A sample of carbon-12 has a mass of 3.0 g. Which expression gives the number of atoms in the sample? (NA is the symbol of the Avogadro costant)
Correct answer: B. 0.25 NA
- A. 0.0030 NA
- B. 0.25 NA
- C. 3.0 NA
- D. 4.0 N A
Explanation
The number of atoms in a sample can be determined by first calculating the number of moles using the formula: Number of moles = Mass / Molar mass. For carbon-12, the molar mass is 12 g/mol, so the number of moles in a 3.0 g sample is 3/12 = 0.25 moles. The number of atoms is then found by multiplying the number of moles by Avogadro's constant (NA), which gives 0.25 NA. Therefore, option B is correct.Option A incorrectly calculates the moles, leading to a wrong number of atoms. Option C incorrectly assumes that mass in grams is equivalent to moles, and Option D makes an incorrect calculation of moles.
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About Moles and Avogadro's Number
The mole connects the microscopic number of particles with measurable mass, using Avogadro's number, 6.022 × 10²³ particles per mole. Questions involve molar mass, conversion between mass, moles and particles, percentage composition, empirical and molecular formulas, and distinguishing atoms, molecules, ions and formula units.
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