Counting by Weighing
A teaspoon holds 5.0 g of water. That one reading on a balance tells a chemist the spoon holds about 1.7 × 10²³ water molecules, far too many to count one by one.
How can a balance count molecules?
Page 1
Science · Chemistry
Chapter 1: Matter and Measurement
Let the units tell you if you are right.
A teaspoon holds 5.0 g of water. That one reading on a balance tells a chemist the spoon holds about 1.7 × 10²³ water molecules, far too many to count one by one.
How can a balance count molecules?
Make your guess to turn the page.
Still to come: 10 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students memorize conversion directions, "big to small, multiply", and the rules fail on anything unfamiliar, such as grams to moles. The book writes every conversion as a fraction equal to one, shows an upside-down factor leaving m²/cm, and runs a lab where cm × cm comes out as cm². Ask: which unit do you want to remove, and where must it go to cancel? The idea to reach is that units are algebra and the remaining unit is the check.
Chains of any length work the same — Grams to moles to particles is two factors in a row, and density, molar mass and Avogadro’s number chain the same way in every later chapter. Since the 2019 SI revision, Avogadro’s number is exactly 6.02214076 × 10²³, so it never limits significant figures; molar masses are measured and do. The orbiter mishap board (1999) blamed both the unit mismatch and the checks that failed to catch it.