Atomic mass, relative atomic mass and average atomic mass sheet4-mark answer frameWhat is average atomic mass? Calculate the average atomic mass of chlorine, which has Cl-35 (75.77%) and Cl-37 (24.23%).
How to lay out the answer. Follow the frame, then write it in your own words.
- 1 · Define in one line. Average atomic mass is the mean mass of the atoms of an element, counting each isotope by its natural percentage.
- 2 · Say why it is needed. Most elements are mixtures of isotopes, so the mass of one atom does not stand for the whole element.
- 3 · Write the formula. Average atomic mass = (A₁ × p₁ + A₂ × p₂) ÷ 100
- 4 · Substitute and work out.
- (35 × 75.77 + 37 × 24.23) ÷ 100
- = (2651.95 + 896.51) ÷ 100
- = 3548.46 ÷ 100 = 35.48 u
- 5 · Real-life link. Tables round this to 35.5, the value used to find the molar mass of common salt, NaCl = 58.5 g.
- 6 · Close in one line. Therefore the average atomic mass of chlorine is 35.48 u.
Scoring tip: Write the formula first and the unit u at the end. Show each multiplication on its own line.
