomic ion element) name of polywith name of 3. name of second atomic ion second element with -ide ending element with -ide ending or name of polyatomic ion * no prefix on first word if there is no subscript

4 EXAMPLES: 1.) P2O5 2.) MgSO4 3.) CuCl2 4.) (NH4)3PO4 5.) FeCO3 6.) K2O Alternate Notes for Naming Compounds: 1.) How many elements does the compound contain? If compound has two elements, go to # 2 If compound contains more than two elements, go to # 3 2.) The compound is binary. Is the first element a nonmetal or a metal? If the first element is a nonmetal, go to # 4 If the first element is a metal, go to # 5 3.) The compound is tertiary (or ternary). Is the first element a nonmetal or a metal? If the first element is a nonmetal, go to #6 If the first element is a metal, go to # 5 4.) To name the compound: ~ 1 st word = name of 1 st element with prefix telling the # of the subscript* ~ 2 nd word = prefix telling subscript, then name of 2 nd element with -ide ending *no prefix on first word if there is no subscript 5.) Is the metal in Group 1, 2, or Al, Zn, or Ag? If metal is in Group 1, 2, or Al, Zn, or Ag, go to # 7 If metal is NOT in Group 1, 2, Al, Zn, or Ag, go to # 8 6.) To name the compound: ~ 1 st word = ammonium ~ 2 nd word = name of polyatomic ion or name of 2 nd element with -ide ending 7.) To name compound: ~ 1 st word = name of metal ~ 2 nd word = name of 2 nd element with -ide ending (if binary) OR name of polyatomic ion (if tertiary) 8.) To name compound: ~ 1 st word = name of metal (Roman numeral indicating the oxidation number of the metal) ~ 2 nd word = name of 2 nd element with -ide ending (if binary) OR name of polyatomic ion (if tertiary) EXAMPLES: 1.) P2O5 2.) MgSO4 3.) CuCl2 4.) (NH4)3PO4 5.) FeCO3 6.) K2O 4

5 Naming Compounds Worksheet 1. NaCl 14. Ba3(PO4)2 2. Al2O3 15. Ni(ClO)2 3. CuSO4 16. ZnCr2O7 4. MgCl2 17. LiOH 5. (NH4)3PO4 18. AgHCO3 6. Pb(NO3)2 19. Fe(C2H3O2)3 7. K2CO3 20. CO2 8. NO2 21. Cu2CrO4 9. SrSO3 22. Al(OH)3 10. RbNO2 23. N2O 11. FeBr2 24. AgClO2 12. P2O5 25. CaI2 13. Cu(ClO3)2 26. SO2 Polyatomic Ions (Listed Alphabetically) Name Formula Name Formula acetate C2H3O2-1 hydroxide OH -1 ammonium NH4 +1 hypochlorite ClO -1 bromate BrO3-1 iodate IO3-1 carbonate CO3-2 nitrate NO3-1 chlorate ClO3-1 nitrite NO2-1 chlorite ClO2-1 perchlorate ClO4-1 chromate CrO4-2 permanganate MnO4-1 cyanide CN -1 phosphate PO4-3 dichromate Cr2O7-2 sulfate SO4-2 hydrogen carbonate (or bicarbonate) HCO3-1 sulfite SO3-2 hydrogen sulfate HSO4-1 thiocyanate SCN -1 5

6 Oxidation Numbers Notes Any uncombined element (element not in a compound) has an oxidation number of 0. Fluorine always has an oxidation number of -1 in a compound. Oxygen has an oxidation number of -2 in all compounds except when it is part of a binary compound with a halogen. Hydrogen has an oxidation number of +1 except when it is with metals. The algebraic sum of the oxidation numbers in a compound is zero. EXAMPLE: Find the oxidation number of carbon (C) in Na2CO3. ~ Let x = oxidation # of carbon ~ Na s oxid. # is +1 ~ O s oxid. # is -2 ~ sum of (# of each element in cmpd. multiplied by its oxid. #) = 0 ~ 2 (+1) + 1 (x) + 3 (-2) = 0 Na C O ~ 2 + x 6 = 0 ~ x 4 = 0 ~ x = +4 (Carbon s oxidation number in Na2CO3 is +4.) Find the oxidation number of the underlined element in each compound. 1. KMnO4 2. LiNO3 3. NaClO 4. MnO2 5. Ca(NO2)2 6. Ba(ClO4)2 6

7 OXIDATION NUMBERS WORKSHEET 1.) HCl Cl: 11.) H2SO3 S: 2.) KNO3 N: 12.) H2SO4 S: 3.) Fe(OH)3 Fe: *13.) BaO2 O: 4.) Mg3N2 N: 14.) KMnO4 5.) KClO3 Cl: *15.) LiH Mn: H: 6.) Al(NO3)3 N: 16.) MnO2 Mn: 7.) S8 S: 17.) OF2 O: 8.) Li2CO3 C: 18.) SO3 S: 9.) PbO2 Pb: 19.) NH3 N: 10.) NaHSO4 S: 20.) Na Na: Percent Composition Worksheet Determine the percent composition of each of the following compounds. 1. KMnO4 K = Mn = O = 2. HCl H = Cl = 3. Mg(NO3)2 Mg = N = O = 4. (NH4)3PO4 N = H = P = O = 5. Al2(SO4)3 Al = S = O = Solve the following problems. 6. How many grams of oxygen can be produced from the decomposition of 100. g of KClO3? 7. How much iron can be recovered from 25.0 g of Fe2O3? 8. How much silver can be recovered from 125 g of Ag2S? 7

8 Percent Composition Notes PERCENT COMPOSITION: the percentage by mass of each element in a compound FORMULA FOR % COMPOSITION: % composition = mass of element in compound x 100 molar mass of compound EXAMPLES: Q1. Find the % composition of copper (I) sulfide, Cu2S. A1. Finding % composition means that you have to find the % of each element in the cmpd. molar mass of Cu2S = Cu: x = S: x = MM of Cu2S = % Cu = x 100 = % S = x 100 = A1 = Cu: % and S: % * For % comp. problems, you can round to 3 SF's. * Q2. Find the percent of oxygen in calcium phosphate, Ca3(PO4)2. A2. molar mass of Ca3(PO4)2 = Ca: x = P: x = O: x = MM of Ca3(PO4)2 = % O = x 100 = A2 = % 8

9 Empirical Formulas Notes opposite of percent composition use % to find formula for compound EMPIRICAL FORMULA: simplest formula; #'s of subscripts are reduced to lowest terms MOLECULAR FORMULA: subscripts are multiples of empirical formula subscripts MOLECULAR FORMULA C6H6 C6H12O6 EMPIRICAL FORMULA C12H16O4N TO SOLVE EMPIRICAL FORMULA PROBLEMS: A sample of a compound is found to contain 36.0 % calcium and 64.0 % chlorine. Calculate the empirical formula. Step 1: Rewrite % as grams g Ca 64.0 g Cl Step 2: Find moles of each element. Ca: 36.0 g Ca 1 mole Ca = moles Ca Cl: 64.0 g Cl 1 mole Cl = 1.80 moles Cl 40.1 g Ca 35.5 g Cl Step 3: Find mole ratio. (Divide by smallest number of moles.) Ca: moles = 1 Cl: 1.80 moles = * These whole numbers are subscripts in formula.* Step 4: Write the formula. Ca1Cl2 ====> CaCl2 Example 2: A sample of a compound contains 66.0 % calcium and 34.0 % phosphorus. What is the empirical formula? Ca: 66.0 g Ca 1 mole Ca = 1.65 moles Ca P: 34.0 g P 1 mole P = 1.10 moles P 40.1 g Ca 31.0 g P Ca: 1.65 = 1.5 P: 1.10 = Q: So, what happens now? I can't write Ca1.5P1. And 1.5 is not close enough to round to 2. A: The easiest way to get 1.5 to a whole # is to multiply by 2. Remember to multiply both #'s by 2 to get your answer. Ca: 1.5 x 2 = 3 P: 1 x 2 = 2 So, formula is Ca3P2 PRACTICE - A compound contains 43.4 % sodium, 11.3 % carbon, and 45.3 % oxygen. What is the empirical formula for this compound? 9

10 Empirical Formulas Worksheet Find the empirical formula for each of the following substances. The percent composition is given % copper & 11.2 % oxygen % carbon, 0.84 % hydrogen, & % chlorine % chromium & % chlorine % potassium, % nitrogen, & % oxygen * % iron & % oxygen Part 2 Determine the empirical formula of the following compounds using the given data. 6. Find the empirical formula for sodium sulfite. Sodium sulfite contains 36.5 % sodium, 25.4 % sulfur, and 38.1 % oxygen. 7. What is the empirical formula for a compound which contains % iron and % sulfur? 8. What is the empirical formula of a compound if the percentage composition is: aluminum %, sulfur %, and oxygen %? 9. If 8.87 grams of phosphorus react with grams of oxygen, what is the empirical formula of the compound formed? 10. Phosgene, a poisonous gas used during World War I, contains 12.1 % C, 16.2 % O, and 71.7% Cl. What is the empirical formula for phosgene? MOLECULAR FORMULAS NOTES To find the molecular formula, one more piece of information must be given - the molar mass (also called molecular mass or formula mass). EX. 1- An organic compound is found to contain 92.25% carbon and 7.75% hydrogen. If the molecular mass is 78, what is the molecular formula? STEP 1: Find the empirical formula. C: g C 1 mole C = 7.69 moles C H: 7.75 g H 1 mole H = 7.75 moles H 12 g C 1 g H 7.69 moles C = moles H = So... empirical formula is CH. STEP 2: Find molar mass of the empirical formula. C: 1 x 12.0 = 12.0 H: 1 x 1.0 = 1.0 MM =

11 STEP 3: Find "multiple" number. MM of molecular formula = multiple # 78 = 6 MM of empirical formula 13 STEP 4: Write molecular formula. Multiply "multiple" # by all subscripts in the empirical formula. So... molecular formula is C6H6. PRACTICE - An oxide of nitrogen contains 30.4 % nitrogen and 69.6 % oxygen. If the molar mass of this compound is 92 g/mole, what is the molecular formula? Molecular Formulas Worksheet Solve the following problems. 1. A compound is found to be 40.0 % carbon, 6.7 % hydrogen, and 53.5 % oxygen. Its molecular mass is 60. grams per mole. What is its molecular formula? 2. A compound is 64.9 % carbon, 13.5 % hydrogen, and 21.6 % oxygen. Its molecular mass is 74 grams per mole. What is its molecular formula? 3. A compound is 54.5 % carbon, 9.1 % hydrogen, and 36.4 % oxygen. Its molecular mass is 88 grams per mole. What is its molecular formula? 4. If the molecular mass of an oxide of nitrogen is 108. What is the molecular formula of a compound that contains 4.02 grams of nitrogen and grams of oxygen? 5. There are two different oxides of phosphorus. Both oxides can exist in different forms depending on the temperature and pressure. Calculate the empirical and molecular formulas from the following data: (A) P: 56.4 %, O: 43.7 %, molecular mass = 220 (B) P: 43.6 %, O: 56.4 %, molecular mass = Nicotine is a compound that contains 74.0 % carbon, 8.7 % hydrogen, and 17.3 % nitrogen. If the molecular mass is 162, what is the molecular formula? HYDRATES NOTES Hydrates are compounds with a certain number of water molecules attached to them. Their formulas look the same except that there is a. # H2O after it. Example: MgSO4. 7 H2O When determining the empirical formula for a hydrate, generally you will be determining the number in front of the H2O in the formula. In order to determine this number, you will need to find the mole ratio between the moles of the compound and moles of water. EXAMPLE: A hydrated sample of sodium carbonate has a mass of grams. The sample is then heated and all water is removed. The anhydrous salt that remains has a mass of grams. What is the empirical formula for the hydrated sodium carbonate? STEP 1: Determine the formula for sodium carbonate. Na +1 CO3-2 Na2CO3 STEP 2: Find moles of sodium carbonate g Na2CO3 1 mole Na2CO3 = moles Na2CO3 Na: 2 x 23.0 = g Na2CO3 C: 1 x 12.0 = 12.0 O: 3 x 16.0 =

12 STEP 3: Find moles of water. First, find grams of water g H2O 1 mole H2O = moles H2O 18 g H2O hydrated sample = grams Anhydrous sample = grams Mass of water = grams STEP 4: Find mole ratio of water to sodium carbonate moles = 10 So empirical formula for this hydrate is Na2CO3. 10 H2O moles Composition of Hydrates Worksheet 1. A hydrated sodium salt containing 39.7 % water is analyzed as follows: Na 16.9 %, C 17.7 %, H 6.67 %, and O 58.8 %. What is the empirical formula of this salt? 2. Nicotine is a compound that contains 74.0 % carbon, 8.7 % hydrogen, and 17.3 % nitrogen. If the molecular mass is 162, what is the molecular formula? 3. Strontium hydroxide is isolated as a hydrate, which means that a certain number of water molecules are included in the solid. When 6.85 grams of the hydrate are dried in an oven, 3.13 grams of anhydrous Sr(OH)2 are formed. 4. A 2.5 gram sample of a hydrate of Ca(NO3)2 was heated, and only 1.7 grams of the anhydrous salt remained. What percentage of water was in the hydrate? 5. A 5.0 gram sample of Cu(NO3) 2. n H2O is heated, and 3.9 gram sample of the anhydrous salt remains. What is the value of n? 12

13 EMPIRICAL FORMULA OF A HYDRATE LAB Many salts crystallized from water solutions appear to be perfectly dry, yet when heated, they liberate large quantities of water. The crystals change form, even color, as the water is driven off. Such compounds are called hydrates. The number of moles of water present per mole of anhydrous salt (salt minus water of crystallization) is usually a whole number. One example is the hydrate of copper (II) sulfate. Its blue crystals look and feel dry. Yet each mole of hydrate contains 5 moles of water. Its formula is CuSO4. 5 H2O. The dot between the CuSO4 and the 5 H2O does NOT mean multiplication. It indicates that 5 water molecules are bound to the other atoms. The molar mass of CuSO4. 5 H2O is g/mole. In this experiment, you will determine the empirical formula of a hydrate. You will do so by heating the hydrate to drive off the water. You will measure the mass of the water driven off the hydrate by heating it and you will measure the mass of the anhydrous salt that remains. By calculating the number of moles of water driven off and the number of moles of anhydrous salt remaining, you will be able to find the empirical formula of the hydrate. Materials " glass test tubes ring stand, wire gauze, iron ring, & burner hydrated salt wood splint spatula for transferring solid to test tubes Procedure *PUT YOUR GOGGLES & APRON ON NOW!* 1. Obtain and label three dry test tubes #1, #2, and #3. Weigh the test tubes to the nearest 0.01 g. Record these data on the data table below. Remember to zero the balance each time you use it. Also remember to use the same balance throughout the experiment. 2. Add g of the hydrate to each of the test tubes. Do not pack the crystals. 3. Reweigh the three test tubes and record the masses, again to the nearest 0.01 g. 4. Lay the test tubes on the square wire gauze. Turn the corners of the wire gauze up slightly so the test tubes do not roll off. You must be able to heat the entire length of the test tube with the burner to drive off all the water of hydration. Place the wire gauze and the three test tubes on the iron ring and heat the three samples gently and evenly along their entire lengths. 5. Heat the test tubes by gently waving the burner flame back and forth underneath the wire gauze. Record any observations you make during the heating process. 6. Heat for approximately 5 minutes. Allow the test tubes to cool before handling. This may take several minutes. (Remember that the glass is very hot even though it may appear to be cool.) 7. Weigh the three samples and record the data. 8. Reheat the three samples for an additional five minutes, cool, and reweigh until the mass of the anhydrous salt and test tube is constant. What does anhydrous mean? 13

14 DATA TABLE 1 mass of empty test tube (g) 2 mass of hydrate & test tube (g) 3 mass after 1st heating (g) 4 mass after 2nd heating (g) 5 constant mass of anhydrous salt & test tube (final mass, g) 6 mass of water driven off (g) 7 # of moles of water 8 molar mass of anhydrous salt ( ) 9 grams of anhydrous salt (g) 10 moles of anhydrous salt 11 mole ratio of H2O to anhydrous salt 12 Empirical formula of hydrate - experimental 13 Empirical formula of hydrate - theoretical Trial 1 Trial 2 Trial 3 Calculations Show all calculations necessary to calculate the empirical formula for the hydrated salt. You have to show your work for only one trial. Grading To receive full credit, the data table must be filled in completely and correctly for all three trials. You must include a sample calculation sheet along with your data table **CRITERIA IN ORDER TO RECEIVE CREDIT FOR THIS ASSIGNMENT: * All numbers are clear & legible. * All numbers are labeled with their correct units. * Calculations are shown in detail. * Calculations are shown in a neat and logical order. * No messy cross-outs or eraser marks. * Failure to follow these criteria will result in your having to re-submit your lab. It will be considered a late grade when you re-submit. THIS LAB IS DUE ON: 14