Section 12.1
Law of Conservation of Mass
1. (Exercises 1 and 2) In a simple acid-base reaction, hydrochloric acid (HCl) combined with sodium hydroxide (NaOH) will form a salt (NaCl) plus water (H
20). If 110 grams of hydrochloric acid reacts with 120 grams of sodium hydroxide and 54 grams of water are formed, how much salt is produced?
Here we must use the law of conservation of mass. We started with 110 grams of hydrochloric acid and 120 grams of sodium hydroxide, so the total mass of substances before the reaction was 230 grams. This means that there must be 230 grams of total substance present after the reaction as well. Since 54 grams of water were formed, the mass of salt will be:
230 g - 54 g = 176 g
12.2Law of Definite Proportions
2. (Exercises 11 and 12) When an 11.2-g iron nail rusts but only 2.4 grams of oxygen is available to react with the nail, how much of the iron will be converted into iron(II) oxide?
The law of definite proportions tells us that the correct ratio of combination for iron and oxygen is determined by taking the ratio of their atomic masses, 56 to 16, or 3.5 to 1. Since oxygen is given as the limiting substance in this reaction, all of the oxygen will be used up and part of the mass of iron will react with it in the ratio of 3.5 parts iron to 1 part oxygen. This means that 3.5 times the amount of the limiting reactant oxygen, in this case 2.4 g, will give us the actual amount of iron that will take part in this reaction.
2.4 g x 3.5 = 8.4 g
In this reaction, 8.4 grams of iron will rust and the other 2.8 grams (11.2 g - 8.4 g) of iron will remain in its original form.
12.4Ionic Bonding
3. (Exercises 13 and 14) Using the periodic table, indicate the ionic charge expected for each of these representative elements: (a) Si, (b) In, (c) Sr, (d) Xe, (e) As, (f) Te, (g) Cl, (h) Na.
Using the column headings in the periodic table as a guide:
| (a) Si --> 0 | (b) In --> 3+ | (c) Sr --> 2+ | (d) Xe --> 0 |
| (e) As --> 3- | (f) Te --> 2- | (g) Cl --> 1- | (h) Na --> 1+ |
12.5Covalent Bonding
4. (Exercises 25 and 26) Use your knowledge of the appropriate number of covalent bonds to predict the formula for a simple compound formed between iodine and (a) sodium, (b) arsenic, (c) tellurium, and (d) silicon.
Sharing electron pairs to make each element isoelectronic with a noble gas, the chemical compounds for these compounds should be:
| (a) NaI | (b) AsI3 |
| (c) TeI2 | (d) SiI4 |
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