Question
Chemistry textbooks routinely assign water a value of 15.74 for this quantity, even though decades of experiments have shown that estimate is too high. For 10 points each:
[10m] Name this molecular property which decreases as a compound becomes more acidic. This quantity and the pH determine the degree of ionization in the Henderson-Hasselbalch equation.
ANSWER: pKa [or the negative logarithm of the acid dissociation constant; or the negative logarithm of the Ka, but reject any partial answers that do not include all three parts; prompt on pKw; reject “Ka” or “acid dissociation constant” by themselves]
[10e] Water’s actual pKa is this two-digit integer, the typical upper bound of the pH scale.
ANSWER: 14
[10h] The confusion stems from incorrectly using water’s molarity in the law of mass action, rather than this dimensionless quantity, which is defined to be 1 for all pure solids and liquids.
ANSWER: activity [reject “activity coefficient”]
<Adam Silverman, Science - Chemistry> ~20758~ <Editor: Adam Silverman>
Summary
| 2023 PACE NSC | 06/10/2023 | Y | 33 | 16.36 | 100% | 64% | 0% |
Data
| Canyon Crest | Woodland | 10 | 10 | 0 | 20 |
| Carbondale | Chattahoochee B | 10 | 10 | 0 | 20 |
| Choate Rosemary Hall | Chattahoochee A | 10 | 10 | 0 | 20 |
| Detroit Catholic Central B | Detroit Catholic Central C | 10 | 10 | 0 | 20 |
| Hunter B | Elkhorn North | 10 | 10 | 0 | 20 |
| Heights | Irvington A | 0 | 10 | 0 | 10 |
| Hinsdale Central | New Brighton | 0 | 10 | 0 | 10 |
| Mercer County | Hoover B | 10 | 10 | 0 | 20 |
| Hunter A | Belmont | 10 | 10 | 0 | 20 |
| Innovation Academy A | Carl Sandburg | 10 | 10 | 0 | 20 |
| Innovation Academy B | John Adams A | 0 | 10 | 0 | 10 |
| Johns Creek A | Dunbar | 10 | 10 | 0 | 20 |
| Cincinnati Hills Christian | Lake Highland | 0 | 10 | 0 | 10 |
| Barrington B | Maggie Walker C | 0 | 10 | 0 | 10 |
| Midtown | Lincoln-Way East | 10 | 10 | 0 | 20 |
| Thomas Jefferson B | Mira Loma | 10 | 10 | 0 | 20 |
| Moberly | Waukee | 0 | 10 | 0 | 10 |
| Northmont A | Thomas Jefferson C | 0 | 10 | 0 | 10 |
| Northmont B | West Point | 0 | 10 | 0 | 10 |
| Parkway West | Mexico | 0 | 10 | 0 | 10 |
| Dallas County | Plymouth | 0 | 10 | 0 | 10 |
| Detroit Catholic Central A | Richard Montgomery | 0 | 10 | 0 | 10 |
| Rockford Auburn A | Barrington A | 10 | 10 | 0 | 20 |
| Maggie Walker B | Rockford Auburn B | 0 | 10 | 0 | 10 |
| St. Mark's | Winston Churchill A | 10 | 10 | 0 | 20 |
| Winston Churchill B | Stevenson A | 10 | 10 | 0 | 20 |
| Thomas Jefferson A | Detroit Country Day | 10 | 10 | 0 | 20 |
| Thomas Jefferson D | Walter Payton | 10 | 10 | 0 | 20 |
| Troy | Solon | 10 | 10 | 0 | 20 |
| Kinkaid | University Lab | 10 | 10 | 0 | 20 |
| Washington | Belvidere | 10 | 10 | 0 | 20 |
| Maggie Walker A | William Fremd | 10 | 10 | 0 | 20 |
| Johns Creek B | Winnebago | 10 | 10 | 0 | 20 |