Which number is called Avogadro's constant, named after the 19 th century scientist Amedeo Avogadro?

Correct — C. Avogadro's constant is 6.022 × 10²³ per mole, the number of elementary entities in one mole of any substance.

The four choices differ only in their digits, so the decision is between 6.022 and the near misses 6.032, 6.012 and 6.020.

Since the 2019 revision of the SI, the constant is fixed by definition at 6.02214076 × 10²³ per mole. Rounded to three decimals that is 6.022, which is option (c).

Why the others are wrong

• (a)
  6.032 × 10 — 6.032 raises the hundredths digit. The accepted value runs 6.0221, so no rounding of it puts a 3 in that place.
• (b)
  6.012 × 10 — 6.012 lowers the same digit. The constant never rounds below 6.02, so this is outside the value altogether.
• (d)
  6.020 × 10 — 6.020 is the closest miss. The value continues 6.0221, so the third decimal place is a 2 and the figure to remember is 6.022.

A mole is an amount of substance containing a fixed number of elementary entities, and the Avogadro constant, 6.022 × 10²³ per mole, is that number.

It is the bridge between what a chemist can weigh and what a chemist wants to count. Because of it, a formula mass expressed in grams is one mole: 12 g of carbon-12, 18 g of water and 2 g of hydrogen gas each hold that many entities.

Amedeo Avogadro did not measure the number himself. It was named in his honour by later workers who established its value.

Key facts

• Avogadro's constant is 6.022 × 10²³ entities per mole.
• Since the 2019 revision of the SI, the value is fixed at exactly 6.02214076 × 10²³ per mole.
• One mole of a substance has a mass in grams equal to its formula mass.
• Amedeo Avogadro did not determine the constant himself, and it was named in his honour later.

Common traps

• Swapping a digit and writing 6.022 as 6.020 or 6.032.
• Treating the mole as a mass when it is a count of particles.

No directly related past PYQ was found.