Teaching Tips for Lesson 17: Formula Weights

Transcription

1 Teaching Tips for Lesson 17: Formula Weights Game Plan: A. Review how moles are utilized in chemistry to measure amounts of elements. B. Introduce how to find the number of grams in one mole of a compound (finding formula weights). C. Play Compound Intensity board game. D. Assign Lesson 17 Practice Pages. E. Lab activity: Weighing Moles of Compounds. F. Give Lesson 17 Test. A. Review how moles are utilized in chemistry to measure amounts of elements. Take time at the beginning of class to review how moles are used to measure out portions of elements. Get out your Styrofoam cups with packing peanuts and sand (or whatever the items you used in Lesson 16 to represent moles of elements). Probe for understanding by asking how the number of atoms vary in a mole an element (they don t vary, only the mass of a mole varies!). Ask for some mole to gram conversions and then some gram to mole conversions. Continue by introducing how to find the mass of one mole of a compound (finding formula weights). B. Introduce how to find the number of grams in one mole of a compound (finding formula weights). Introduce finding formula weights by reading the text portion together. Be careful to note how a subscript outside a parentheses has its effect upon the entire ion that it follows. A great way to practice finding formula weights is to play Compound Intensity. 1

2 C. Play Compound Intensity board game. Compound Intensity (Level 1) allows your students a fun way to practice finding formula weights of compounds as well as reviewing the names of s, anions and formation of compounds. The board for play is found on pages 4-5 (Volume 2). You may wish to make copies of the game board and laminate them into one flat game board. There are two card decks which are needed for play which are found in the manipulative set. You will also need a die plus playing pieces (one playing piece per team) and sheets of blank paper on which to write answers. If small children are in the playing area, note that the die and playing piece can be a choking hazard. After cutting out the cards, separate them into an anion deck and a deck and place them face down in their respective los on the game board. Each team or individual should place his playing piece at the starting square. We recommend each team be no larger than two members. Play begins by each team rolling the die. The team with the highest roll, goes first. After rolling the die, the team moves their playing piece the appropriate number of spaces down the board. There are five different kinds of spaces on the board on which a team might land. Each space requires the team complete a particular task. Should a team land on a space marked anion, the team draws one card from the anion deck. The card is revealed to all teams. After reading the card, all teams make an attempt to write the correct anion symbol and charge on their papers. When all are complete, it is your job to see if the team whose turn it is, has the correct answer. If they do, they earn the point value of the card. If they have an incorrect answer, announce to the opposing teams that they have a chance to steal points. Check their answers and award teams with correct responses the points on the card. Teams who have incorrect responses earn no points. Play continues with the team to the right rolling the die. Should a player land on a space marked, play proceeds as above with the exception that a card from the deck is drawn. Should a player land on a space marked compound formula, both an anion and a card is drawn. Cards are revealed to all teams. Using both cards, all teams then write the correct compound formula. Check the work of the team whose turn it is for 2

3 accuracy. If their work is correct, double the total point value of the cards and award accordingly. If the answer is incorrect, announce the steal opportunity and check the work of the opposing teams. Award points accordingly. Should a team land on a formula weight space, an anion and card are drawn and revealed to all teams. Everyone calculates a formula weight for the compound. Play continues as above. If the team whose turn it its calculates a correct formula weight, the total points of both cards is tripled! If incorrect, other teams can steal and points are awarded accordingly. Finally, there are some special spaces on the board with varying instructions. Most are self-explanatory and are similar to other board games (lose a turn, go ahead spaces, etc.) One space which needs explanation is the any two cards space. When a team lands on this space, they have the option of choosing any two cards from the top of the decks. They can draw two anion cards, two cards or one of each card. After revealing these cards, they have three options: (1) they can write only the symbol and charge for each revealed ion and earn points accordingly, or (2) they can write the compound formula for the two revealed cards (they must have taken one card of each) for double point values, or (3) calculate the formula weight (again they must have taken one card of each) for triple point values. As an added bonus, the total point value for the round is then doubled! Stealing by opposing teams in NOT an option for landing on this space. In the anion and decks are each found a wild card. When a team draws one of these cards, they can designate which ever anion (if it was drawn from the anion deck) or (if it was drawn from the deck) they would like for it to be. Once decided upon, all opposing teams are informed. Because it was a wild card, it carries maximum point values which are awarded accordingly. Play continues until a team reaches 1000 points (or another point goal set by you.) Consider playing Compound Intensity over several days. A tremendous amount of practice can be made which will improve your student s skills. To make your life easier, we have compiled an answer key for all possible combinations of anion and cards. This card includes the ion symbols and charges, the compound formula and the formula weight. Because Compound Intensity has a level 2 3

4 Any 2 cards, double points! Cation Compound formula Lose a Turn Formula Weight Anion Formula Weight Level 1 Compound formula Cation Place Cation Cards Here 50 free points and roll again! Begin Play here Anion Cation Compound formula Cation Formula Weight 4

5 Compound formula Anion Formula Weight Go back two spaces! Formula Weight Compound formula Formula Weight Place Anion Cards Here Anion Compound formula Level 1 Formula Weight Cation Compound formula Formula Weight Formula Weight Compound formula Anion 5

6 (finding the percent composition of a compound), this answer key will be used again later in the course. This answer key can be found on pages 8-9. To use this key, note that the s are listed across the top of the chart while the anions are listed down the left side. Find the desired and then go down that column until you match it with the needed anion. In that square you will find the correct chemical formula for that compound. Beneath the compound formula you will find the formula weight. Beneath that number you will see the percent composition values for each constituent of the compound. Note that these percent composition values appear in the same order as the elements do in the compound. Answers your students generate may vary slightly up or down from our answers due to rounding. D. Assign Lesson 17 Practice Pages. E. Lab Activity: Weighing Moles of Compounds This is a simple lab activity which gives your students some hands-on experience in weighing out the moles of various household compounds. Materials needed are: a scale which will weigh in grams (triple-beam, double-pan or electric work equally well; a kitchen scale will work also as long as it has gram measurements), some sort of weighing dish (Styrofoam cups or bowls work fine, Petri dish lids work well), a spoon or scoop to transfer chemicals and then your compounds. If all you have available is a scale that does not read in grams, you can still use it knowing that 1 pound equals 454 grams and 1 ounce equals 28.3 grams. The purpose of this lab is to have your students weigh out a specified number of moles of a compound. In order to do so, they must calculate the formula weight first. Once that step is completed, depending upon the number of moles you requested, they then weigh out the grams necessary to meet that requirement. For example, if you requested 2 moles of sodium chloride (table salt), they would first calculate the formula weight of the salt (58 grams). Since you requested 2 moles, they would multiply their formula weight by two to get the required amount of salt necessary. To check their work, you can weigh the amounts of compound they pro- 6

7 vide you. In this example, they would weigh out 116 grams (58 g x 2) of table salt. A variation on the lab is to prepare some compound samples with a known number of moles (to you, that is). For example, before class time, weigh out a certain number of moles of various compounds. Allow your students to make their observations (amount of grams present would be a vital bit of information to be gathered). Tell your student the identity of each compound but then require that they tell you the number of moles present. Compounds that work well for this lab include table salt (sodium chloride), baking soda (sodium carbonate) and Epsom salts (magnesium sulfate). E. Give Lesson 17 Test. 7

8 Compound Intensity Friendly Answer Chemistry Card Page 1 Cations Anions Magnesium Mg +2 Barium Ba +2 Copper (II) Cu +2 Lead (IV) Pb +4 Tin (II) Sn +2 Ammonium (NH 4) +1 Iron (III) Fe +3 Tin (IV) Sn +4 Potassium K +1 Bromide Mg +2 Br 2 Br /87 Ba +2 Br /54 Cu +2 Br /71 Pb +4 Br /61 Sn +2 Br /57 (NH 4) +1 Br 98 14/4/87 Fe +3 Br /81 Sn +4 Br /73 K +1 Br /67 Oxide Mg +2 O O 40 60/40 Ba +2 O /10 Cu +2 O 80 80/20 Pb +4 O /13 Sn +2 O /12 (NH 4) +1 O 52 54/15/31 Fe 2 +3 O /30 Sn +4 O /21 K 2 +1 O 94 83/17 Fluoride Mg +2 F 2 F 62 39/61 Ba +2 F /22 Cu +2 F /37 Pb +4 F /29 Sn +2 F /24 (NH 4) +1 F 37 38/11/51 Fe +3 F /50 Sn +4 F /39 K +1 F 58 67/33 Chloride Mg +2 Cl 2 Cl 94 25/75 Ba +2 Cl /34 Cu +2 Cl /52 Pb +4 Cl /40 Sn +2 Cl /37 (NH 4) +1 Cl 53 26/8/65 Fe +3 Cl /65 Sn +4 Cl /54 K +1 Cl 74 53/47 Iodide Mg +2 I 2 I 278 9/91 Ba +2 I /65 Cu +2 I /80 Pb +4 I /71 Sn +2 I /68 (NH 4) +1 I /3/87 Fe +3 I /87 Sn +4 I /81 K +1 I /77 Cyanide Mg +2 (CN) 2 (CN) 76 31/31/37 Ba +2 (CN) /13/15 Cu +2 (CN) /21/24 Pb +4 (CN) /15/18 Sn +2 (CN) /14/16 (NH 4) +1 (CN) 44 32/9/27/32 Fe +3 (CN) /27/31 Sn +4 (CN) /22/15 K +1 (CN) 65 60/18/22 Sulfide Mg +2 S S 56 43/57 Ba +2 S /19 Cu +2 S 96 67/33 Pb +4 S /24 Sn +2 S /21 (NH 4) 2 +1 S 68 41/12/47 Fe 2 +3 S /46 Sn +4 S /35 K 2 +1 S /29 Peroxide Mg +2 (O 2) (O 2) 56 43/57 Ba +2 (O 2) /19 Cu +2 (O 2) 96 67/33 Pb +4 (O 2) /24 Sn +2 (O 2) /21 (NH 4) 2 +1 (O 2) 68 41/12/47 Fe 2 +3 (O 2) /46 Sn +4 (O 2) /35 K 2 +1 (O 2) /29 Nitrate Mg +2 (NO 3) 2 (NO 3) /19/65 Ba +2 (NO 3) /11/37 Cu +2 (NO 3) /15/51 Pb +4 (NO 3) /12/42 Sn +2 (NO 3) /12/40 (NH 4) +1 (NO 3) 80 18/5/18/60 Fe +3 (NO 3) /17/60 Sn +4 (NO 3) /15/52 K +1 (NO 3) /14/48 Sulfate Mg +2 (SO 4) (SO 4) /27/53 Ba +2 (SO 4) /14/27 Cu +2 (SO 4) /20/40 Pb +4 (SO 4) /11/21 Sn +2 (SO 4) /15/30 (NH 4) 2 +1 (SO 4) /6/24/48 Fe 2 +3 (SO 4) /24/48 Sn +4 (SO 4) /21/41 K 2 +1 (SO 4) /18/37 Hypochlorite (ClO 1) Mg +2 (ClO 1) /56/25 Ba +2 (ClO 1) /29/13 Cu +2 (ClO 1) /42/19 Pb +4 (ClO 1) /34/16 Sn +2 (ClO 1) /34/14 (NH 4) +1 (ClO 1) 69 20/6/51/23 Fe +3 (ClO 1) /50/23 Sn +4 (ClO 1) /43/20 K +1 (ClO 1) 90 43/39/18 Phosphate Mg (PO 4) 2 (PO 4) /24/48 Ba 3 +2 (PO 4) /10/21 Cu 3 +2 (PO 4) /16/34 Pb 3 +4 (PO 4) /12/26 Sn 3 +2 (PO 4) /11/23 (NH 4) 3 +1 (PO 4) /8/21/43 Fe +3 (PO 4) /21/42 Sn 3 +4 (PO 4) /17/35 K 3 +1 (PO 4) /15/30 Sulfite Mg +2 (SO 3) (SO 3) /31/46 Ba +2 (SO 3) /15/22 Cu +2 (SO 3) /22/33 Pb +4 (SO 3) /17/26 Sn +2 (SO 3) /16/24 (NH 4) 2 +1 (SO 3) /8/31/47 Fe 2 +3 (SO 3) /27/41 Sn +4 (SO 3) /23/34 K 2 +1 (SO 3) /27/41 Chlorite Mg +2 (ClO 2) 2 (ClO 2) /45/40 Ba +2 (ClO 2) /26/24 Cu +2 (ClO 2) /35/32 Pb +4 (ClO 2) /29/27 Sn +2 (ClO 2) /28/25 (NH 4) +1 (ClO 2) 85 16/5/41/38 Fe +3 (ClO 2) /41/37 Sn +4 (ClO 2) /36/33 K +1 (ClO 2) /33/30 Hydroxide Mg +2 (OH) 2 (OH) 58 41/55/3 Ba +2 (OH) /19/1 Cu +2 (OH) /33/2 Pb +4 (OH) /23/1 Sn +2 (OH) /21/1 (NH 4) +1 (OH) 35 40/11/46/3 Fe +3 (OH) /45/3 Sn +4 (OH) /34/2 K +1 (OH) 56 70/29/2 Chlorate Mg +2 (ClO 3) 2 (ClO 3) /37/51 Ba +2 (ClO 3) /23/32 Cu +2 (ClO 3) /30/42 Pb +4 (ClO 3) /26/36 Sn +2 (ClO 3) /25/34 (NH 4) +1 (ClO 3) /4/35/48 Fe +3 (ClO 3) /34/47 Sn +4 (ClO 3) /31/43 K +1 (ClO 3) /29/39 Nitrite Mg +2 (NO 2) 2 (NO 2) /24/55 Ba +2 (NO 2) /12/28 Cu +2 (NO 2) /18/41 Pb +4 (NO 2) /14/33 Sn +2 (NO 2) /13/30 (NH 4) +1 (NO 2) 64 22/6/22/50 Fe +3 (NO 2) /22/49 Sn +4 (NO 2) /18/42 K +1 (NO 2) 85 46/16/38 Carbonate Mg +2 (CO 3) (CO 3) 84 29/14/57 Ba +2 (CO 3) /6/24 Cu +2 (CO 3) /10/39 Pb +4 (CO 3) /7/29 Sn +2 (CO 3) /7/27 (NH 4) 2 +1 (CO 3) 82 17/10/15/59 Fe 2 +3 (CO 3) /12/49 Sn +4 (CO 3) /10/40 K 2 +1 (CO 3) /9/35 Permanganate (MnO 4) Mg +2 (MnO 4) /42/19 Ba +2 (MnO 4) /29/34 Cu +2 (MnO 4) /36/42 Pb +4 (MnO 4) /32/37 Sn +2 (MnO 4) /31/36 (NH 4) +1 (MnO 4) /3/40/47 Fe +3 (MnO 4) /40/46 Sn +4 (MnO 4) /37/43 K +1 (MnO 4) /35/41 8

9 Compound Friendly Intensity Chemistry Answer Card Page 2 Cations Anions Zinc Zn +2 Sodium Na +1 Hydrogen H +1 Nickel Ni +2 Calcium Ca +2 Lithium Li +1 Silver Ag +1 Chromium Cr +3 Aluminum Al +3 Lead (II) Pb +2 Bromide Zn +2 Br 2 Br /71 Na +1 Br /78 H +1 Br 81 1/99 Ni +2 Br /73 Ca +2 Br /80 Li +1 Br 87 8/92 Ag +1 Br /43 Cr +3 Br /82 Al +3 Br /90 Pb +2 Br /44 Oxide Zn +2 O O 81 80/20 Na 2 +1 O 62 74/26 H 2 +1 O 18 11/89 Ni +2 O 75 79/21 Ca +2 O 56 71/29 Li 2 +1 O 30 47/53 Ag 2 +1 O /7 Cr 2 +3 O /32 Al 2 +3 O /47 Pb +2 O /7 Fluoride Zn +2 F 2 F /34 Na +1 F 42 55/45 H +1 F 20 5/95 Ni +2 F /39 Ca +2 F /49 Li +1 F 26 27/73 Ag +1 F /15 Cr +3 F /52 Al +3 F /71 Pb +2 F /16 Chloride Zn +2 Cl 2 Cl /52 Na +1 Cl 58 40/60 H +1 Cl 36 3/97 Ni +2 Cl /54 Ca +2 Cl /64 Li +1 Cl 42 17/83 Ag +1 Cl /24 Cr +3 Cl /67 Al +3 Cl /80 Pb +2 Cl /25 Iodide Zn +2 I 2 I /80 Na +1 I /85 H +1 I 128 1/99 Ni +2 I /81 Ca +2 I /86 Li +1 I 134 5/95 Ag +1 I /54 Cr +3 I /88 Al +3 I /94 Pb +2 I /55 Cyanide Zn +2 (CN) 2 (CN) /21/24 Na +1 (CN) 49 47/21/29 H +1 (CN) 27 4/44/52 Ni +2 (CN) /22/25 Ca +2 (CN) /26/30 Li +1 (CN) 33 21/36/42 Ag +1 (CN) /9/10 Cr +3 (CN) /28/32 Al +3 (CN) /35/40 Pb +2 (CN) /9/11 Sulfide Zn +2 S S 97 67/33 Na 2 +1 S 78 59/41 H 2 +1 S 34 6/94 Ni +2 S 91 65/35 Ca +2 S 72 56/44 Li 2 +1 S 46 30/70 Ag 2 +1 S /13 Cr 2 +3 S /48 Al 2 +3 S /65 Pb +2 S /13 Peroxide Zn +2 (O 2) (O 2) 97 67/33 Na 2 +1 (O 2) 78 59/41 H 2 +1 (O 2) 34 6/94 Ni +2 (O 2) 91 65/35 Ca +2 (O 2) 72 56/44 Li 2 +1 (O 2) 46 30/70 Ag 2 +1 (O 2) /13 Cr 2 +3 (O 2) /48 Al +3 2(O 2) /65 Pb +2 (O 2) /13 Nitrate Zn +2 (NO 3) 2 (NO 3) /15/51 Na +1 (NO 3) 85 27/16/56 H +1 (NO 3) 63 2/22/76 Ni +2 (NO 3) /15/52 Ca +2 (NO 3) /17/59 Li +1 (NO 3) 69 10/20/70 Ag +1 (NO 3) /8/28 Cr +3 (NO 3) /18/61 Al +3 (NO 3) /20/68 Pb +2 (NO 3) /8/29 Sulfate Zn +2 (SO 4) (SO 4) /20/40 Na 2 +1 (SO 4) /23/45 H 2 +1 (SO 4) 98 2/33/65 Ni +2 (SO 4) /21/41 Ca +2 (SO 4) /24/47 Li 2 +1 (SO 4) /59/28 Ag 2 +1 (SO 4) /10/21 Cr 2 +3 (SO 4) /24/49 Al 2 +3 (SO 4) /28/56 Pb +2 (SO 4) /11/21 Hypochlorite (ClO 1) Zn +2 (ClO 1) /42/19 Na +1 (ClO 1) 74 31/47/22 H +1 (ClO 1) 52 2/67/31 Ni +2 (ClO 1) /43/20 Ca +2 (ClO 1) /49/23 Li +1 (ClO 1) 58 12/60/28 Ag +1 (ClO 1) /22/10 Cr +3 (ClO 1) /51/23 Al +3 (ClO 1) /59/27 Pb +2 (ClO 1) /23/10 Phosphate Zn (PO 4) 2 (PO 4) /22/31 Na 3 +1 (PO 4) /19/39 H 3 +1 (PO 4) /32/65 Ni 3 +2 (PO 4) /17/35 Ca 3 +2 (PO 4) /20/41 Li 3 +1 (PO 4) /27/55 Ag 3 +1 (PO 4) /7/15 Cr +3 (PO 4) /21/44 Al +3 (PO 4) /26/53 Pb 3 +2 (PO 4) /8/15 Sulfite Zn +2 (SO 3) (SO 3) /22/31 Na 2 +1 (SO 3) /25/38 H +1 (SO 3) 82 2/39/59 Ni +2 (SO 3) /23/35 Ca +2 (SO 3) /27/40 Li 2 +1 (SO 3) 94 15/34/51 Ag 2 +1 (SO 3) /11/16 Cr 2 +3 (SO 3) /28/42 Al 2 +3 (SO 3) /33/49 Pb +2 (SO 3) /11/17 Chlorite Zn +2 (ClO 2) 2 (ClO 2) /35/32 Na +1 (ClO 2) 90 26/39/35 H +1 (ClO 2) 68 1/51/47 Ni +2 (ClO 2) /36/33 Ca +2 (ClO 2) /40/37 Li +1 (ClO 2) 74 9/47/43 Ag +1 (ClO 2) /20/18 Cr +3 (ClO 2) /42/38 Al +3 (ClO 2) /46/42 Pb +2 (ClO 2) /21/19 Hydroxide Zn +2 (OH) 2 (OH) 99 66/32/2 Na +1 (OH) 40 57/40/2 H +1 (OH) 18 11/89 Ni +2 (OH) /34/2 Ca +2 (OH) /43/3 Li +1 (OH) 24 29/67/4 Ag +1 (OH) /13/1 Cr +3 (OH) /47/3 Al +3 (OH) /62/4 Pb +2 (OH) /13/1 Chlorate Zn +2 (ClO 3) 2 (ClO 3) /30/42 Na +1 (ClO 3) /33/45 H +1 (ClO 3) 84 1/42/57 Ni +2 (ClO 3) /31/43 Ca +2 (ClO 3) /34/47 Li +1 (ClO 3) 90 8/39/53 Ag +1 (ClO 3) /18/25 Cr +3 (ClO 3) /35/48 Al +3 (ClO 3) /38/52 Pb +2 (ClO 3) /19/26 Nitrite Zn +2 (NO 2) 2 (NO 2) /19/37 Na +1 (NO 2) 69 33/20/46 H +1 (NO 2) 47 2/30/68 Ni +2 (NO 2) /19/42 Ca +2 (NO 2) /21/48 Li +1 (NO 2) 53 13/26/60 Ag +1 (NO 2) /9/21 Cr +3 (NO 2) /22/51 Al +3 (NO 2) /26/59 Pb +2 (NO 2) /9/21 Carbonate Zn +2 (CO 3) (CO 3) /10/38 Na 2 +1 (CO 3) /11/45 H 2 +1 (CO 3) 62 3/19/77 Ni +2 (CO 3) /10/40 Ca +2 (CO 3) /12/48 Li 2 +1 (CO 3) 74 19/16/65 Ag 2 +1 (CO 3) /4/17 Cr 2 +3 (CO 3) /13/51 Al 2 +3 (CO 3) /16/62 Pb +2 (CO 3) /4/18 Permanganate (MnO 4) Zn +2 (MnO 4) /36/42 Na +1 (MnO 4) /39/45 H +1 (MnO 4) 120 1/46/53 Ni +2 (MnO 4) /37/43 Ca +2 (MnO 4) /40/46 Li +1 (MnO 4) 126 6/44/51 Ag +1 (MnO 4) /24/28 Cr +3 (MnO 4) /40/47 Al +3 (MnO 4) /43/50 Pb +2 (MnO 4) /25/29 9

10 NAME DATE FRIENDLY CHEMISTRY Lesson 17 Test Finding Formula Weights Find the formula weight of each compound listed below. Show your work! 1. Sodium chloride Formula weight = 2. Barium oxide Formula weight = 3. Calcium sulfate Formula weight = 4. Magnesium phosphate Formula weight = 5. Zinc nitrate Formula weight = 6. Lithium carbonate Formula weight = 7. Copper (II) chlorite Formula weight = 8. Ammonium sulfide Formula weight = 9. Aluminum hydroxide Formula weight = 10. Lead (IV) hypochlorite Formula weight = 10

11 Lesson 17: Finding Formula Weights In the last lesson, you learned that chemicals are measured in amounts called moles. You learned that a mole of any chemical contains 6.02 x atoms but the weight of a mole varies from element to element. For example, you learned that a mole of oxygen weighs about 16 grams while a mole of lead weighs about 208 grams. On your practice problems you were asked to weigh-out a specified number of moles of different chemicals. In order to do those problems, you referred to your periodic table of elements. In this lesson, we will move one step further and apply the concepts we learned in the last lesson to finding amounts of compounds. This process is called finding a formula weight. Let s begin by looking at the compound calcium chloride. Recall that in the compound calcium chloride, we had to take two chloride ions for every one calcium ion to achieve a stable compound. Ca +2 1 molecl 2 moles Or Ca +2 1 Cl 2 11 S147

12 To find the formula weight, we examine all components of the compound. In this case, we have moles of calcium and chlorine. Begin by listing those components: Ca: Cl: Next, write the number of moles of each component. Ca: 1 mole Cl: 2 moles Next, multiply the number of moles present by the atomic mass of each element present. Ca: 1 mole x 40 grams per mole = Cl: 2 moles x 35 grams per mole = Complete the arithmetic and then add the results together to find the formula weight. Ca: 1 mole x grams per mole = 40 grams Cl: 2 moles x grams per mole = 70 grams Formula weight = 110 grams We can then say that one mole of calcium chloride has the mass of 110 grams. From the example above, you can see that the formula weight for a compound is the total number of grams found when the atomic masses of each ingredient are added. We can also say that the formula weight of a compound is equal to the mass of one mole of that compound. Knowing the formula weight of a compound will be quite useful as we continue to explore how to go about preparing compounds in the laboratory. Before we continue, take some time to practice finding formula weights by working some practice problems. Below are three more examples. S148 12

13 Example 1. Find the formula weight of sodium hydroxide. Step 1. Write the formula for the stable compound. Sodium: Na +1 Hydroxide: (OH) Sodium Hydroxide: Na +1 1(OH) 1 We can say that to make one mole of sodium hydroxide, one mole of sodium ions and one mole of hydroxide ions are required. Step 2. List the elements present in the compound. Na: O: H: Step 3. Find the number of moles present of each element and multiply by the atomic mass values for each element present. Na: 1 x 23 grams = 23 grams O: 1 x 16 grams = 16 grams H: 1 x 1 gram = 1 gram Step 4. Add up the grams. Na: 1 x 23 grams = 23 grams O: 1 x 16 grams = 16 grams H: 1 x 1 gram = 1 gram Formula weight = 40 grams From this calculation we can say that one mole of sodium hydroxide would have the mass of 40 grams. If you needed one mole of sodium hydroxide, you would weigh-out 40 grams. 13 S149

14 Example 2. Find the formula weight of potassium sulfate. Step 1. Write the formula for the stable compound. Potassium: K +1 Sulfate: (SO 4 ) Potassium sulfate: K +1 2(SO 4 ) 1 Step 2. List the elements present in the compound. K: S: O: Step 3. Find the number of moles present of each element and multiply by the atomic mass values for each element present. K: 2 x 39 grams = 78 grams S: 1 x 32 grams = 32 grams O: 4 x 16 grams = 64 grams Step 4. Add up the grams. K: 2 moles x 39 grams/mole = 78 grams S: 1 mole x 32 grams/mole = 32 grams O: 4 moles x 16 grams/mole = 64 grams Formula weight = 174 grams From these calculations, one mole of potassium sulfate would have the mass of 174 grams. S150 14

15 Example 3. Find the formula weight for one mole of chromium (III) nitrate. Step 1. Write the formula for the stable compound. Chromium: Cr +3 Nitrate: (NO 3 ) Chromium nitrate: Cr +3 1(NO 3 ) 3 Note that there are three nitrate ions in this compound. You can envision the compound like this: (NO 3 ) (Cr) +3 (NO 3 ) (NO 3 ) Cr +3 1(NO 3 ) 3 Steps 2-4. Cr: 1 mole x 52 grams/mole = 52 grams N: 3 moles x 14 grams/mole = 42 grams O: 9 moles x 16 grams/mole = 144 grams (three sets of three oxygens) Formula weight = 238 grams Practice finding formula weights by playing Compound Intensity or completing the following practice pages. 15 S151

16 Name Date Friendly Chemistry Lesson 17: Finding Formula Weights Practice 1 Find the formula weights for the following compounds. 1. Sodium oxide 2. Barium fluoride 3. Aluminum sulfide 4. Nickel carbonate 5. Calcium chlorate S152 16

17 6. Barium permanganate 7. Zinc hydroxide 8. Potassium nitrate 9. Sodium phosphate 10. Lithium sulfite 11. Calcium acetate 12. Chromium perchlorate 17 S153

18 Name Date Friendly Chemistry Lesson 17: Formula Weight Practice 2 1. Joel needed to prepare a mole of potassium oxide. How many grams should be get from the stock container? 2. Mary was asked to prepare 3 moles of sodium chloride for a lab activity. How many grams should she get? 3. Frank had 3 moles of lead (II) oxide. How many grams of this compound did he have? 4. Sarah had 10 moles of hydrogen phosphate. How many grams of this compound did she have? 5. Horace had 12 moles of carbon. Julie had 144 grams of carbon. Who had the most carbon? S154 18

19 6. Francis was conducting a lab exercise which required she use 9 moles of iron (III) sulfate. She looked in the chemical closet and found she had a full container of the compound. The label said there was 500 grams in the bottle. Did she have enough? Show your work as proof of your answer. 7. Joey was given 1000 grams of calcium carbonate (limestone). How many moles did he have? 8. Terry had two containers of copper (I) sulfite. One said it had 3.5 moles and the second container s label said it contained 4.75 moles of the compound. How many grams of copper (I) sulfite did Terry have? 19 S155

20 Notes S156 20