Question 1

What is atomic mass?

Accepted Answer

Atomic mass is the average mass of the naturally occurring isotopes of an element, usually expressed in atomic mass units (u). For an isotope mixture, average atomic mass is calculated as the weighted sum of isotope masses and their fractional abundances. By convention, 1 u is one-twelfth of the mass of one carbon-12 atom.

Question 2

How is the average atomic mass of chlorine calculated?

Accepted Answer

Using 75% ³⁵Cl and 25% ³⁷Cl, average atomic mass = 35 × 0.75 + 37 × 0.25 = 35.5 u.

Question 3

How is molecular mass calculated?

Accepted Answer

Molecular mass is the sum of the atomic masses of all atoms in one molecule. For H₂O using rounded values, it is 2 × 1 u + 16 u = 18 u.

Question 4

What is the difference between a formula unit and a molecule?

Accepted Answer

A molecule is the smallest particle of a covalent compound that retains its identity. A formula unit is the smallest electrically neutral unit of an ionic compound, such as Na⁺ and Cl⁻ in NaCl.

Question 5

What is the relationship between molecular and empirical formula mass?

Accepted Answer

Molecular mass equals n multiplied by empirical formula mass. Here n is the number of empirical formula units in one molecule’s molecular formula; it is not the number of separate particles being totalled. For H₂O and CO₂, the empirical and molecular formulas are the same, so n = 1.

Question 6

What does the 3D mass display show?

Accepted Answer

The Play view shows average atomic mass from the selected chlorine isotope abundances. The other views show the sum of atomic masses for one molecule or one ionic formula unit, and the total mass for the selected number of copies.

Isotope Cloud & Molecule Balance — Average and Molecular Mass

About this lab

Practice