Formula weight, molar mass, counting particles by weighing them, and empirical and molecular formulas. The in-class questions, practice and rounds of ten.
Wednesday 23 September
Formula mass, the mole and molar mass, then percent composition. Empirical and molecular formulas follow.
Homework: Formula Mass and the Mole Concept, in the Chapter 3 folder.
A mole is a count, 6.022 × 1023 of anything. One mole of a substance weighs its formula mass in grams, and that one fact is what lets you count atoms on a balance.
Atomic masses are the course periodic table’s, as printed on the exam equations sheet: most to two decimal places, hydrogen to three, 1.008. Add them up as many times as the subscripts say, and that sum is the formula weight in amu and the molar mass in grams per mole. Parentheses multiply everything inside them, and the dot in a hydrate such as CuSO4·5H2O means five whole water molecules are along for the ride.
Every one of these problems is the same walk along that line. Find where the question starts, find where it ends, and take the steps between them one at a time.
The percentages of all the elements add to 100, which is a free check on your arithmetic. Percent composition needs no sample size, because the ratio is the same in a gram as in a ton.
A molecular compound such as C₂H₆ comes in molecules. An ionic one such as NaCl has no molecules, so a mole of it is 6.022 × 10²³ formula units.
The atomic masses are table values. The measured mass is what limits the answer, so 24 g of anything gives two significant figures and 9.23 g gives three.
Written 6.022 × 10²³, four significant figures. On the exam sheet it is given, so use it as written rather than a rounder 6 × 10²³.
The course table gives hydrogen as 1.008, so glucose is 180.16 g/mol; with 1.01 it is 180.18. Either passes, but the significant figures follow the mass you used: 4 × 1.01 = 4.04 has three, 4 × 1.008 = 4.032 has four. Keep one set of masses and do not mix them.
The eight questions from the Chapter 3 in-class sheet, one box at a time. Answers are checked for significant figures as well as value. A calculation such as 25.6/98.09 shows what it comes to, then write the number out. The answer stays hidden until you have tried.
A fresh compound and a fresh sample every time, worked the way the homework asks: formula weight, moles, particles, atoms of one element, then percent by mass.
Empirical and molecular formulas, the steps from the Chapter 3 notes: moles of each element, divide by the smallest, then clear any fraction. By combustion, carbon comes from the CO₂, hydrogen from the H₂O and oxygen by difference. A molar mass turns the empirical formula into the molecular one.
Molar masses are the course table’s, on the exam equations sheet.